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8. Keeping the valve stems closed, adjust the two regulators on the nitrogen
manifold to the recommended back pressure for your test (refer to the
chart below). Open (loosen) the top valve stem ½ turn to pressurize the
sample. Maintain this pressure on the fluid until the desired temperature
is stabilized as indicated by the thermometer. The heating time of the
sample should never exceed one hour.
The upper and lower limits of the test pressure differential are determined
by the test temperature. As this temperature exceeds 212°F (100°C), the
back pressure must be increased in order to prevent vaporization of the
filtrate. The 1,000 PSI differential pressure must be maintained, so the top
pressure will have to be increased accordingly. The table below shows
the pressures recommended for various test temperatures.
Recommended Minimum Back Pressure
Test Temperature
Vapor Pressure
Minimum Back Pressure
°F
°C
PSI
kPa
PSI
kPa
212
100
14.7
101
100
690
250
121
30
207
100
690
300
149
67
462
100
690
350
177
135
932
160
1,104
400*
204
247
1,704
275
1,898
500*
260
680
4,692
700
4,830
*For tests above 400°F, use Teflon o-rings.
9.
When the fluid sample reaches the desired test temperature, increase the
primary pressure (the regulator on the left) to 1,000 PSI (3,448 kPa) more
than the back pressure. Open (loosen) the bottom valve stem ½ turn to
initiate filtration.
10.
Collect the filtrate for 30 minutes maintaining the selected test
temperature within ± 5°F (± 3°C). If the back pressure rises above the
recommended setting during the test, cautiously reduce the pressure by
opening the valve on the receiver and drawing off some of the filtrate into
the graduated cylinder.
11. At the end of the test, close (tighten) the top and bottom valve stems to
seal off the cell. Turn the regulator t-screws counter-clockwise to close off
the flow of pressurized gas. Open the valve on the back pressure receiver
to collect all of the filtrate in the graduated cylinder. Release the remaining
pressure from the nitrogen manifold by opening the bleeder valve on the
manifold block.
