Chapter 3 HVAC Tools, Equipment, and Service Information 41
Gauges
The gauges used with a refrigeration gauge manifold
are either analog (indicator needle), which resemble other
pressure gauges, or digital. See Figure 3-3. In an analog
gauge, the position of the needle in relation to the numbers
on the gauge face indicates the pressure or vacuum in the
refrigeration system. Digital gauges provide a numerical
reading indicating system pressure or vacuum.
All gauge manifolds have high and low pressure
gauges. A few older gauge manifolds have three gauges.
The third gauge was used to measure compressor output.
Older high side gauges are calibrated from 0-500 psi
(3445 kPa). Newer high side gauges may be calibrated from
0-250 psi (1723 kPa). Low side gauges are calibrated from
0 to 100-250 psi (689-1723 kPa). In addition to the pres-
sure scale, low side gauges have a provision for mea suring
0-29.9” of vacuum (approximately 50 microns). Vacuum
measurements are explained later in this chapter.
R-134a and R-12 Gauges
There are no major differences between R-134a and
R-12 gauges. The internal operation of each type of gauge
is the same. The difference is the calibration of each gauge,
and the markings on the gauge faces. R-134a and R-12
gauge manifolds cannot be interchanged.
Manifold Hoses
Manifold hoses are tubes of high strength nylon or fab-
ric cord covered by neoprene rubber. Most hoses are rated
to withstand 500 psi (3445 kPa) pressures. At each end of
the hose is a connector that allows it to be attached to the
gauge manifold and the refrigeration system. Hoses used
with a gauge manifold have connectors designed to match
the refrigerant being measured by the gauges. Compare the
Figure 3-2. This cross-section of a gauge manifold shows the
internal passages that allow the manifold to be used to check
pressure, add and remove refrigerant, and many other functions.
(Robinair)
O-rings (2)
Stem seal
Figure 3-3. Digital manifold gauges are available. They give accurate pressure readings, however, they do
not provide some of the diagnostic advantages of analog gauges.