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Recommended Minimum Back Pressure
Test Temperature
Vapor Pressure
Minimum Back Pressure
°F
°C
psi
kPa
psi
kPa
200-299
95-149
12.1-67
84-462
100
700
300-374
150-189
67-184
462-1269
200
1400
375-399
190-199
184-247 1269-1704
275
1900
400-424
200-219
247-326 1704-2245
350
2500
425-450
220-230
326-422 2245-2912
450
3100
The upper and lower limits of the test pressure differential are determined
by the test temperature. If the test is conducted at a temperature less
than the boiling point of water, then no back pressure will be required and
filtrate may be collected directly into a graduated cylinder. If however tests
are conducted above the boiling point of water, then as this temperature
exceeds 212°F (100°C), the back pressure must be increased above the
vapor pressure, at the test temperature, in order to prevent vaporization of
the filtrate. If running a standard API test, the 500 psi differential pressure
must be maintained, so the top pressure will have to be increased 500
psi over whatever the back pressure is. The table below shows the back
pressures recommended for various test temperatures.
19. Open (loosen) the Inlet valve stem ½ turn to pressurize the sample. Main
-
tain this pressure on the fluid until the temperature has stabilized. This will
prevent vaporization of the test solution if heating above the boiling point
of the liquid.
20. After the one hour heat up time, increase the pressure on the inlet pres
-
sure unit to 500 psi (3,448 kPa) more than the back pressure.
21. Open (loosen) the bottom valve stem ½ turn to initiate filtration as soon as
possible. Start the filtration test at this time. Set a timer to 30 minutes. If
spurt loss is being measured, set a second timer to 7.5 minutes.
Closely monitor the pressure gauges. If at any time during the test the
pressure inside the cell rises above the setpoint, carefully open the
needle valve on the top pressure assembly just enough to bleed off the
excess pressure. Then close the valve. If the cell pressure decreases due
to collection of filtrate, increase the pressure with the inlet regulator.
22. To collect filtrate, carefully open the needle valve on the bottom of the
back pressure receiver while holding a graduated cylinder to the valve
port. Close the valve immediately after the outlet pressure just begins to
avoid having to replace a CO
2
bulb in the middle of the test.
