Glossary of High-Pressure Compressed Gas & Breathing-Air Terms

reprinted from the High pressure Breathing Air Handbook with permission from Breathing Air Systems

ABSOLUTE PRESSURE: The atmospheric pressure, at sea level is 14.7 psi (actually 14.69 psi.) A pressure gauge, at sea level, would, of course, read ZERO. There remains this same 14.7 psi difference between absolute pressure and gauge pressure, at any pressure.

ACFM or ACTUAL CUBIC FEET PER MINUTE: This is the measurement of a compressor's static cfm output, with the back-pressure on the compressor held constant at a certain pressure. When the ACFM is measured with the compressor working against its highest intended working pressure, a true comparison of the compressor's efficiency can be made. Another term used for this measurement is FREE AIR DELIVERY, (FAD), at a stated back- pressure.

ABSORB: This speaks to the property of a media placed in the compressor's purification system. A media which ABSORBS a contaminant actually soaks it up and draws it within itself, much as a sponge absorbs water.

ADSORB: This is the property of a material which has the propensity to collect a contaminant by means of causing it to adhere to its outer surface.

ACDS, (AUTOMATIC CONDENSATE DRAIN SYSTEM): By means of an electric timer, a solenoid valve is momentarily energized every 15-18 minutes of operation, and each of the system's separators automatically drained of accumulated liquid. Without this feature, the operator must manually drain the separators four times during each hour of operation. The ACDS is a very convenient feature, and also provides assurance that the separators will be drained. The consequences of the failure to timely drain these separators are mechanical damage to the compressor, or the disabling of the system's purification system, through saturation. However, the ACDS itself requires regular monitoring and routine maintenance.

ACTIVATED CARBON: Charcoal obtained by the destructive distillation of vegetable matter and important as an adsorbent of gases and vapors.

AFTERCOOLER: Heat exchangers used for the cooling of the air after it leaves the final stage of the compressor. With air-cooled compressors, the aftercooler is usually a length of finned tubing, coiled in the flow of the cooling air.

AIR CONTROL PANEL: May be wall-mounted, but is more typically associated with the fill station. Valves, gauges, and regulators are mounted in the air control panel to regulate and control the flow to and from the storage bank, fill hoses, etc. While extremely elaborate air control panels are often used, it is my opinion that the most “basic” design that will accomplish the need is usually the best approach. Most system operators appreciate an air control system that is straight-forward and easy to think through.

AIR-COOLED COMPRESSORS: This design incorporates a fan or blower which directs a high-flow stream of air over the compressor block, heads, intercoolers, and aftercooler.

AIR FLOW: Usually measured in cubic feet per minute, (CFM), in our applications; generally used to speak of the output of a compressor or the amount of air flowing through a line. It can be measured with a flow meter, or by means of timing the flow of a known quantity of air.

AIR LINE RESPIRATOR: This describes a breathing system in which the user is provided air through an air line, or “umbilical” hose. Most often, the pressure in this hose is no more than 75-125 psi. The air can be supplied with a compressor, or from a bank of storage cylinders, (with the pressure reduced by a regulator to the required line pressure), or a combination of both. For extremely long runs of hose, using these relatively low pressures, larger hose interior diameters must sometimes be used. Consult the hose manufacturer for these recommendations. Air-line respirators differ from respirators in that an air supply is involved; generally, the description of a respirator means only a face mask with attached filter canisters.

AIR PURITY ANALYSIS: Usually conducted in a laboratory, where test conditions can be carefully controlled. (The NFPA recommends the use of only accredited laboratories). The laboratory usually provides the apparatus with which the air sample is taken and shipped. Upon completion of the test, the laboratory usually provides a print-out of all of the constituents of the air, and, if it meets all of the standards, a certificate. If the sample fails the analysis, the laboratory sometimes can suggest the means of correcting the situation which caused it. Since MOISTURE CONTENT is not a stated factor of most air purity standards, some laboratories will not test for moisture except as an extra cost. Even so, the compressor owner must insist on a moisture measurement. The NFPA currently recommends at least four air purity tests each year.

AMBIENT: Surrounding, encircling, encompassing. The air surrounding you can be called the ambient air. The temperature in a room where a compressor is located can be called the ambient temperature. Humidity and pressure can also be referred to as ambient, at a given location.

BLEEDER VALVE: Usually smaller than a line valve, this valve is most commonly used to bleed the trapped air from a line or vessel. For example, when the supply valve to a pressurized fill line is closed, and the full cylinder's valve is closed, the bleeder valve, positioned between the two, will relieve the pressure from the fill hose, so that an easier disconnection of the filler attachment can be made. Sometimes also referred to as a PURGE valve.

BLOCK: Used within the industry to describe the assembly comprised of the compressor crankcase with cylinders, cylinder heads, intercoolers, aftercooler, interstage separators, interstage safety relief valves, and drive pulley attached. With low pressure compressors, this assembly is more often called the HEAD.

BOOSTER PUMP: A one or two stage closed-loop mini-compressor which is capable of accepting air at a lower pressure and boosting it to a higher pressure. These are available powered by an electric motor, or powered by 100-150 psi of air pressure. The majority of those in service are of the latter type, utilizing a fraction of the supply air for power, making them completely self-contained.

BURST DISC: This is descriptive of a one-time safety device, typically built into cylinder valves. Instead of a spring-loaded poppet valve, it is a disc of frangible metal, (often copper, or an alloy thereof), held in place over an orifice. The size of this orifice and the strength of the disc are such that the disc will burst, at a certain predetermined pressure. This design burst pressure is usually 5/3, (166%), of the cylinder's working pressure. Once ruptured, the entire burst disc assembly must be replaced. In most modern cylinders, the air escaping from a ruptured burst disc will be directed in two or four opposing directions, so the cylinder will not be “jet propelled” violently. In cylinders that are repeatedly emptied and refilled, the burst disc can sometimes be flexed so often that some fatigue occurs, allowing the disc to eventually rupture at a pressure well below its design pressure. Burst disc assemblies are relatively inexpensive, and should be replaced periodically, if these conditions exist. Some authorities recommend that the burst disc assembly on a cylinder be replaced each time the cylinder is hydrostatically retested.

BURST PRESSURE: All compressor components, pressure vessels, hoses, tubing, etc., are constructed to a design burst pressure, meaning that the product can be expected to FAIL at that pressure. This has an entirely different meaning than the designed working pressure for that product. Depending on the intended safety ratio, the stated working pressure of the product will be a fraction of the burst pressure.

CARBON MONOXIDE INDICATOR: The CO indicator was typically used as an in-line visual check of gross Carbon Monoxide contamination in air. A color-change element was viewed through a transparent port, in which a usually high CO level was expected to change the color of the element from (usually) yellow to gray. The indicator, of course, was incapable of calling attention to itself, and tended to be rather hard to interpret, particularly by those people with some degree of color-blindness.

CARBON MONOXIDE MONITOR: An electronic device with a gas-sensitive electronic sensor. Quite accurate, when calibrated properly, and capable of providing a digital read-out of the exact Carbon Monoxide content of the air, in parts per million (ppm), and, if desired, providing for audible or visual alarms when the CO level exceeds a pre-set level. Most electronic CO monitors can also be used to automatically shut the equipment off, when this threshold is exceeded. The heart of the monitor is the sensor. These eventually must be replaced, typically every 2-5 years. One of two different types of sensors is usually used. The metal oxide type of sensor requires that the monitor be constantly powered, with a constant flowing air sample. When power or air flow is interrupted, these sensors may take several hours to re-stabilize. Electro-chemical sensors, on the other hand, only require about a minute or less to stabilize, when turned on.

COMPRESSOR BLOCK: In industry terminology, the compressor block consists of the compressor crankcase, cylinders, heads, intercoolers, aftercoolers, and inter-stage separators, all as an assembled unit ready to be powered and operated. Also known as a compressor head, usually when speaking of lower-pressure compressors.

CAPACITY: Usually a term for the quantity that a container or vessel can hold, this term also is used to describe the volume rate of a compressor's output.

CARBON: In respect to compressors, carbon is a potential service concern; generally caused by a residue of the lubricating oil carbonizing on surfaces that are too hot. When a carbon deposit forms between a head valve and its seat, or a small piece of dislodged carbon lodges in that position, the seal will be lost, and the compressor will malfunction.

CARTRIDGE-IMBEDDED SENSOR SYSTEMS: A component of advanced purification systems, an electronic sensor device placed directly in the final purification cartridge senses when the cartridge is approaching the end of its service life. At this point, a yellow light alarm is provided to the operator. If the operator does not replace this cartridge within the 4 or 5 hours still remaining in its service life, the sensor provides a red light, and shuts down of the compressor system. With this device, the entire service life of the cartridge can be utilized, and, at the same time, it is impossible to operate the air system with a spent cartridge in place.

CARTRIDGE, PURIFICATION: The word cartridge, in this application, indicates a pre-packed expendable replacement cartridge intended to be discarded when spent. Depending on its duty function, the purification cartridge will be packed with different materials to perform different functions in the processing of the air. If the purification system incorporates only one cartridge, layers of all of these materials will be packed in that one cartridge. In multi-chamber purification systems, all of the cartridges prior to the final cartridge will usually contain only desiccant materials.

Cartridges should not be reused, dried out, or repacked. All cartridges have an “expiration date”, beyond which they should be discarded and not used. When replacing the cartridges in a purification system, the date and hour meter reading should be recorded on the system's maintenance log, and many operators also mark this data on the new cartridge itself.

CASCADE METHOD: This refers to the working of a multi-vessel air storage system utilizing only one vessel at a time, in sequence, and closing the valve on that vessel before using the next. In this respect, the word “cascade” is a verb. When refilling from a multi-vessel storage system into smaller bottles, more small bottles will be completely refilled using this method, than would be affected by opening all of the storage vessels at the same time.

CASCADE SYSTEM: A term still commonly used to describe a multi-vessel air storage system.

CATALYTIC CONVERSION: The process by which the molecules of a gas are converted to a different structure, forming a different gas. In the case of the air system's purification system, this process is functioned by a catalyst packed in the final purification cartridge, and converts Carbon Monoxide to Carbon Dioxide. This catalyst will continue to function as long as it is absolutely dry, so the catalyst is always located after the dissicant. This conversion, however, can only process a certain level of Carbon Monoxide content, and if the incoming air contains more than approximately 400 ppm of Carbon Monoxide, the excess will simply pass on through, unconverted.

CFM (Cubic Feet per Minute): A cubic foot, of course, would be the free air in a space measuring 12" X 12" X 12". A telephone booth measuring 4' X 4' X 8' would contain 128 cubic feet. A compressor capable of pumping 10.7 cfm would pump 128 cu. ft. in approximately 12 minutes.

CHARGING RATE: This is the measure of a compressor's output based on its average output over the span of time required to fill a cylinder of a known capacity from a certain pressure to a certain pressure. To date, however, there exist no industry standards for this measurement, and manufacturers are free to choose the criteria which produce the results most conducive to the desired claims.

CHASSIS: In compressor terms, this is the frame on which the compressor block and the driver are mounted. In most modern units, the purification system will also be mounted on the chassis,along with the electrical control enclosure, the final separator, the gauge panel (if so equipped), and the other accessories that pertain to the operation of the compressor and purification system. Often, the chassis is described as being of vertical configuration, with the driver mounted under the compressor, or horizontal, meaning that the compressor and the driver are mounted side-by-side.

CHECK VALVE: A valve that automatically permits air flow in one direction only.

CLOSED LOOP: Describes an air flow completely contained in a closed circuit, without the possibility of contamination from air outside of the loop. Usually used to describe the method of boosting air pressure from a bank of purified stored air into the cylinders being refilled, thereby eliminating the entry of contaminants into the air stream.

COMPOSITE CYLINDER: A pressure vessel combining a rather thin aluminum alloy “shell”, over-wrapped with a resinoid fabric of exceptional strength. The benefit: lighter weight. Composite cylinders are somewhat more subject to damage from exterior abuse, and should be removed from service if the over-wrap shows evidence of undue wear. As of this date, composite cylinders are approved for a maximum service life of 15 years.

COMPRESSIBILITY: A factor which expresses the deviation of a gas from the laws of hydraulics. Gases vary in their compressibility, while most liquids are, for most practical purposes, incompressible.

COMPRESSION RATIO: The ratio of the absolute discharge pressure to the absolute inlet pressure. Absolute pressure at the inlet of a compressor, at sea level, would be 14.7 psi. (Gauge pressure would be zero.) If the outlet pressure at the discharge side of a compressor's first stage was 88 psi (absolute), this would be said to be a 6 to 1 (6:1) compression ratio. Remember, however, that 88 psi absolute pressure would be read on a gauge as about 73 psi.

CONDENSATE: The liquid which accumulates in the separators, and which must be drained regularly. In an oil-lubricated compressor, this liquid will usually be the color of skim milk, though it may be somewhat dark during the compressor's break-in period. After the compressor is broken in, if this condensate liquid turns quite dark or exceptionally oily, this condition may be a first indication that a problem could exist with the compressor.

CONDENSATE TRAP, CONDENSATE DRAIN: (See SEPARATOR) The condensate drain is the outlet at the lowest point of the separator, from which the accumulated liquid can be drained. On units not equipped with an automatic drain system, a petcock will be located at this point. (This petcock is to be opened momentarily for draining while the chamber is pressurized, and should never be left in an effort to provide constant draining.)

CONTAMINANTS: Any element in the breathing air than is potentially harmful to the breather of the air. Carbon Monoxide and oil droplets or oil vapor are two of the contaminants of the greatest concern.

CRANKSHAFT THROW: The point on the crankshaft to which the connecting rods are attached. Unlike automobile engines, most multi-stage air compressors have only one throw, to which all of the connecting rods are attached. The measurement by which the center line of the throw is different from the center line of the crankshaft determines half the stroke of the piston.

CYLINDER: Two definitions: A cylinder is a container or vessel in which a pressurized gas is stored. (always cylindrical) On a compressor or engine, the cylinder is the bore through which the piston moves.

CYLINDER HEAD: The compression cylinder is closed on top with a removable head, into which are usually built the intake and outlet valves for that cylinder. In the case of air-cooled compressors, the cylinder head is usually finned, to help transfer heat away.

DEALER: A seller of equipment, usually working through a manufacturer's regional distributor. There are some dealers who can provide field service, either limited or full-scale, while others may rely on the distributor to provide this; Responsive, capable dealer sale able to provide pre-specification consultation, budget estimates, and application expertise.

DEFLECTOR TUBE: A part of the Fill Station, the deflector tube accepts the bottle to be filled and holds it in place during the refilling process. In most fill stations, the deflector tube(s) are circular, and constructed of thick-walled metal, to help contain the fragments of a rupturing cylinder.

DESICCANT: A drying agent or media, as used as the first element in a modern purification cartridge.

DISPLACEMENT: The displacement of a compressor is the volume swept by the piston per unit of time. Usually expressed as cubic feet per minute. This is not a direct measurement of a compressor's output, however, as compressors vary greatly in their air leakage and valve efficiency.

DISTRIBUTOR: Most major manufacturers have established a network of servicing distributors. To qualify, the distributor must stock systems and parts, maintain a full-scale repair shop staffed with factory-certified technicians, and provide field service. Distributors, in turn, usually establish a network of regional dealers within their marketing area, and provide end-user service for those dealers without this capability. In those regions in which a dealer is not established, the distributor may sell directly to the end users.

DOOR INTERLOCK: Usually denotes the device installed on a Class 2 fill station which precludes the filling of cylinders if the protective door is left open. May also describe micro switches which are sometimes installed on room or compartment doors for the purpose of causing the door to be opened (for ventilation), before the compressor can be operated.

DOUBLE-ACTING PISTON: The double-acting piston is designed to compress the air in more than one stage at a time, either by “stacking” one cylinder or stage on top of another, or by compressing air in separate chambers on both the “up” and the “down” stroke. Thus, one connecting rod is used to compress air in two chambers. It is more difficult to carry away the added heat that this method produces.

DRAIN CYCLE: Refers to the interval between the recommended manual draining of the separator chambers, or between the automatic drain system's activation. In general, it is suggested that the drain cycle interval be shortened, somewhat, under unusually high ambient humidity conditions.

EFFICIENCY, VOLUMETRIC: Ratio of actual capacity (output) to the piston displacement, stated as a percentage. In high pressure multi-stage compressors, one design may have an efficiency rate of only around 60%, while another may achieve 75%, or more.

EXTENDED WARRANTY: A contract with the supplier in which the original factory warranty is extended, for an annual fee. With the extended warranty, some suppliers also include routine scheduled on-site maintenance service calls, and perform any needed interim service on-site, at the compressor's location. Others may provide no field service at all, and perform any needed warranty work in their own shop, or at the factory, with the owner paying for the transportation to the shop. Most extended warranties provided by compressor suppliers do not cover electric motors or fueled engines.

FABRICATOR: Commonly used to describe a manufacturer who utilizes various other brands of “off-the-shelf” components to construct a finished assembly, usually sold under the fabricator's own brand name. Oddly, this same description is also used to describe an O.E.M. (Original Equipment Manufacturer.)


FILL HOSE: This usually describes the flexible line which is connected to the valve of the small cylinder for refilling. The fill hose should be as short as is practical, to minimize the effects of “whipping”, should the hose ever rupture. It should be replaced whenever any signs of wear become apparent.

FILL RATE: Refers to the time required to refill a cylinder. Sometimes stated in terms of cfm, but if only one size of smaller bottles is usually being refilled, the fill rate is often spoken of in terms of minutes required to fill a cylinder of that size.

For example; an empty “30-minute” SCBA cylinder refilled in less than about four minutes, will have enough of a temperature rise that it will only contain 80-85% of its intended capacity when it cools back down to room temperature. To provide the next user with its intended duration, it will then have to be “topped-off” after it cools down (be re-connected to the air system to bring the pressure back up to the cylinder's working pressure.)

In some situations, taking four minutes to refill each small cylinder will seem time-consuming. In this case, if a means is available to connect and fill two cylinders at once, the recommended rate of four minutes will hold the temperatures down, but still result in an AVERAGE fill time of two minutes per cylinder. If the air source is adequate, even more cylinders can be refilled simultaneously, even further reducing the average fill time.

FILL STATION: Technically, the fill station is the protective device into which small cylinders are placed for the refilling process. However, the entire breathing air production system is also still sometimes referred to as a “fill station”.

FILTERS: Today, this usually refers to the pre-inlet devices that separate and remove dust and dirt before it enters the first stage of the compressor. In the past, the media through which the compressed air was flowed after compression was also known as a filter system, but modern systems which better improve the quality of the air are known as purification systems.

FLOATING PISTON: Used in the final stage of certain compressors, the floating piston is not secured to the connecting rod, but is moved with a non- compressing “push piston” that is so attached. Once this stage is pressurized by the intermediate pressure being supplied by the previous stage, the floating piston is held in contact with the push piston on the down stroke. By design, the floating piston can be ringless. Its seal on the cylinder walls is made with a thin film of oil between the two precision-machined surfaces. This creates less heat, less friction, better compression, and longer service life. It also tends to require less start-up torque.

FLOW METER: A device, usually hand-held, with which the CFM flow of air can be measured. Flow meters are available in a number of cfm ranges, typically 0-15 cfm, 0-30 cfm, etc. (The narrower the range, the more accurately they can be read.) While in-line flow meters for permanent installation are available, they are cost-prohibitive for pressures above 1000 psi. The flow meter is one of the primary diagnostic tools used by the compressor technician.

FLOW RESTRICTOR: To avoid harmful high-velocity air flows, or to limit the fill rate of small cylinders to a rate which will not create too much heat, the concept of a flow restrictor at first seems like good logic. Flow restrictors are available, and all operate on the principle of creating a restricted orifice through which only a limited amount of air can flow. However, in the act of refilling small cylinders, we are dealing with ever-increasing pressures on the downstream side of the restrictor valve, as the small cylinder progresses from empty to full. A restricted orifice will allow more air to pass through it when the pressure differential is the greatest. An orifice about the size of the period at the end of this sentence, for example, might allow an air flow of 10 cfm, when the upstream pressure is 5000 psi, and the downstream pressure is zero. However, when the downstream pressure climbs to 2500 psi, the flow would only be about 5 cfm. When the back-pressure reaches around 4000 psi, the flow would be even further reduced. The end result, using a fixed orifice, is that we would have a too-high air flow at the start of a refill, and such a small trickle of air as the refill neared completion that it would take 15-20 minutes to finish. While fixed orifice flow restrictors are useful for certain applications, they work poorly (or too well) for slowing down the refill procedure. Perhaps an “automatically variable orifice” flow restrictor will someday be developed.

FLUIDICS: The general subject of instruments and controls that are dependent upon low rate air flow of air or gas at low pressure as the operating medium.

FREE AIR: Free air is defined as air at atmospheric conditions at any specific location; also often referred to ambient air.

GAS: From a physics point of view, the form of air is that of a gas, as one of the three basic phases of matter. However, in pneumatics, a special meaning is assigned to air, and the term gas refers to any gas other than air. Air is actually composed of a number of gases, most notably Oxygen (19- 23%), and Nitrogen (77-81%).

GAUGE: The device by which pressure is indicated. In general, pressure gauges should not be used for pressures greater than approximately 75% of their maximum face pressure. It is generally recommended that one should face the gauge away from himself when it is pressurized.


HEAD: In compressor terminology, especially in lower-pressure compressors, the assembled compressor unit is known as the head. With high pressure units, the assembly is more often referred to as the block. See CYLINDER HEAD for more common usage.

HEAT, HIGH TEMPERATURES: Unduly high temperatures are probably the single most destructive element regarding compressor service life and air quality. The air compressor used for the production of breathing air must be designed for the coolest operation possible, and the installation of the air system must provide for an adequate and unrestricted flow of cooling air. The compressor and all of its cooling coils must be kept free of dirt accumulation at all times.

HIGH PRESSURE: Always a relative term, with no fixed description. For example, one might own some cylinders rated for 2400 psi, and some rated for 4500 psi, and speak of them as “low pressure” and “high pressure”. A compressor manufacturer might make “low pressure” compressors which produce 100-150 psi, and “high pressure” compressors rated to 6000 psi. A plumber might consider 300 psi fittings to be “high pressure”, and anything lower than that to be “low pressure”. In other words, never accept anyone else's terminology of either description.

HORSEPOWER, BRAKE: This is the horsepower input to the compressor shaft, or more generally, to any driven machine shaft.

HOUR METER: As used in breathing air compressor applications, the hour meter is usually of the “accumulative, non-resettable” type. That is, like an automobile odometer, the hour meter will always show the total number of operating hours on the compressor. Hour meters are available in any voltage or even in a “vibration” mode for engine-powered units with no electrical system. (Vibration-type hour meters, however, when mounted on a compressor system installed in a vehicle, sometimes log the vehicle's operation, even when the compressor is not operating.) For proper maintenance, of both the compressor and the purification system, ALL compressors used in our applications should be equipped with recording hour meters.

HUMIDITY, SPECIFIC: Specific humidity is the weight of water vapor in an air vapor mixture per pound of dry air.

HUMIDITY, RELATIVE: The relative humidity of an air mixture is the ratio of the partial pressure of the vapor to the vapor saturation pressure at the dry bulb temperature of the mixture.

HYDROSTATIC TEST: The means of measuring the elasticity of a high pressure cylinder. This is done by pressurizing the cylinder to a prescribed pressure higher than its working pressure, and measuring the temporary expansion of its dimensions, and the permanent expansion, if any, that remains when it is de-pressurized.

INTERCOOLERS: These are heat exchangers used for removing some of the heat of compression from the air between the stages of a compressor. These also serve to help remove a considerable amount of moisture, as well. In an air-cooled compressor, the intercoolers are usually seen in the form of lengths of finned tubing positioned in the flow of cooling air.


MAINTENANCE CONTRACT: An arrangement in which the supplier's technician, on a prescribed schedule, visits the air system to perform operational checks and routine maintenance, leaving a copy of his checklist for the owner's records. If such contracts are available from different sources, they may have different inclusions and exclusions, and the compressor owner will do well to have a complete understanding of the terms. Typically, the Maintenance Contract does not include the cost of any needed emergency service calls that come up between scheduled visits.

MAINTENANCE COORDINATOR: This is the individual assigned to be responsible for the air system's maintenance, and the permanent recording of all maintenance procedures performed. In some cases, he may also serve as the Operations Coordinator.

MAINTENANCE PROCEDURES: Usually drafted by the Maintenance Coordinator, in conjunction with the supplier and the equipment manuals, this is a printed schedule of the routine maintenance to be performed on the air system. Often in the form of a working chart, on which all routine service intervals are listed, then logged on completion.

MAINTENANCE TIMER: A resettable timer clock, which provides a visual or audible alarm, or a compressor shut-down, after a pre-determined number of hours or minutes. The concept is that the operator will therefore be reminded that a certain maintenance procedure is due. However, since various maintenance procedures are performed at different intervals, a single maintenance timer serves only to call attention to one operation.

MOBILE AIR SYSTEM: Generally describes an air refill system, or air-line respirator system, which is transported to the scene-of-the-need for operation. While the mobile air system can include a compressor, the recent trend is more towards the transportation of a quantity of stored air which has been processed by a stationary compressor located away from the scene-of-the-need.


MOISTURE INDICATOR: (See CARBON MONOXIDE MONITOR) In-line color-change moisture indicators are also available, but are incapable of drawing attention to an alarm situation.

MOISTURE MONITORS: These are electronic devices which, by means of an electronic sensor, or humidistat, are capable of alarming the operator when a pre-set moisture level is exceeded. Those which function with a sensing of the relative humidity are considered somewhat inaccurate, but can usually sense a gross moisture condition. (See CARTRIDGE-IMBEDDED SENSOR SYSTEMS)

MOLECULAR SIEVE: A media commonly used in modern purification cartridges, primarily as a desiccant (drying agent).

MUFFLER, SILENCER: For fueled engines, this is the device used to silence the exhaust. On compressors, it refers to the device, usually resembling a filter element that is in place to muffle the sound of the exhausting of the automatic condensate drain system.

MULTI-STAGE COMPRESSORS: These are compressors employing two or more stages.

MURPHY'S LAW: Yes, it's in the dictionary, and it belongs here. It is the principle that whatever can go wrong will (and usually at exactly the worst possible time). This is the rule that causes the electrical power to fail just when the air system operator has a stack of empty cylinders that need refilled quickly. It is the recognition of Murphy's Law that makes it practical, in this example, for emergency personnel to have an ample air storage system fully pressurized.

NEEDLE VALVE: A line valve in which the flow of the media is metered by the controlled insertion of a tapered shaft through an orifice of fixed size. When used for complete shut-off, the tapered shaft completely closes off the orifice, creating a metal-to-metal seal. With most such designs, it is difficult to attain a bubble-tight shut-off of high pressure air, so this is not a popular design for most air system applications.

ON-SITE START-UP: With the installation of a new air system, the buyer should insist that the supplier provide the services of a factory-certified delivery technician. The technician shall make a post-delivery integrity check of the entire system, check for correct installation and wiring, calibrate the system to the buyer's exact application needs, perform a thorough operational check of the entire system, and train the personnel who will be operating and maintaining the system. The buyer should beware of sellers who promise an “on-site start-up”, but are actually capable of little more than having a representative present at the time the system is turned on.

OPERATIONS COORDINATOR: The person assigned to develop the operating procedures for the air system. The drafting of these procedures is often done in consultation with the supplier, and the Maintenance Coordinator. The Operations Coordinator is responsible for seeing that the system is operated in accordance with these procedures. If other personnel are to be involved in the operation of the system, the coordinator is responsible for training these people and enforcing the correct procedures. In some cases, the Operations Coordinator may also serve as the Maintenance Coordinator.

OPERATING PROCEDURES: A written set of instructions for the correct and safe operation of the system. Usually separate from the MAINTENANCE PROCEDURES. All personnel who are authorized to operate the system are to be made thoroughly familiar with these procedures, and the key features of the procedures are to be prominently displayed in the vicinity of the air system. Since air systems are comprised of several components, and since applications can vary, the Operating Procedures must be exactly tailored to the individual system and the applications.

OXIDATION: The process or state of undergoing combination with Oxygen. In our applications, this usually translates to rust or corrosion.

OXYGEN CLEAN: Applies to fittings, hoses, regulators, etc. that are intended for use with pure Oxygen. For this application, all equipment must be chemically cleaned to remove all traces of any type of oil or oil residue. Such equipment, when new, is usually individually packaged, and labeled OXYGEN CLEAN.

PHASING: Electric-driven compressors will be equipped with either a single-phase motor, or three-phase. The electrical supply must match the type of motor. Some older three-phase supplies involve one phase (wire), which is of higher voltage than the other two. This is known as the “wild leg”, and must be connected to the proper terminal in the compressor's electrical enclosure, as per the manufacturer's instructions. Motor rotation of a three-phase motor can be reversed by switching any two of the three power leads. Thus, rotation of the motor must be checked upon installation, and any time thereafter when any wiring changes are made in the building. Though most compressors will pump air and seem to be functioning when the motor rotation is incorrect, damage and high operating temperatures will soon occur.

PIGTAIL: The most common use of this term, in our applications, is to describe the connections between the cylinders in a multi-cylinder air storage system. Usually “looped”, to provide some protection against vibration or movement of the cylinders, the pigtail may be made of hose or of tubing.

POD: This generally refers to a cluster of storage vessels contained in a mounting rack which can be lifted (with a crane or fork-lift), and transported as a single unit.

PLUG VALVE: In the field of high pressure air control it describes a valve design which incorporates a fixed orifice, and onto this a seal element is forced by a clock-wise torque of a threaded stem, for shut-off. A complete bubble-tight shut-off is easily achieved. The seal element most used is of dense nylon, or similar.

When the valve handle is turned counter-clockwise, the seal element is lifted from the orifice in tiny increments, and a metered air flow is readily controllable. In most designs, the usual air flow is intended to be through the orifice TOWARD the seal element. Virtually all cylinder valves are of this design, as are the majority of line valves and control panel valves in use. This is also known as the globe style.

PPM: Parts Per Million.

PRESSURE, DISCHARGE: Discharge pressure is the absolute total pressure at the discharge outlet of a compressor. Compressors are rated at a maximum design discharge pressure, and should not be operated at pressures greater than that, even if they are mechanically capable of generating higher pressures.

PRESSURE LUBRICATION: A feature in which oil pressure from an oil pump is directed to the internal points of the compressor which require absolute lubrication. Typically, in a high pressure compressor, the working oil pressure may be 800-1200 psi. Pressure lubrication is often combined with “splash” and “mist” lubrication for various other points within the crankcase. Without pressure lubrication, though, it is difficult for the lubricant to travel into the upper cylinder wall area of the high pressure stages.

PSI (Pounds Per Square Inch): The amount of pressure exerted on one square inch of surface; often stated as PSIA, or PSIG, signifying ABSOLUTE or GAUGE pressure.


PURIFICATION SYSTEM: This is a chamber or series of chambers, in which various media are packed (usually in disposable cartridges), through which the compressed air flows after it has passed through the final moisture separator. On most models of newer compressors, the purification system is mounted directly on the chassis with the compressor. Unlike earlier “filter” systems, which removed only certain contaminants, the modern purification system actually improves the quality of the air by changing some of its constituents (mainly, Carbon Monoxide molecules are converted to Carbon Dioxide molecules, by catalytic conversion.) On a stand-alone purification system, as for retrofit with an older compressor, a final separator chamber is also usually included.

QUARTER-TURN VALVE: A line valve that is FULLY-OPEN, or FULLY-CLOSED, with only a 90° rotation. This is an appealing concept, since such valves can quickly be seen to be open or closed. However, most quarter-turn valves do not lend themselves well to the act of metering the flow of air, which is a very common need in handling high pressure air. In fact, most quarter-turn valves will perform poorly when used for metering air flow, and will often fail prematurely. The quarter-turn valve should be used generally in an application where the air flow is to be only 100% ON or 100% OFF; also known as the ball valve.

RADIAL BLOCK: This term has to do with the positioning of the compression cylinders on the compressor's crankcase. In the radial design 4-stage unit, the stages are lined up 90° from each other. As a piston is on its up-stroke in one cylinder, it is one the down-stroke in the cylinder exactly opposite. The radial configuration tends to provide better balance of the moving parts, and is more effectively cooled.

RECEIVERS: Tanks or vessels used for the storage of air discharged from compressors. They also serve to dampen discharge line pulsations.

RECIPROCATING COMPRESSORS: The reciprocating compressor's element is a piston, or pistons, having a reciprocating motion inside a cylinder (as opposed to rotary or screw-type compressors, which are seldom used in high pressure breathing air applications).

REGULATOR: Usually adjustable, the regulator reduces a higher pressure to a lower pressure. Most regulators incorporate an IN pressure gauge (indicating the supply pressure), and an OUT pressure gauge (indicating the adjusted/regulated pressure).

All regulators are designed for a certain MAXIMUM inlet pressure, and a certain range of adjustable outlet pressure. Simply substituting higher or lower pressure gauges in the regulator will not change its performance characteristics or design safety!

In general, the higher the inlet pressure rating of the regulator, the higher its cost. FIXED RATIO non-adjustable regulators are also available, for certain applications. These reduce the supply pressure to a certain percentage; regulated pressure therefore varies with the supply pressure.

REMOTE INTAKE: This refers to the extension of the compressor's intake port to another area. Before modern purification systems were developed, this practice was rather common, by running the extension to outside of the building.

Today, the concept is that the compressor should not be installed in a location in which the quality of the ambient air would be hazardous to the health of the operator. If the ambient air at the compressor's location is good enough for the operator to breathe safely, then the compressor's purification system can readily improve this to Grade “E” air.

When unusual circumstances dictate the need for a remote intake, the compressor manufacturer's literature should be consulted for the appropriate dimensions of the extension. An extension of the wrong design or dimensions can very easily “throttle” the intake air, and cause reduced output and high operating temperatures.

SAFETY RELIEF VALVE: This describes a spring loaded valve which is normally in the CLOSED position, and which releases air when the upstream pressure is greater than the pre-selected relief pressure. Most such valves will re-close, when the upstream pressure is reduced. Safety valves are available in a wide range of pressure ratings, are usually adjustable for the exact desired relief pressure within that range, and should never be used to control a pressure other than the prescribed range.

A second factor pertaining to safety valves is the rated flow rate of the valve, meaning the maximum amount of air that can flow through the valve, at its rated pressure. For example, a safety valve that could only flow a maximum of 15 cfm, when opened, would be absolutely inappropriate for use in conjunction with a 20 cfm compressor! One-time BURST DISCS, which are used with cylinder valves and certain other applications, are sometimes referred to as safety valves, but they technically are not.

SCBA: An acronym for Self-Contained Breathing Apparatus. Usually descriptive of the unit consisting of the high pressure cylinder, the regulator(s), the connecting hoses, the wearer's face mask assembly, and the harness used to don the apparatus. SCBA units are to NEVER be used as SCUBA (underwater).

SCUBA: An acronym for Self-Contained Underwater Breathing Apparatus. SCUBA usually describes the high pressure cylinder, the diver's regulator and mouthpiece, and the back-pack or assembly used for attaching the unit to the diver.

SEPARATOR: Also known as a condensate trap. A chamber in which moisture is removed from the air, by impingement, coalescence, expansion, temperature change, or any of a number of other mechanical means. In the typical high pressure compressor used for breathing air applications, separator chambers are usually used between each of the stages of compression, and another final separator is used just prior to the air's entry into the purification system. All separator collection chambers must be emptied of their accumulated moisture regularly, without fail!

SERVICE CONTRACT: Unlike the less expensive Maintenance Contract, the service contract includes the cost of any and all needed emergency service calls. Most suppliers incorporate regularly scheduled routine maintenance visits as part of the service contract.

SINGLE-ACTING PISTON: The single-acting piston compresses air only in one chamber, on only one stroke (usually the “up” stroke). A double-acting piston, on the other hand, compresses air in more than one chamber (as in stacked cylinders), some on the same stroke, and some on both the up and down stroke. The single-acting piston tends to create less heat, but does require a separate connecting rod for each stage of compression.

SOLENOID VALVE: A valve, of any pressure, activated by an electrical surge from a remote source. Most automatic condensate drain systems incorporate one or more solenoid valves; can be thought of as serving the same purpose as a relay in an electrical circuit.

SPECIFIC GRAVITY: This property is the ratio of the specific weight of a gas to that of dry air at the same pressure and gravity. The specific gravity of dry air is 1.0, or the “benchmark” for the other gases. The specific gravity of some other gases are: Oxygen 1.105; Nitrogen .9672; Carbon Dioxide 1.529; Carbon Monoxide .9762; Helium .1381.

SPEED: (or RPM). The speed of a compressor refers to the revolutions per minute of the compressor shaft.

SPLASH LUBRICATION: A method of lubrication in which moving parts “dip” into the reservoir of oil in the crankcase, causing droplets to be thrown around the interior of the crankcase.

STACKED CYLINDERS OR STACKED STAGES: In this design, unlike the “radial” block design, one cylinder/stage is located on top of another, both compressed with one piston, and one connecting rod. While less costly than providing a separate rod and piston for each stage of compression, this design tends to make it difficult to carry away the heat of compression as efficiently.

STAGES: Stages are the steps of compression of a gas. Each stage usually requires a separate cylinder, and the air is compressed by the stages in succession.

STANDARD AIR: Defined as air at a temperature of 68°F, a pressure of 14.7 psi, and a relative humidity of 36%.

SUPPLIER/SOURCE: Generally considered to be the party who is paid for the purchased equipment. A supplier may be the manufacturer of the equipment; a distributor serving as the manufacturer's service and distribution point; a dealer who represents either a manufacturer or a distributor; or a broker, who usually has no service capability. In general, the buyer will obtain the best long-term value by choosing a source capable of providing expert ready field service. Most manufacturers will not sell directly to end users because the factories are not well positioned to provide end user field service.

TEMPERATURE, DISCHARGE: This is the total temperature at the discharge outlet of the compressor. In our applications, this more often refers to the temperature of the air at the outlet of the aftercooler. Well designed compressors will usually affect an (aftercooler) discharge temperature of no more than 15-17° over the compressor's inlet (ambient) temperature.

TWO-WAY VALVE: This is a line valve which is used to send the air flow in one direction or another. As an example, a two-way valve on an air control panel might be used to send the air to a high pressure circuit, when in one position, or to a lower-pressure circuit when in the other position. Like quarter-turn valves, however, the two-way valve is not intended for the metering of the air flow. Three-way valves are also available.

UNITIZED AIR SYSTEM: Also sometimes referred to as the MODULAR air system. A relatively recent development, the unitized air system incorporates the compressor, the driver, the purification system, the air storage system, the air control panel, the electronic monitors, and the fill station all in one cabinet. Unitized systems tend to have a neat appearance, sometimes can cost slightly less than the components purchased separately, and, in some cases, require less total floor space for installation.

VALVES, HEAD: These valves, usually metal-to-metal, control the flow of air into and out of each of the compression cylinders of the compressor. Except for extremely large compressors, these valves are not mechanically opened and closed, but are simply spring-loaded “check valves”, in effect. The head valve is one of the more critical parts of the compressor, and, especially on the higher-pressure stages, must be made of extremely durable metals, and machined to exacting close tolerances. Over a span of operation, the high pressure compressor's head valves are likely to require the most service.

VALVES, CONTROL: Line-mounted or panel mounted, the control valve regulates the flow of air, and are usually manually operated by the system operator.

VALVES, SOLENOID: Solenoid valves are usually actuated by electrical pulse or pneumatic flow or pressure differentials, and are used to function controls such as the automatic condensate drain system, automatic storage system sequential refilling systems, and some of the other automated devices.

“VEE” BLOCK: This refers to the design of the placement of the compression cylinders on the compressor's crankcase. The VEE block, when viewed from the front or back, resembles the letter “V”. If the compressor is of 3 or 4 stages, one or more of the stages will be “stacked” on top of another.

WATER BATH: This usually refers to the practice of immersing the small cylinders to be refilled into a container of cool water, to limit the temperature rise that will result from the compressing of the air. This is not recommended for composite cylinders.

WORKING PRESSURE: According to the desired SAFETY RATIO, a product's design working pressure will be a fraction of the product's design burst pressure. Some products are marked with their working pressure, while others aren't. If in doubt about an unmarked product's intended working pressure, the manufacturer's charts must be consulted. Never pressure any component beyond its rated working pressure.