Refrigeration & Air Conditioning Technology, 8e - 27

Unit 1

Heat, Temperature, and Pressure

Figure 1.22 The aneroid barometer uses a closed bellows that expands
and contracts with atmospheric pressure changes.
29.6 in. Hg

27

Figure  1.24 The gauge on the left, called a compound gauge, reads
pressures above and below atmospheric pressure. The gauge on the
right, called the high-pressure (high-side) gauge, will read up to 500 psi.

29.92 in. Hg

LINKAGE

PIVOT

Photo by Eugene Silberstein

THE CLOSED BELLOWS RESPONDS
TO ATMOSPHERIC PRESSURE BY
EXPANDING AND CONTRACTING.
LINKED TO A NEEDLE, ATMOSPHERIC
PRESSURE CAN BE MEASURED.

1.13 PRESSURE GaUGES
Measuring pressures in a closed system requires a different method-the Bourdon tube, figure 1.23. The Bourdon
tube is linked to a needle and can measure pressures above
and below atmospheric pressure. A common tool used in
the refrigeration industry to take readings in the field or
shop is a combination of a low-pressure gauge (called the
low-side gauge) and a high-pressure gauge (called the highside gauge), figure 1.24. The gauge on the left reads pressures above and below atmospheric pressure and is called
a compound gauge. The gauge on the right will read up to
500  psi and is called the high-pressure (high-side) gauge.

SAFETY PRECAUTION: Working with temperatures that are
above or below body temperature can cause skin and flesh damage. Proper protection, such as gloves and safety glasses, must be
used. Pressures that are above or below the atmosphere's pressure can cause bodily injury. A vacuum can cause a blood blister
on the skin. Pressure above atmospheric can pierce the skin or
inflict damage when blowing air lifts small objects like filings.●● ●
These gauges read 0 psi when opened to the atmosphere.
If they do not, then they should be calibrated to 0 psi. The
gauges are designed to read psig (pounds-per-square-inch
gauge pressure). Atmospheric pressure is used as the starting

Figure 1.23 (A) The Bourdon tube is made of a thin substance such as brass. It is closed on one end, and the other end is fastened to the pressure
being checked. When pressure increases, the tube tends to straighten out. When attached to a needle linkage, pressure changes are indicated. (B) An
actual Bourdon tube.

BOURDON TUBE

Photo by Eugene Silberstein

GEAR

VESSEL OR PIPE WITH PRESSURE
(A)

(B)



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
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