SM0486
of scale up to 75 percent of scale) and 1 1/2 percent of the scale range over the
remainder of the range.
In a grade A high grade commercial gage, the middle
portion of the scale is often referred to as the "working" portion because it is in
this portion that pressure indications are most frequently made. Grade A gages are
commonly used in power plants, refrigeration systems, hydraulic presses, and a
number of other applications where steam, water, oils, and gases are the principal
measured media. They often represent a good compromise between high accuracy and
initial cost.
(5) With a grade B commercial gage, the error in pressure indication is not
to exceed 2 percent of scale range at any point within the middle half of scale
and, for the remainder, 3 percent of scale range (see Figure 7c).
Most of the
gages that are installed on air compressors, fire extinguishers, and similar
equipment fall into the Grade B classification. The accuracy is generally adequate
for such application, and the gage can be made suitably rugged, as well as compact
and economical.
(6) Classification by specific end use. A number of gage names have come
into being because they were designed for a specific end use.
They use dictates
certain design features which may make the gage quite suitable for other uses, but
often the original name sticks.
a. One example is the ammonia gage, which has stainless steel internal
parts to withstand ammonia and its fumes.
It also has two sets of scale
graduations: one for the pressure, and the other for the ammonia temperature
equivalent. The A.S.A. standards recognize this specific type, specifying that the
gage shall have "plainly inscribed" on its dial the word "Ammonia."
Other gages
designed for specific service in refrigeration or air conditioning equipment have
similar design features. The dual scale usually has pressure graduations in black
numbers, and the refrigerant's temperature equivalent in red numbers. Arising from
the service needs, such gages usually have a convenient zero adjustment and
cellulose acetate crystals.
b. Oxygen gages are another type gage specifically designed for
measurement of oxygen pressure. A.S.A. standards specify that such a gage shall:
Be equipped with a reliable and efficient safety device to minimize
danger in the event of a ruptured element.
When delivered, be free of grease, oil, or any other substance which
reacts explosively with oxygen.
Bear the following inscription on the face of the dial:
"OXYGEN--USE NO
OIL."
c. The hydraulic gage is specifically constructed for service at high
pressures, where water or noncorrosive liquid is the pressure medium, such as gages
for hydraulic presses.
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