SOR Thermal Differential Flow Switch - Manual (Page 12)

12/12 Form 1024 (08.13) ©SOR Inc.
14685 West 105th Street, Lenexa, KS 66215  913-888-2630  800-676-6794 USA  Fax 913-888-0767
Registered Quality System to ISO 9001
Printed in USA sorinc.com
• Turn the unit around so that the other side of the probes faces the flow
- RTD’s have a front and a back. Sometimes turning the probes around so that
the other side is facing the flow will produce better results
... the unit calibrated well but at some later point my setpoint has shifted?
• Make sure the process has not changed
• Changes in the thermal properties of the fluid will affect set point accuracy
• Check the probe tips for uneven build-up
- If the build up on one probe (usually the hot) is much greater than the other
then the thermal differential relationship is compromised.
• Check inside of pipe for build-up
- If the process sticks and builds-up on the inside of the pipe, the diameter can
get smaller causing a change in process velocity.
• If all else fails, check the integrity of the sensor and boards (for step-by-step
instructions see the Thermal Differential Point Switch_1024 General Instruction
Manual on sorinc.com)
Power and Continuity Verifi cation

Turn the power off to the T21 Switch.

Remove the instrument enclosure cover.

Loosen the PC board capture screws. (See , page 4)

Unplug the PC board from the instrument enclosure by pulling straight out on the transformer.

Reapply power and verify correct voltage at pins 7 (positive for DC) and 8 (negative for
DC) of TBB. (See , page 4)

If voltage is correct, verify the fuse (F1) on the PC board is not blown. (See , page 9)
If fuse is not blown, proceed to Sensor/Electronics Functionality verification step 2.

If fuse is blown, replace with the appropriate fuse.
Sensor/Electronics Functionality Verifi cation

Turn the power off to T21 Switch.

Allow a 5-minute cool down.

Measure the resistance of each RTD at pins 1 and 6 of TBA (See , page 4) for the
first RTD, and pins 3 and 5 of TBA for the second RTD. These resistances should
be 110 ± 10 ohms (with sensors at approximately 70°F) and within 5% of each other
in value.

Measure the insulation resistance between pin 1 of TBA and the case of the Switch. It
should be greater than 20 megohms.

If the switch sensor assembly resistances are not as specified above, the switch sensor
assembly must be replaced.

If the switch sensor assembly resistances are as specified, the switch PC board
must be replaced.