Zero Check; High Pressure Operation - Teledyne HASTINGS HFM-300 Instruction Manual

2.6.2. Zero Check

Turn the power supply on if not already energized. Allow for a 1
hour warm-up.
minutes. Caution:
completely shut off the flow. Even a slight leakage will cause an
indication on the meter and an apparent zero shift.
standard 0-5 VDC output, adjust the zero potentiometer located on
the inlet side of the flow meter until the meter indicates zero (Fig
2.3). This zero should be checked periodically during normal
operation. Zero adjustment is required if there is a change in
ambient temperature, or vertical orientation of the flow meter
/controller.

2.6.3. High Pressure Operation

Thermal mass flow meters measure the heat transfer caused by gas flow. This heat transfer is a
function of the constant pressure specific heat (C
is essentially a constant for all normal pressure/temperature conditions. As the gas structure becomes
more complex and as the molecular weight of the gas increases the specific heat increases as the
pressure inside of the sensing tube increases. This is compensated somewhat by the effect of the hoop
strain of the tube wall caused by the internal pressure on the resistance heaters wrapped around the
sensing tube.
Larger tubing will have more strain than the smaller tubing and a larger associated pressure affect.
Charts of the effects of various gases on the span of the flow instruments as a function of pressure are
shown below for each of the available sensor diameters. The sensor tubing diameter is written on a
label mounted on the side of the sensor and is listed in the options in the order acknowledgement.
Manual: 151-032015 300-302 Series
Stop all flow through the instrument and wait 2
Do not assume that all metering valves
Fig. 2.4
For the
) of the gas. For monatomic gases the specific heat
p
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