MKS 1579A, Instruction Manual

Page: 43 / 87

1579A/579A Chapter 6: Operation (Profibus Units)
43
Zeroing the Flow Controller
1. If no gas is flowing and the mass flow controller has stabilized (ref. to Appendix A,
Specifications, Warm Up) the flow output signal can be zeroed. This can be done in
different ways:
a) using the push button AUTOZERO, located at the gas inlet side of the unit.
Use this means only when gas flow is completely stopped. Check that the flow signal
has been set to zero!
or
b) using the potentiometer ZERO, located at the gas inlet side of the unit.
The potentiometer allows to adjust the zero signal to a known value of leak flow if
completely shut off is not possible.
or
c) via the Profibus interface
The Profibus version provides different modes for zero setting. If the mode switch A/B
on the inlet side of the unit is set to mode B then parameter USER_ZERO in the
PROFIBUS Setup is used. If set to mode A then 5 different options are provided,
depending on the configuration of the address switches. Refer to
Zero Modes in this
chapter.
The command for zero setting also sets the analog flow output signal to zero (pin 2 on
the connector ANALOG).
Note
Zeroing should be done only on units that are installed in final position.
It is recommended to completely stop the gas flow prior to any zero
adjustment.
If a pressure difference exists at the mass flow control unit then a small
flow might occur even if the integrated control valve is closed. This is
more likely with special units for low inlet pressures.
Do not adjust the flow signal then to zero because it is a real flow but
use a positive shutoff valve to definitely stop the flow.
The integrated control valve may not completely shut off the gas flow.
2. Check that the respective flow signal (analog or Profibus) has been set to zero!
3. Periodically check the zero adjustment of the unit, e.g. on maintenance intervals. The zero
adjustment may be affected by thermal effects and especially by contamination.
Beside the need for measurements of highest accuracy the zero signal is very important for
diagnosing the flow sensor and the control valve.