Troubleshooting the [c]FP-TC-120

This document describes common problems that can occur when using the [c]FP-TC-120, and explains how to fix them.

1. Erratic Temperature Readings

Symptoms:
Unstable readings, showing spreads or offsets, appear on some or all channels when several thermocouples are connected to the module. The readings are correct if only one thermocouple is connected to the module.

Causes:
Erratic readings usually occur when stray currents run through the module. The [c]FP-TC-120 input channels are differential, which means that they measure the difference between the two thermocouple leads. However, the channels are not isolated from each other. If two or more of the thermcouples are connected to different potientials (such as one to a power outlet and another to a ground, or both to different grounds), current runs from one thermocouple into the module and out to the other thermocouple. Depending on the range used, if more than 0.5 to 2.0 V is run through the thermocouples, one or more of the readings can become erratic. In areas of high noise, common-mode voltages can be induced on the thermocouple wires. If these voltages are more than 0.5 to 2.0 V and another thermocouple is grounded, has a common-mode voltage, or is connected to a voltage source, erratic readings will also occur.

Because the [c]FP-TC-120 is isolated from the rest of the bank, noise from thermocouples on other modules do not affect the readings. Please note that connecting the V and C terminals between modules defeats isolation between the modules. To determine if stray currents are causing erratic readings, connect only one thermocouple to the module. If each thermocouple alone does not have a problem, most likely the erratic readings are due to stray currents.

Possible Solutions

Use shielded thermocouple wires, and connect the shield of each thermocouple wire to a C terminal of the [c]FP-TC-120. Do not connect the shield at the thermocouple end of the wire.

Do not connect the V or C terminals of the [c]FP-TC-120 to the V or C terminals of any other FieldPoint module. Connecting to other FieldPoint modules defeats isolation between the modules.

Keep thermocouple wires as far as possible from sources of electrical noise.

If you are spot welding thermocouples in place, weld with the same metal the thermocouples are made of. This avoids inaccuracy caused by cold junctions between thermocouples and dissimilar metals. Make sure that you do not create a ground loop when welding a thermocouple.

Do not ground the ends of the thermocouples. If they must touch a metal object, use an insulator and thermal conduction grease.

Route thermocouple cables through grounded metal conduit.

2. Unrealistic Temperature Readings or Reading 0 Degrees

Symptoms:
Channels show unrealistic temperatures when monitoring a known temperature or show around 0 degrees.

Causes:
Some channels may be misconfigured in FieldPoint software. Thermocouple channels may be misconfigured for thermocouple type, the range may be set to voltage, or the cold-junction compensation (CJC) channel may be misconfigured. If you are using a cFP-TC-120 with an external terminal block, attach an external thermistor that meets the proper specifications.

Possible Resolution

Make sure that each channel is configured for the proper thermocouple type. In MAX, go to the Channel Configuration tab for the module. Under Channels, select Type 1: Analog Input. For each channel 0 to 7, select the Range under Data Configuration, and select the Thermocouple Type under Channel Attributes.

Make sure that the CJC channel (channel 8) is configured properly. In MAX, go to the Channel Configuration tab for the module. Under Channels, select Type 2: Analog Input #2. Select the Range under Data Configuration, and select Internal for the Value of Cold Junction Compensation under Channel Attributes. Internal indicates that the [c]FP-TC-120 module uses the current measured temperature from the CJC sensor on an FP-TB-3 terminal base or cFP-CB-3 connector block. Alternatively, you can select 0 or 25 °C. This setting determines how measurements are linearized.
Note: Only channels that are configured for thermocouple readings are linearized. If a channel is configured for voltage measurements, the CJC value is ignored.

If you are using an external terminal block to wire thermocouples to the cFP-TC-120, make sure that the thermistor for cold-junction compensation is of the proper type. Refer to KnowledgeBase 2SCBTB2P: What Thermistor Should be Used if I Am Connecting an External Terminal Block to the cFP-TC-120?

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