Proline t-mass 65 PROFIBUS DP/PA Commissioning
Endress+Hauser 83
6.9 Adjustment
6.9.1 Zero point adjustment
Calibration takes place under reference operating conditions. ( ä 109).
Consequently, the zero point adjustment is generally not necessary!
At zero flow conditions, the output of most thermal mass flow devices has a strong dependency on
the process pressure. The effect on the true zero point of the device, by the static line pressure, is
dependant on the gas type and the application demands and in many cases the use of the low flow
cut-off function is adequate to zero the device output.
However, with some gases and /or a combination of high static line pressures, the zero point may
need to be adjusted under process conditions to restore the very low measurement capability of the
device.
Therefore, the zero point adjustment is advisable in the following special cases:
• To achieve highest measuring accuracy with very small flow rates.
• Under process or operating conditions where the gas properties (heat capacity and thermal
conductivity) will change e.g. Hydrogen and Helium.
Preconditions for a zero point adjustment
Note the following before you perform a zero point adjustment:
• A zero point adjustment can be performed only with gases that contain no solid contents or
condensate.
• The adjustment is performed with the process gas at zero flow and at operating pressure. This can
be achieved, for example, with shut-off valves upstream and/or downstream of the sensor.
– Normal operation valves 1 and 2 open
– Zero point adjustment with pump pressure Valve 1 open / valve 2 closed
– Zero point adjustment without pump pressure Valve 1 closed / valve 2 open
a0003601
Fig. 44: Zero point adjustment and shut-off valves
!
Note!
You can view the currently valid zero point value using the ZERO POINT function in the SENSOR
DATA group (see the "Description of Device Functions" manual, BA000114D/06/…).
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