Troubleshooting / FAQs
8.4 Step 3: Calculating the measurement error
SITRANS F C MC2
Operating Instructions, 10/2009, A5E02154544-02
33
E
= Z x 100% / Qm
E
= measurement error in % of flow rate
Z
= Zero Sigma value in kg/h
Qm
= current Flow rate in (kg/h)
Example 1: Low flow rate application
● Di 15 sensor. The sensor is specified to max. 5600 kg/h
● Zero point error/ ZERO SIGMA value is specified as 0.2 kg/h
● Flow: Min. 10 kg/h - Max. 100 kg/h
After the zero point adjustment, the ZERO SIGMA value 'Z' is read as 1 kg/h, i.e. a 5 times
greater than that specified for the sensor.
The error for a flow rate of 10 kg/h is estimated as:
● E = 1 kg/h x 100%/ 10 kg/h = 10%.
For a flow rate of 100 kg/h the error is estimated as:
● E = 1 kg/h x 100% / 100 kg/h = 1 %
For this application it is necessary to investigate more closely what the cause of the relatively
high ZERO SIGMA value is, in order to establish what needs to be done to improve the
measurement accuracy.
Example 2 : High flow rate application
● Di 15 sensor. The sensor flow rate is specified as max. 5600 kg/h
● The zero point error/ZERO SIGMA value is specified as 0.2 kg/h
● Flow rate: Min. 1000 kg/h - Max. 3000 kg/h
After the zero point adjustment, the ZERO SIGMA value 'Z' is read as 1 kg/h, i.e. a 5 times
greater value than that specified for the sensor !
The error at a flow rate of 1000 kg/h is estimated as:
● E = 1 kg/h x 100%/ 1000 kg/h = 0.1%
At a flow rate of 3000 kg/h the error is estimated to be:
● E = 1 kg/h x 100% / 3000 kg/h = 0.03 %
Plus the linearity error of 0.1 %
As can be seen, in this case it is not so important that the zero point, i.e. Zero Sigma Value
is 1kg/h. The error due to the zero point is only 0.1 % for a flow rate of 1000 kg/h, and even
less for a higher flow rate.
So for this installation with the given flow rate and zero point error (ZERO SIGMA value), you
should typically choose not to spend more time finding ways to improve the application.