Chandler Engineering AMETEK 7200, Инструкция по эксплуатации

Страница: 43 / 77

SECTION 4 – CALIBRATION 4 - 3
Flow Rate Calibration
The two flow meters used by the Model 7200 CHA are very-high-accuracy (+/- 1% of Full
Scale) gas mass flow meters (transducers). These transducers output a linear 0-5VDC signal
corresponding to 0-1000 sccm (standard cubic centimeters per minute). This signal is read by
the data-acquisition module and the CHA-Control software converts this voltage to a flow
rate value according to the calibration scaling entered on the calibration screen for that meter.
The procedure is identical for both meters.
The flow rate is expressed as an equivalent volumetric flow rate of a specific gas (N
2
, in this
case) at a chosen Standard Pressure (760 mm Hg = 14.696 psia = 1.00 atm. = 101.325 kPa =
1.01325 bar) and Standard Temperature (70°F = 21.1°C).
Figure 14 - Flow Rate Calibration
Unlike the pressure transducers, the flow meters can only be calibrated by manually. From
the manufacturer’s calibration sheet for th e appropriate meter, note the full-scale output
voltage at full-scale flow rate. Divide t he “Full S cale Flow” rate (units of s ccm) by this full-
scale output voltage and enter that scaling factor (sccm/Volt) in the Slope control For
example, if the full-scale output voltage for the transducer is 5.0 Volts DC, then Slope = 5
sccm / 5.0 Volts = 1.000 sccm/Volt.
To Zero the flow rate, close the appropriate Valve to stop all flow. Allow the meter reading
to stabilize. If the
Calibrated Value is not zero, enter a value in the Intercept entry box to
make the Calibrated Value equal to 0.0 sccm. That is, if the Intercept is currently 0 sccm,
and the Calibrated Value is currently shown as 0.2 sccm, enter a value of -0.2 for the
Intercept . The Calibrated Value should now read 0 sccm.
Note that each flow meter is calibrated by its manufacturer to be within stated accuracy for
only a specific gas (N
2
), only at the calibrated inlet pressure to the flow meter, and only
within a certain range of ambient temperature.
Each flow meter is calibrated for an inlet pressure of 1000 psig, which is the maximum
working pressure of the CHA instrument. If conducting cement hydration tests at lower
pressures, the flow rate will not be accurate within the stated accuracy but will be sufficiently
accurate for the purposes
of the CHA instrument’s use of the measured flow rate.