Devair DRD 400 Specifications download pdf (Page 4)

General Definitions

To follow are basic terms and concepts that are used throughout this booklet. These are helpful in that they give a

general understanding of compressed air.

psia = gauge pressure + atmosp. pressure

= 175 psig + 14.7 psi

= 189.7 psia

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The air we breath, and that which surrounds the earth, has a weight associated with it. 'Atmospheric pressure' is a

term that relates the weight of the air (ultimately the weight of the atmosphere) to a particular area on the earth's

surface.

Area can generally be defined as a tract with both length and width. For arguments sake, lets use 1 inch long by 1 inch

wide. In this example, the area of a tract 1" Long x 1" Wide is 1 square inch.

Now, the air we breath (the atmosphere) puts a weight of 14.7

pounds on every square inch of the earth's surface (at sea level.

We'll deal with other elevations in the next paragraph.).(We didn't

just decide this; it's one of the many Laws of Physics.) In other

words, the 'atmospheric pressure' is 14.7 pounds per square inch

at sea level, or more simply put, 14.7 psi. See Figure 1.

The figure of 14.7 pounds per square inch is the atmospheric

pressure at sea level only. As you increase in altitude, the density of

the air becomes less, and therefore the atmospheric pressure also

becomes less. The actual pressure drop per 1000 feet in elevation

is about 1/2 psi. For example, at 6000 feet above sea level, the

atmospheric pressure will be 11.7 psi.

Due to the lower density and pressure at higher altitudes,

Compressor output is also affected. A good rule of thumb is that air

delivery is reduced by approximately 3% for every 1000 feet in

elevation above sea level.

'Figure 1'

Atmospheric Pressure

(At Sea Level)

'Figure 2’

14.7 lb

1”

Gauges found on Compressor Units simply measure the pressure of the air

inside the Air Receiver above atmospheric pressure.This is commonly

termed 'gauge pressure' or 'psig'. When disconnected from a Unit, a gauge

will read '0 psi'.

A pressure gauge reading 175 psi simply indicates that the air pressure at

that particular point is 175 psi above atmospheric.

Though not commonly used, 'absolute pressure' (psia) is the addition of

both the gauge pressure and atmospheric pressure. In our example above:

General Information

What is atmospheric Pressure?

What is ‘gauge pressure’?

Information Bulletin IFB-01

March ‘07

For Sales & Service contact:

CENTRAIR Air Systems & Supplies 705-722-5747 [email protected] www.centrair.ca

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Devair DRD 400 Specifications Page 4

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