Alicat PCU Portable Calibration Unit - Manual (Page 8)

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Once zeroed the PCU can now be used to check the calibration of other mass flow meters. Providing
there are no leaks in the lines, the PCU can be used to check another mass flow meter either upstream
or downstream of the PCU.
Note: The PCU cannot be used to directly check volumetric flow meters (such as rotameters) because
the density of the gas changes with the pressure drop present in all systems.
Gas Absolute Pressure: The PCU flow meters utilize an absolute pressure sensor to measure the line
pressure of the gas flow being monitored. This sensor references hard vacuum and accurately reads
line pressure both above and below local atmospheric pressure. This parameter is located in the upper
left corner of the display under the dynamic label “PSIA”. This parameter can be moved to the primary
display by pushing the button just above the dynamic label (top left). The engineering unit associated
with absolute pressure is pounds per square inch absolute (PSIA). This can be converted to gage
pressure (psig = the reading obtained by a pressure gauge that reads zero at atmospheric pressure) by
simply subtracting local atmospheric pressure from the absolute pressure reading:
PSIG = PSIA – (Local Atmospheric Pressure)
The flow meters use the absolute pressure of the gas in the calculation of the mass flow rate. For
working in metric units, note that 1 PSI = 6.89 kPa.
Gas Temperature: The PCU flow meters also utilize a temperature sensor to measure the line
temperature of the gas flow being monitored. The temperature is displayed in engineering units of
degrees Celsius (°C). The flow meters use the temperature of the gas in the calculation of the mass
flow rate. This parameter is located in the upper middle portion of the display under “°C”. This parameter
can be moved to the primary display by pushing the top center button above “°C”.
Volumetric Flow Rate: The volumetric flow rate is determined using the Flow Measurement Operating
Principle described elsewhere in this manual. This parameter is located in the lower left corner of the
display over “Volume”. This parameter can be moved to the primary display by pushing the “Volume”
button (lower left). In order to get an accurate volumetric flow rate, the gas being measured must be
selected (see Gas Select Mode). This is important because the device calculates the flow rate based on
the viscosity of the gas at the measured temperature. If the gas being measured is not what is selected,
an incorrect value for the viscosity of the gas will be used in the calculation of flow, and the resulting
output will be inaccurate in direct proportion to the ratio between the two gases viscosities.
Mass Flow Rate: The mass flow rate is the volumetric flow rate corrected to a standard temperature
and pressure (typically 14.696 psia and 25°C). This parameter is located in the lower middle portion
of the display over “Mass”. This parameter can be moved to the primary display by pushing the button
located below “Mass” (bottom center). The meter uses the measured temperature and the measured
absolute pressure to calculate what the flow rate would be if the gas pressure was at 1 atmosphere and
the gas temperature was 25°C. This allows a solid reference point for comparing one flow to another.
Flashing Error Message: Our flow meters and controllers display an error message (MOV = mass
overrange, VOV = volumetric overrange, POV = pressure overrange, TOV = temperature overrange)
when a measured parameter exceeds the range of the sensors in the device. When any item flashes
on the display, neither the flashing parameter nor the mass flow measurement is accurate. Reducing
the value of the flashing parameter to within specified limits will return the unit to normal operation and
accuracy.