Bronkhorst Mass Flow - Manual (Page 21)

BRONKHORST HIGH-TECH B.V.
3.6 Instrument performance
3.6.1 Sensors
Assuming that the transfer function of a system is an exponential shaped curve, the time constant is defined
as follows:
time constant = time for the signal to reach 63.2 % of its final output value. Approx. five time constants is the
time to reach the final value.
Each flow sensor has a time constant of 5…10 seconds, which can be electronically improved to approx.
1…3 seconds.
For gas flow meters factory standard adjustment is 3 seconds typical.
For liquid flow meters the actual response depends on model and flow rate.
Pressure sensors have a time constant of some milliseconds. However the actual response is determined by
the pneumatic response of the system which the pressure meter is part of.
3.6.2 Controllers
The dynamic response of a controller is factory set. Standard settling time is defined as the time to reach the
setpoint (and stay) within ± 2% of the initial setpoint.
The control mode is factory set in such a way that after a step change, there will be little overshoot.
Note:
In pressure control systems the system widely determines the response behaviour of the control loop. During
testing the customer system is simulated as closely as possible. In some cases however readjustment is
needed for optimum performance under actual conditions.
9.17.001 page 21