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300-302 Series Page 11 of 31
Span Error Vs. Pressure
0.017" Sensor
-2%
-1%
0%
1%
2%
3%
4%
5%
0 100 200 300 400 500 600 700 800 900 1000
Pressure (psig)
Span Error (% reading)
Mean
Max
Min
If the system pressure is higher than 250 psig (1.7 MPa) the pressure induced error in the span reading
becomes significant. The charts above show the mean error enveloped by the minimum/maximum
expected span errors induced by high pressures. This error will approach 16% at 1000 psig. For
accurate high pressure measurements this error must be corrected.
The formulae for predicting mean error expressed as a fraction of the reading are:
)"026.0(,)10*3288.8()10*4154.3()10*887.9(
527311
26
SensorPPPError
−−−
+−=
)"017.0(,)10*8313.1()10*304.3()10*(1.533
4273-10
17
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:
Where the Indication is the indicated flow and Error is the result of the previous formula (or read from
charts above).
2.6.4. Blending of Gases
This section describes two methods by which to achieve a controlled blending of different gasses. Both
methods use the flow signal (Output) from one flow instrument as the Master to control the Command
signal (Input) to a second unit.
The first method requires that the two controllers use the same signal range (0 to 5 VDC or 4 to 20 mA)
and that they be sized and calibrated to provide the correct ratio of gasses. Then, by routing the
actual flow Output signal from the primary meter/controller through the secondary controller’s
(
)
ErrorIndicationIndicationCorrected *
−
=
Fig. 2.5