301/303/305/306/307 Flowmeters and Controllers Instruction Manual
EAR99 Technology Subject to Restrictions Contained on the Cover Page
Page 15 of 32
Operation
The standard instrument output signal is 0 - 5 VDC and is proportional to the flow (0 volts = zero flow, 5 Volts =
100% flow). The 4 - 20 mA option is also proportional to flow (4 mA = zero flow and 20 mA = 100% flow).
2.6.1. Operating Conditions
For proper operation, the combination of ambient temperature and gas temperature must be such that
the flow meter temperature remains between -20 and 70°C. The most accurate measurement of flow will
be obtained if the flow meter is zeroed at operating temperature as temperature shifts result in some
zero offset. Hastings 300 series instruments are intended for use in non-condensing environments only.
Condensate or any other liquids which enter the flow meter may destroy its electronic components.
2.6.2. Zero Check
Turn the power supply on if not already energized. Allow for a 1 hour
warm-up. Stop all flow through the instrument and wait 2 minutes.
Caution:
Do not assume that all metering valves completely shut off
the flow. Even a slight leakage will cause an indication on the meter and
an apparent zero shift. For the standard 0 - 5 VDC output, adjust the
zero potentiometer located on the inlet side of the flowmeter until the
meter indicates zero. For the optional 4 - 20 mA output, adjust the zero
potentiometer so that the meter indicates 4 mA. 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
p
) of the gas. For monatomic gases the specific heat 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 pressure inside of the sensing tube
increases and so does the specific heat of the gas. 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.