3-4
EN
Potentiometer Adjustment
The procedure for calibrating the Model PX2088 is different from that of any
other pressure transmitters and takes some time to become accustomed to.
However, this method of calibration is considerably faster than other calibration
methods. In addition, there is no interaction between the zero and span adjustments.
An important feature of this design is that the potentiometer is an active part of the
circuit only during calibration. The potentiometer is removed from the active circuit
when the switch is placed in the “RUN” position. This unique design eliminates the
temperature drift and stability shifts often associated with common potentiometers.
The potentiometer is marked “DN” (Down), “FINE,” and “UP.” The coarse
adjustment regions are at each end, and a fine adjustment region is in the center.
The output of the transmitter increases or decreases automatically when the
potentiometer is placed in the coarse adjustment regions. The longer the
potentiometer is held in the coarse adjustment region, the faster the rate of change
in output.
NOTE
When the transmitter output is saturated below 4 mA or above 20 mA, the
potentiometer may appear to have no effect on the transmitter calibration because
the microprocessor is adjusting the calibration, but the current-limiting circuitry is
maintaining the output at the saturation levels. Place the potentiometer in the
coarse-adjust region and wait at least 15 seconds for the output to change.
Setting the Zero
Setting the zero point of the transmitter involves applying pressure and adjusting
the potentiometer accordingly. To set the zero point of the transmitter, use the
following procedure:
1. If the transmitter does not have a readout device, attach an ammeter using
the test terminals in either compartment (see Figure 3-2 on Page 3-3).
2. Ensure that the selector switch is in the “RUN” position. Apply the pressure to
which the zero point will be calibrated.
3. Set the selector switch to “Z.”
4. Using the potentiometer, adjust the transmitter output until the readout
device reads the 4 mA.
5. Return the selector switch to the “RUN” position.
The potentiometer is a ¾-turn device and has a mechanical stop to prevent full rotation. Do
not exert large twisting forces against the mechanical stop or damage will result.
4690.bk : 4690.s3 Page 4 Tuesday, May 26, 1998 5:28 PM
