7.5Vacuum Comparison of Cal=Trak with Sonic Nozzle Transfer Standards
The Cal=Trak operates similarly in both pressure and suction applications. Sometimes, however, users wish to compare the Cal=Trak under suction (vacuum) conditions. With a calibrated sonic nozzle, a simple setup such as that shown in Figure 10 can be used with the inlet to the Cal=Trak flow cell at atmospheric pressure. There is one precaution, however. The sonic nozzle’s pressure must be measured during the Cal=Trak cycle to obtain the actual flow that the Cal=Trak is measuring. In turn, the pressure transducer must not be upset by the Cal=Trak’s initial pressure pulse and must have rapid response relative to the Cal=Trak cycle time. This method is therefore most suitable for flows in the lower part of a Cal=Trak’s range.
It should be noted that cycling of the Cal=Trak would cause a slight change in the nozzle’s inlet pressure. Even though the pressure transducer is read during the Cal=Trak cycle, the nozzle may not achieve internal thermal equilibrium during the cycle, slightly reducing accuracy.
Pressure
Transducer
Sonic Nozzle | Vacuum Pump |
DryCal
Cal=Trak
Stable Pressure
>Critical
Figure 10 Vacuum Setup for Sonic Nozzle Transfer Standard
7.6Calibration of Mass Flow Controllers (MFCs)
Modern mass flow controllers have fast response times on the order of milliseconds. They can simply be connected to an appropriate inert gas source and their output stream applied to the Cal=Trak, as in Figure 11. Proper calibration consists of comparing the Cal=Trak reading to the MFCs actual indicated flow and not to its control signal.
If a slow flow controller is to be calibrated, it is best to calibrate it in its metering mode. Apply the appropriate signal to fully open the controller’s internal valve (full scale or digital “open), and calibrate the device as shown for mass flow meters (MFMs), below.
Remember that the
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