MKS 1579A, Instruction Manual

Page: 40 / 87

Chapter 6: Operation (Profibus Units) 1579A/579A
40
Valve Override
By means of an analog override signal applied to the respective pins at the connector ANALOG the
control valve can be driven completely open or closed, independent of the setpoint command
signal ( only in analog mode ). Refer to figure 9 and its explanations for the detailed description.
In Profibus mode the override however is only possible via the bus:
Set the parameter VALVE_OVERRIDE to NORMAL to allow the mass flow controller establishing
a flow in accordance to the setpoint command FLOW_SETPOINT.
Set the parameter VALVE_OVERRIDE to FLOW_OFF to completely close the control valve.
Set the parameter VALVE_OVERRIDE to PURGE to completely open the control valve. This
command may be used to purge or vent the instrument or the system.
Auto Zero
If VALVE_OVERRIDE is equal to FLOW_OFF and THERMAL_MASS_FLOW_RATE is smaller
than 5 % of full scale, the flag AUTOZERO (0 to 1 transition) will take the actual sensor signal as
zero and will subtract this value from all future measurements (refer also to
USER_ZERO ).
Note: Zero adjust can be performed also using the AUTOZERO button.
Alarm Limits
LOW_TRIP_POINT and HIGH_TRIP_POINT define the limits for LOW_LIMIT_ALARM and
HIGH_LIMIT_ALARM flags. Consider that there is a 0.5% hysteresis for each limit switch. The
hysteresis of 0.5% is divided in a slope 2.5% above and another slope 2.5% below the defined
limit.
Temperature Measurement
The internal temperature of the device is measured in °C and given as INTERNAL_TEMP.
Valve Drive Level
The position of the valve is given in VALVE_DRIVE_LEVEL. 0% means that the valve is closed
and 100% means that the valve is fully opened (e.g. at the PURGE command).
A typical value under normal operating conditions is 40…60 % with new units. The value depends
on many parameters, e.g. pressure conditions, temperature, contamination, age etc., and may
change also during operation even at the same flow rate.