User Guide

11
Gas Absolute Pressure: The M Series ow meters utilize an absolute pressure sensor to measure
the line pressure of the gas ow 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 ow meters use the absolute pressure of the gas in the calculation of the mass ow rate. For
working in metric units, note that 1 PSI = 6.89 kPa.
Gas Temperature: The M Series ow meters also utilize a temperature sensor to measure the line
temperature of the gas ow being monitored. The temperature is displayed in engineering units of
degrees Celsius (°C). The ow meters use the temperature of the gas in the calculation of the mass
ow 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 ow 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 ow rate, the gas being measured must be
selected (see Gas Select Mode). This is important because the device calculates the ow 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 ow, and the resulting
output will be inaccurate in direct proportion to the ratio between the two gases viscosities.
Mass Flow Rate: The mass ow rate is the volumetric ow 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 ow 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 ow to another.
Flashing Error Message: Our ow 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 ashes
on the display, neither the ashing parameter nor the mass ow measurement is accurate. Reducing
the value of the ashing parameter to within specied limits will return the unit to normal operation and
accuracy.