Brooks Instrument Mfi Series - Manual (Page 37)

3-9
Brooks ® Mf Series
Section 3 Operation Installation and Operation Manual
X-TMF-Mfi-Mfx-MFC-eng
Part Number: 541B074AAG
August, 2009
seconds after setpoint change requires the use of a fast response
flowmeter (500 millisecond response to be within 0.2% of final value or
better) in series with the Mf Series and a storage oscilloscope or
recorder.
a. Allow the flow controller to stabilize at 0% setpoint for at least thirty
seconds. Make a step in setpoint to the controller from 0-100% of full
scale flow and record the output signal of the fast response flowmeter.
b. If this signal shows more than 4% overshoot, adjust the anticipate
potentiometer one-half to one turn counterclockwise. If the signal does
not show overshoot but is not within 2% full scale of final value after
three seconds, adjust the anticipate potentiometer one-half to one turn
clockwise. Set command potentiometer for 0% of flow.
c. Repeat Steps a and b until the fast response flowmeter output signal
meets the specified response requirements.
Note: With the above equipment, the anticipate potentiometer can be
adjusted to give optimum response characteristics for any process.
Response (Flow Output Signal for Meter)
To achieve the proper response characteristics the response
compensation circuit must be adjusted. This adjustment is performed by
observing the output signal of the meter when the flow is suddenly stopped.
Place a metering valve upstream of the Mf Meter to control the flow rate. A
Brooks NRS
TM precision metering valve is well suited for this application.
Also place a fast acting shut-off valve immediately downstream of the
flowmeter. A solenoid valve is ideal for this, but a manual toggle valve will
do. Keep the length of interconnecting tubing as short as possible between
the valves and the Mf Meter since the tubing can have a dampening effect
on the flow and the gas may not stop flowing the instant the downstream
valve is closed. Adjustment of the fast response circuit will not alter the
steady state accuracy of the flowmeter as adjusted in Section 3-4.
Note: This procedure requires an oscilloscope chart recorder, or a DVM
with a sampling speed of three samples per second or greater to monitor
the rate of change of the output signal during the test. Monitor the output
signal at TB1-5. TP1-4 may be used for circuit common.
a. With the shut-off valve open adjust the metering valve so that the
output voltage of the Mf Meter is 4.0505 to 5.000 Vdc. Allow the output
to stabilize at this setting.
b. Close the shut-off valve to stop the flow. Observe the output signal as it
decays.
c. The behavior of the output signal during the transition between 100%
and 0% flow indicates the adjustment required of the anticipate
potentiometer. Refer to Figure 3-6.
1. If the flow signal decays to -0.05 to -0.5 V then arises to 0.0 V, the
anticipate potentiometer is properly adjusted.
2. If the flow signal decays rapidly and goes below -0.5 V before rising
to 0.0 V, the anticipate potentiometer must be adjusted clockwise
and Steps a and b repeated.
3. If the flow signal decays slowly and does not go below -0.05 V, the
aniticipate potentiometer must be adjusted counterclockwise and
steps a and b repeated.