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Manual: 152-082010 301-305-306_303-307 Series Page 12 of 35
2.7. Operation
The standard instrument output is a 0 - 5 VDC and the signal is proportional to the flow i.e., 0 volts =
zero flow and 5 volts = 100% of rated flow. The 4 - 20 mA option is also proportional to flow, 4 mA =
zero flow and 20 mA = 100% of rated flow.
2.7.1. Operating Conditions
For proper operation, the combination of ambient temperature and gas temperature must be such that
the flow meter temperature remains between 0 and 60°C. 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. The 300 Series is intended for use in non-condensing environments only. Condensate or any other
liquids which enter the flow meter may destroy its electronic components.
2.7.2. 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 lower inlet side of the flow
meter until the meter indicates zero. For the optional 4 - 20 mA output, adjust the zero potentiometer
so that the meter indicates slightly more than 4 mA, i.e. 4.03 to 4.05 mA. This slight positive
adjustment ensures that the 4 - 20 mA current loop transmitter is not in the cut-off region. The error
induced by this adjustment is approximately 0.3% of full scale. This zero should be checked periodically
during normal operation. Zero adjustment is required if there is a change in ambient temperature or
orientation of the flow meter/controller.
2.7.3. High Pressure Operation
If the instrument is not mounted in a level position when operating at high pressure, the increased
density of the gas will cause natural convection to flow through the sensor tube. This natural convection
flow will be proportional to the system pressure and will be seen as a shift in the zero flow output that is
directly proportional to the system pressure.
If the system pressure is higher than 250 psig (1.7 MPa) the pressure induced error in the span reading
becomes significant. The following charts show the mean error and the minimum/maximum expected span
errors induced by high pressures. This error can approach 4.5% at 400 psig. for 0.017” sensors. For accurate
high pressure measurements, this error must be corrected. Consult your packing list to determine if you have a
0.026”, 0.017” or 0.014” sensor. After determining the sensor tube ID, use one of the formulae below to
determine the expected mean error, expressed as a fraction of the reading.
)"026.0(,)
5
10*3288.8(
2
)
7
10*4154.3(
3
)
11
10*887.9(
26
SensorPPPError
−
+
−
−
−
=
)"017.0(,)
4
10*8313.1(
2
)
7
10*304.3(
3
)
-10
10*(1.533
17
SensorPPPError
−
+
−
−=
)"014.0(,)
5
10*929.1(
2
)
7
10*776.1(
3
)
-10
10*(-1.692
14
SensorPPPError
−
−
−
+=
Where P is the pressure in psig and Error is the fraction of the reading in error.
The flow reading can be corrected as follows:
ErrorScaleFullIndicationCorrected *−=
Where the Indication is the indicated flow, Full Scale is the full scale reading and Error is the result of the
previous formula or read from charts below.
CAUTION: The HFM-306 does not have a high pressure option and is only rated for a
maximum working pressure of 300 psig.