Troubleshooting Pressure Transducers - Daikin MicroTech II Installation And Maintenance Manual

Troubleshooting Pressure Transducers

Use the following procedure to troubleshoot a suspect sensor:
1 If the duct static pressure always reads 0" WC on the unit
keypad/display and the discharge inlet vane position or
VFD speed is continuously ramping to 100%, check the
following:
a
If the unit has two duct static pressure sensors (SPS1
and SPS2), verify that they both function properly
following Steps 2 through 5 below. Also check for
faulty wiring connections at analog inputs MCB-AI13
and MCB-AI14. The controller displays and controls to
the lower of the two readings. If a sensor is defective
and inputs 0 volts to the controller, the static pressure
reading on the keypad/display reads 0 and the controller
attempts to increase the 0 value to set point by ramping
the discharge inlet vanes or VFD up.
b
If a second sensor (SPS2) is not installed or the pressure
tubing to it is not connected, make sure the 2nd P
Sensor= parameter in the Unit Configuration menu of
the keypad/display is set to "None" so that the
controller ignores the second static pressure analog
input MCB-AI14.
c
If a second sensor (SPS2) is installed, but it is a
building space sensor rather than a duct static pressure
sensor, make sure the 2nd P Sensor= parameter in the
Unit Configuration menu of the keypad/display is set to
"Bldg."
2 Check the 24 VAC power supply to the sensor.
a
If the sensor is SPS1 or SPS2 (duct static), verify that
there is 24 VAC between the suspect transducer "+" and
"-" terminals.
b
If the sensor is SPS2 (building static), verify that there
is 24 VAC between the "IN" and "CM2" terminals on
the SPS2 terminal block.
c
If 24 VAC supply reads low and/or the MCB is
malfunctioning, the sensor may be drawing too much
current. Disconnect the sensor and recheck the voltage
and the MCB operation.
3 Using an accurate manometer or gauge, measure the same
pressure that the suspect transducer is sensing. To do this,
tap into the transducer high and low pressure tubing or
locate the measurement device taps next to the transducer
taps.
The fittings on the pressure transducers are fragile. Splice a
tee fitting into each tap tube instead of removing each tap
tube from its transducer fitting. Use an airtight cap to cover
the test port after the pressure measurement.
Note – If the suspect sensor is measuring duct static pressure,
verify that the high and low pressure taps are properly
installed. An improper pressure tap installation can
cause severe fluctuations in the sensed pressure. Refer
65
CAUTION
to the model-specific installation manual for pressure tap
installation guidelines (see Table 2 on page 1).
4
Measure the DC voltage output from the transducer.
a If the sensor is SPS1 or SPS2 (duct static), measure the
voltage across the sensor "S" and "-" terminals.
b
If the sensor is SPS2 (building static), measure the
voltage across the "OT2" and "CM2" terminals on the
SPS2 terminal block.
If the measured voltage and pressure do not match, there
may be a wiring problem or the transducer may be
defective. Check the transducer input circuit wiring and
connections for defects.
If the measured voltage and pressure match, the MCB may
be missconfigured or defective.
5
Remove power from the controller by opening system
switch S1. If available, swap a similar good transducer
with the suspect transducer or try installing a new
transducer. Restore power by closing S1 and verify
whether the suspect transducer is defective.
Figure 31. Duct static pressure transducer voltage vs.
pressure
5
4
3
2
1
1
Static Pressure ("W.C.)
Figure 32. Building (space) static pressure transducer
voltage vs. pressure
5
4
3
2
1
-0.25 -0.125 0.000
Static Pressure ("W.C.)
2 3 4
0.125 0.25
Daikin IM 696-4