of 20
11
the pressure. To check operation pull the shuttle valve
outward, close the release valve and operate the pump
handle several times. When handle resistance is felt, push
the shuttle valve inward and continue pumping within the
range of the test device.
Entrapped air will prevent the pump from operating in the
high pressure valve position or cause it to achieve only
partial pressure. Repeat the above bleeding procedure
as necessary to assure all air is removed. Once air is re-
moved the pump will continue to operate without further
attention providing the reservoir level is maintained.
3.5.3 Making The Test
Turn displacement piston handle (41) to mid-position, so
that it may be used for setting an exact pressure on the
gauge. The displacement piston has internal stops which
prevent unscrewing or accidental loss of pressure.
Clockwise rotation of displacement piston will produce
an increase in pressure; counterclockwise will decrease
pressure.
Prior to taking readings, both gauges should be fin-
ger-tapped lightly at the center of the gauge face, to elim-
inate any movement friction. Note the pressure readings
on the test gauge and the gauge under test. If the pres-
sure indicated on the gauge under test is not equal (within
the tolerance of the gauge) to the pressures of the master
test gauge, the gauge being tested requires calibration.
CAUTION:
Do not pump handle to pressures greater than the pres-
sure range of the gauges connected to the test pump, as
this may damage the gauges.
3.5.4 To Release Pressure
Open release valve slowly (37), until pressure returns to
zero. Do not loosen any connections until pressure in the
gauge tester has reached zero, as indicated on the test
gauge.
If additional gauges are to be tested, close the release
valve when the pressure reaches zero. This will prevent
complete drainage of oil in the tester back to the reservoir.
Unseal the gauge that has been tested by unscrew-
ing adapter nut (62) from connector (60) several turns.
Unscrew the gauge from bushing (59) if used, or connec-
tor (60), and remove.
Remove the test gauge in the same manner as the gauge
under test.
3.5.5 To Dismantle
To replace test pump in case, remove gauges and both
tube assemblies. Put shipping plugs into pump and tighten.
Close filler plug (35).
3.5.6 Operating Fluids
Standard Test Pump:
Any medium weight oil may be used (including automotive
oils S.A.E 10, 20 or 30) S.A.E. 20-W recommended.
Hydraulic Test Pump:
Any hydraulic fluid (silicate or phosphate base). Skydrol or
Pydraul is suitable.
Oxygen Test Pump:
Distilled or demineralized water with a compatible corro-
sion inhibitor added. A .1% solution of sodium dichromate
is a suitable inhibitor.
For normal operation it is not necessary to change the
weight of oil for various pressures. A lighter oil may be
used where low pressures are being checked, a heavier
oil where higher pressures are being tested.
The reservoir can be refilled while the test pump is operat-
ing under pressure.
3.5.7 Caution
Standard testers designed for oil service may not be used
with water for oxygen service.
3.5.8
Hydraulic service testers should not be filled with water or
any oil other than hydraulic fluid. Serious pump failure may
occur due to O-ring damage.
4.0 FACTORS AFFECTING OPERATION OF
DEADWEIGHT TESTERS
4.1 Deadweight Tester Accuracy
Excellent accuracy is possible using the deadweight tes-
ter through analysis and control of certain factors. If the
tester’s rated accuracy of 1/10th of 1% is adequate, then
the nominal pressure (sum of the denominations of the
weights loading the piston) may be assumed to be correct.
The pressure normally developed is determined by this
formula: Deadweight Tester Pressure =
Mass of weights plus piston mass
Effective area of piston and cylinder
P = (M-ph) g
A gs
P
= pressure
M = mass of the load on the piston
A
= effective area of the piston in sq. inches, or sq. cm
g/gs = ratio of the value (g) of gravity at the point of use to
the standard value of gravity (gs)
P
= density of liquid used in test
h
= difference in level between gauge being tested, and
the bottom of the piston
The deadweight tester is capable of measuring pressures
to proper corrections after analyzing these factors.
4.2 Gravity, Calibrated Weights
Weights furnished are calibrated at standard gravity of
980.665 gals. If precise accuracy is required, the error
introduced by change of weight due to change in gravity at
the locality should be calculated and included in results.
The mechanism of an Ashcroft Pressure Gauge includes
a Bourdon tube and geared movement, which is unaf-
fected by variations in gravity. Conversely, the pressure de-
veloped by a deadweight tester is proportional to the value
of gravity. Readings of this type pressure gauge will cor-
respond to those of a deadweight tester when the tester
is subjected to standard gravity (gs = 980.665 gals. in the
International System). In southern sections of the United
States, the value of gravity may be several thousandths
less than the standard value, if the latitude (Ø) and the
elevation above sea level (a) for an area are known, the
approximate value of (g) in gals. may be calculated from
this formula:
g = 980.632-2.586 COS 2Ø
+.003 COS 4Ø - .000094a
Ø = Latitude (Degrees)
a = Elevation above sea level (ft)
4.3 Effective Measured Area: Chamber
The effective area of the deadweight tester may be de-
termined by the average of the cross sectional area of
the piston and the area of the cylinder bore. This effective
area is affected somewhat by temperature, and by the
elastic distortion of the piston and cylinder when pressure
is being applied. The effective area of a stainless steel
piston and cylinder increases approximately .068% with
