Fisher 846 - Principle of Operation; Electronic Circuit Operation

Instruction Manual
D102005X012
846 Transducer
June 2017
22
6. Repeat steps 4 and 5 to verify and complete the calibration.
7. If the 12.0 mA (Vm = 3.0 V) cannot be adjusted to 1.0 bar (15.0 psi) in step 4, turn off the air supply. Remove the
module final assembly from the housing. Place the range jumper in the Lo position for Low Range, as shown in
figure 18. Replace the module final assembly. Turn on the air supply.
8. Apply an input of 12.0 mA (Vm = 3.0 V), and adjust the zero screw to achieve an output of 1.0 bar (15.0 psi).
9. Apply an input of 20 mA (Vm = 5.0 V), and note the error (the actual reading versus 3.0 psi). Adjust the span screw
to overcorrect the error by a factor of two. For example, if the reading was 2.95 psi, adjust the span screw to achieve
an output of 3.05 psi.
10. Repeat steps 8 and 9 to verify and complete the calibration.
Transporting the Module Final Assembly
The transducer allows the module final assembly to be removed while the housing is in its installed position. In the
event the transducer does not function properly, an operational module final assembly can be taken to the field and
exchanged with the nonfunctional module.
After the transducer is calibrated in the shop, the module final assembly can be removed from the housing. At the time
the span and zero screws disengage, there will be minimal effect on the calibrated span. The calibrated module can
now be taken to the field. Ensure that the span and zero potentiometers are not moved from their calibrated positions.
Principle of Operation
The following paragraphs describe the functional parts of the 846. Figure 10 shows the block diagram.
Figure 10. Functional Parts Block Diagram
4 to 20 MA
INPUT
ELECTRONIC CIRCUIT
MAGNETIC ACTUATOR
PILOT STAGE
BOOSTER STAGE
SOLID‐STATE
PRESSURE
SENSOR
TO VALVE ACTUATOR
3 TO 15 PSI
OUTPUT, TYPICAL
A6324‐1
Electronic Circuit
During operation, the input current signal is received by the transducer's electronic circuit and compared to the output
pressure from the booster stage. A solid‐state pressure sensor is part of the electronic circuit and monitors the booster
stage output.