Brooks Instrument 5866M - Manual (Page 39)

Brooks Instrument Model 5866 Pressure Controller
6-2
Note: If a vacuum pump is not available and the atmospheric pressure
is accurately known the zero pressure signal output can be adjusted
using the following formula:
(Vdc)
Output at Output at 100% Atmospheric
Atmospheric Pressure = x Pressure
(Vdc) Pressure at 100% (psia)
(psia)
The above formula will be most accurate with high pressure
transducers where the output at atmospheric pressure is a small
fraction of the full scale pressure.
4. Apply the desired full scale pressure to the transducer side of the Model
5866 and adjust the span potentiometer (P3) so the Model 5866 output
agrees with the output of the precision pressure indicator. If the output
cannot be adjusted to the correct value with the span potentiometer
change the course gain setting by moving jumper (J1).
5. Apply a vacuum to the transducer side of the Model 5866 and check
zero pressure output. If the zero pressure output is correct, the
calibration is complete. If zero pressure output is not correct, repeat
Steps 3 and 4. Readjustment will probably only be required if the
atmospheric pressure method of adjustment was used to set zero.
6-2 Gas Conversion Factors
If a mass flow controller is operated on a gas other than the gas it was
calibrated with, a scale shift will occur in the relation between the output
signal and the mass flow rate. This is due to the difference in heat
capacities between the two gases. This scale shift can be approximated by
using the ratio of the molar specific heat of the two gases or by sensor
conversion factor. A list of sensor conversion factors is given in Table 6-1.
To change to a new gas, multiply the output reading by the ratio of the gas
factor for the desired gas by the gas factor for the calibration gas used.
Actual Gas Flow Rate = Output Reading x
Factor of the New Gas
Factor of the Calibration Gas
Example:
The controller is calibrated for Nitrogen.
The desired gas is Carbon Dioxide
The output reading is 75 sccm when Carbon Dioxide is flowing
Then 75 x 0.773 = 57.98 sccm
In order to calculate the conversion factor for a gas mixture, the following
formula should be used:
P
1
Sensor
Conversion
Factor
1
P
3
Sensor
Conversion
Factor
3
P
2
Sensor
Conversion
Factor
2
Sensor Conversion Factor
Mixture
=
100
+ +