Troubleshooting

Ground Loops

This is a common and complex problem people encounter when using thermocouples. It occurs when bare thermocouple wire is used, or a metal thermocouple probe where one of the thermocouple wires inside is connected to the probe (often called a "grounded" probe). If multiple of these are connected to a common conductor (e.g. a system of connected metal pipes), and connected single-ended to the LabJack, you can get ground loops and unexpected thermocouple junctions. 

The typical fix is to use differential inputs with a resistor (100k is typical) from the negative input to GND. As noted in the Differential Readings Application Note, the resistor is sized to be low enough to provide a path for bias currents but high enough to prevent ground loop issues. Differential connections usually prevent this problem, but sometimes further steps are needed (e.g. channel-to-channel isolation). The following methods can also help prevent ground loop issues:

• Place the thermocouple close to, but not touching, the test specimen.

• Use “ungrounded” thermocouple probes. See the probe section of Selecting the Right Hardware for additional information.

• Apply a substance with good thermal conduction but poor electrical conduction between the thermocouple metal and test specimen. This could be Epoxy, tape, etc.

• Use thermocouples with an electrically insulating layer between the thermocouple metal and test specimen.  One example is the SA1 series of self-adhesive thermocouples from DwyerOmega.

Too Much Noise

If you see too much noise (notable swings) in your readings, try the following:

Range and Resolution Configuration Settings:

Measurements that have too much noise are often due to mis-configured range and resolution settings. This is particularly common on the T8, T7, and U6. To test:

1. Disconnect the thermocouple and jumper the analog input to GND (or both analog inputs to GND if using a differential input).

2. For T-Series (T4, T7/T7-Pro, T8): Open Kipling and go to the Analog Inputs tab.
   For UD-Series (U3-LV/U3-HV, U6/U6-Pro, UE9/UE9-Pro): Open LJ Control Panel and go to the Test Panel window.

3. Set the range and resolution as needed and look at the noise level on the applicable channels and compare to the specified typical noise for the LabJack. See the specifications in the appropriate device datasheet(s):

   1. Appendix A of the U3 Datasheet

   2. Appendix B of the U6 Datasheet

   3. Appendix B of the UE9 Datasheet

   4. Appendix A-3-1 of the T-Series Datasheet

   5. Appendix B of the LJTick-InAmp Datasheet

      With the default thermocouple settings of Range=0.1 and Resolution Index=0, the noise level of a T7/U6 will be perhaps 1 or 2 microvolts.  After confirming the proper noise level in Kipling/LJControlPanel, go to your actual software and look at the raw voltage to confirm the proper noise level there.

Floating Differential Input:

When taking differential thermocouple measurements, you must connect a resistor from the negative channel to GND. Note that this does not apply to the isolated inputs on the T8. See the Differential Readings App Note for additional information.

Removing Environmental Factors:

See the Bad ground loops, ground offsets, and EMI susceptibility complications described above.

• Try removing all external connections from the LabJack except one thermocouple and testing. This will help ensure that external connections are not affecting the reading.

• If your thermocouple is mounted to an electrically conductive surface, try a test measurement with a thermocouple that is not mounted (in free air). This could help if there is an issue with a ground loop from your mounting.

• If possible, try testing with the LabJack and thermocouple in a different environment. This can help establish whether something in your test location is causing the issue.

Wrong Temperature

If the issue appears to be a static error, for example regularly reading 5 above the expected temperature, there is typically a problem with either:

• the CJC (cold junction compensation) setup

• the thermocouple hardware setup

• an issue with the conversion math

Is the CJC (Cold Junction Compensation) Temperature Correct?

If using the AIN_EF feature on a supported T-series device, read AIN#_EF_READ_C for the CJC temperature.

If there is a +10 degree error in the measurement of the cold junction, you will see a roughly +10 degree error in the calculated value for absolute temperature of the remote end of the thermocouple.

1. If there appears to be a static error, you may be able to simply add an offset to your CJC temperature reading to account for it.

2. If doing CJC, note that the internal temperature sensors on LabJack devices sometimes do not properly reflect the cold junction temperature, and an external CJC temperature sensor such as an LM34 may be useful. For example, when using an expansion board such as the CB37, the cold junction may be at room temperature whereas the LabJack internal temperature sensor is a few degrees warmer than room temperature due to self heating from the LabJack. These errors in the cold junction temperature reading may be worse in environments with a significant temperature gradient.

Is the Raw Thermocouple Voltage as Expected?

Test against a 0 V condition: Bring the remote end (hot junction) of the thermocouple near the local end (cold junction) to check the condition where both ends are at the same temperature. You should read very close to 0 volts, within a few hundred microvolts typically.  A thermocouple gives you a voltage related to the difference in temperature between the 2 ends, so if the cold junction and hot junction are at the same temperature, the voltage difference measured should be close to 0. This is a good way to test the thermocouple setup if you are uncertain about the cold junction and/or hot junction temperatures.

Test any arbitrary temperature: To check at some arbitrary temperature difference, see the thermocouple voltage and temperature tables provided by NIST ITS90. For example, say you have a Type K thermocouple where the cold junction is at 25 °C and hot junction at 35 °C.

The voltage at the 25 °C end is 1.000 mV and the voltage at the 35 °C end is 1.407 mV, so the difference is 407 µV. If the LabJack is reporting 407 µV, then continue to the next section.  If not, use a DMM to measure across the screw-terminals where the +/- leads are connected to see if the DMM reading is different or agrees with the LabJack. If the DMM does not agree:

• There may be a ground offset issue (see the complications section above).

• If you are taking a differential measurement, ensure that you have a reference to GND (see the Floating Differential Inputs section above).

• If using a differential connection, there could perhaps be an issue with common mode voltages; check the voltage on each AIN input compared to GND.  Both should be near 0 and within the typical measurement range of your device.

Is the Math Correct?

If the CJC temperature is right and the raw thermocouple voltage is right, then there may be something wrong with the conversion math. Note that our conversion functions typically only support the ranges (and associated reverse coefficient polynomials) described on the NIST website for conversion.

Thermocouple Extension Wires

Thermocouple extension wires are used to measure temperatures far away from the measurement hardware. These can introduce additional complications.

Wiring Errors

There are several common errors that can occur when setting up extension wires.

• Ensure that you are using the thermocouple extension wire type that matches your thermocouple type, f.e. type K extension wire for type K thermocouple measurements. Using the wrong type of extension or plain (copper or aluminum) wire can introduce offset voltages that show up as error in the measurements.

• Ensure that the polarity of the thermocouple extension matches the thermocouple itself (thermocouple+ to extension+, thermocouple- to extension-).

• Bad electrical connections can cause excessive noise or completely erroneous measurements. Ensure that there is good continuity in the thermocouple wiring:

  1. Disconnect the thermocouple from the measurement hardware.

  2. Ensure that the remote end (hot junction) is not touching any conductive surfaces.

  3. Use a multimeter/ohmmeter to measure the impedance between the two thermocouple wire leads. If the continuity is good, the impedance should be low, up to a few Ohms. If you see a very high impedance reading, infinity, or something like OL/OC (open circuit) then there is likely a break, damage, or a bad connection somewhere along the thermocouple wire.

  4. Check your connections everywhere. You may need to replace the extension wire and thermocouple.

EMI, Noise Rejection

The longer your extension wire, the greater the surface area for EMI to conduct alongside the signal. The following solutions can help minimize the effects of EMI:

• Take differential or isolated measurements so that noise may be rejected as common mode voltage (and it also helps avoid ground loops).

• Use twisted wire pairs so that noise affects both thermocouple extension wires equally, and can therefore be rejected as common mode voltage when taking an isolated or differential thermocouple measurement. Thermocouple extension wire is commonly available in twisted wire pairs.

• Use shielded extension wires to try to minimize the amount of noise that appears on the wires.

• Lay out any long thermocouple wire runs as far away from power lines as possible. If the wire must be ran close to power lines, it would be beneficial to run them perpendicular to the power lines. This will help minimize the amount of coupling that can occur between the power and thermocouple lines.

• Adding a ferrite choke to the thermocouple extension wire near the measurement hardware can help reject high frequency noise. A similar and superior solution would be to construct a dedicated low-pass RC filter near the measurement hardware to reject lower frequency noise such as 50/60Hz from power mains.

LJTick-InAmp Troubleshooting

The following points can help troubleshoot thermocouple issues on the LJTick-InAmp Revision 3. If you have an LJTick-InAmp Revision 2, see Appendix C.1 of the LJTIA Datasheet instead:

• You can test that the LJTick-InAmp is operating correctly by performing the Operations Tests in the LJTick-InAmp Datasheet.

• Ensure that the thermocouple signal is within the Signal Range of the LJTick-InAmp. This should not normally be an issue unless there is an external voltage being applied to the thermocouple wiring somewhere. You can find V_in and V_cm by measuring between IN+ and GND, then IN- and GND, with a DMM:
  Vin = VIN+ - VIN-
Vcm = (VIN+ + VIN-) / 2