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Thermocouples with the T7 (App Note)

App-Notes - T7 Thermocouple - Getting Started

LabJack T7-Pro, Multifunction USB, Ethernet and 802.11b/g WiFi Thermocouple Data Acquisition and Control DAQ Device, 20+ Channels, Thermocouple Types J, K, R, T, E with Cold Junction Compensation

Getting Started, T7 & Type J Thermocouples

Thermocouple data acquisition (DAQ) app-note

Thermocouple Basics

Information about measuring thermocouple temperature sensors in regards to LabJack devices.

Connect Thermocouple to T7/T7-Pro USB, Ethernet, WiFi Thermocouple DAQ device.

Connect Thermocouple

After going through the T7 quickstart guide, connect the positive wire of the thermocouple to the AIN0 terminal on the T7 and the negative to ground.

Note: for added noise-rejection and preventing ground-offset issues connect the thermocouple so that a differential analog input reading can be performed.  Connect the negative lead of the thermocouple to AIN1 and add a 10k or 1M resistor between the negative lead and a GND screw terminal.

Kipling's Device Selection Tab

Run Kipling

Run Kipling and connect to the appropriate device.

Kipling's Analog Inputs Selection Tab

Configure the analog input channel

Navigate to the analog inputs tab and configure the T7's AIN_EF system for AIN0 for a type J thermocouple. The thermocouple temperature reading should now be displayed. To save these settings, use the Power-Up Defaults tab.

Note: to properly perform a differential analog input reading also configure the "Negative Channel" option from "GND" to "AIN1".

LJLogM: T7/T7-Pro compatable USB, Ethernet, WiFi Data Logging Application

Open LJLogM

Close Kipling and then open the device in LJLogM. Add the channel "AIN0_EF_READ_A" to the list of channels. Thermocouple readings can now be graphed and logged to a .csv file. Repeat this process for enabling and logging other thermocouple on the T7. To save these settings, use the "Power-Up Defaults" tab.

Going Further

The T7 is compatible with multiple thermocouple types including E, J, K, R, T, S, and C. For an up to date list, look at section 14.1.1 Thermocouple which is in the AIN/AIN_EF section of the T7 datasheet. If another thermocouple type is required for your application let us know.

Multiple Thermocouples (2 to 42)

The T7 is capable of measuring up to 42 thermocouples using differential measurement techniques when combined with a Mux80 and any necessary CB37 expansion boards. When connecting more than one thermocouple at a time it is recommended that customers connect them so that differential input analog measurements can be made. For example, connect the positive lead of the thermocouple to a positive analog input channel and then the negative lead of the thermocouple to a negative analog input channel.  Then connect a resistor (somewhere between a 10K and 1M in value) between the negative lead and an available GND terminal.  Collecting data from multiple thermocouples using the differential input mode helps address several of the common thermocouple complications, for more details see the "Thermocouple Complications" section in the general thermocouples app-noteWhen measuring multiple thermocouples, it is important to absolutely avoid issue #5.

For more information about which channels are positive and negative, look at the "Single-ended or Differential" section of the T-Series Analog Inputs datasheet page.  Also look at table 2 in the Mux80 datasheet for channel-mapping/wiring details regarding positive and negative channels as well as the extended channels section in the T-Series devices datasheet.

App-Notes - T7 Thermocouple Details

Device Capabilities

The T7 family of devices has the resolution and amplification necessary to directly measure raw thermocouple signals.  If you save 1 channel for an external cold junction temperature sensor (e.g. LM34) and aren't accounting for any common thermocouple issues by taking differential measurements, a single T7 can measure up to 13 thermocouples itself (you might want the CB37).  To handle more signals, it is easy to use multiple T7s (depending on software) or you can use a Mux80 multiplexer board to handle up to 83 thermocouples with a single T7. We typically suggest that customers stick with the 42 thermocouples limitation.

The -Pro devices have a 24-bit low-speed sigma-delta converter that is excellent for thermocouples.  In addition to the improved resolution, this converter provides excellent rejection of 50/60 Hz noise (with ResolutionIndex = 11 or 12) which can be a common problem in thermocouple applications.

See the information on the main page of the Thermocouple App Note.  In particular, read through the complications 1-5, decide what you will use for CJC, and avoid complication #5 (ground loops) if at all possible.

Resolution:

What's the difference between resolution and accuracy?  See the Resolution and Accuracy app note.

A type K thermocouple provides roughly 37 μV/°C.  Output is -6.458 mV at -270 °C to +54.886 mV at 1372 °C.

The maximum ResolutionIndex for a T7 is 8, and for a T7-Pro is 12, and the typical range used with thermocouples is ±100 mV.  From Appendix A-3-1 of the T7 User's Guide, looking at the ±0.1 range, the typical device resolution at ResolutionIndex=8 is about 6.3 μV noise-free and 1.3 μV effective (0.2 and 0.04 °C for a type K).  At ResolutionIndex=12 it is about 1.2 μV noise-free and 0.2 μV effective (0.03 and 0.005 °C for a type K).  The effective numbers mean that most samples (1 standard deviation) will fall in that range.

Note that the actual signal from a thermocouple will likely have real noise with it, beyond the internal noise of the device itself noted above.  The high-resolution sigma-delta converter on the T7-Pro has excellent noise rejection, and in particular rejects 50/60 Hz noise when set to ResolutionIndex=12.

Also note that temperature in air tends to have lots of small fluctuations.  What looks like noise on a thermocouple signal might be real temperature changes.

Accuracy:

From Appendix A-3 of the T7 User's Guide, the device is calibrated to an absolute accuracy of ±0.01% full-span on the ±0.1 V range.  Full-span is 0.2 V so that equates to an accuracy of ±20 μV, which corresponds to an accuracy of about ±0.5 °C for a type K thermocouple, which is more accurate than the thermocouple itself (per complication #4).

There are other sources of error in a thermocouple system, and in particular any error in cold junction temperature measurement is reflected as error in the thermocouple temperature.  Expect about ±2.0 °C with the Internal Temperature Sensor, or if using the common LM34CAZ sensor it is accurate to about ±0.5 °C at room temperature.  If the local ends of the thermocouples are all at the same temperature, then CJC error will affect them all equally and will not affect relative accuracy between the thermocouples.

 

Tutorial - Type K with T7 (Traditional Technique):

For the traditional technique to using thermocouples, which is handling CJC and voltage-to-temperature math in software, see the U6 Tutorial.  Most of the information is the same, except you should use LJLogM rather than LJLogUD.  Even if you use the AIN-EF technique that follows, the traditional technique can lend useful insight for troubleshooting.  Also, AIN-EF is not supported in stream mode, so in the strange situation where you are streaming thermocouples the traditional technique would be used.

 

Tutorial - Type K with T7 (New AIN-EF Technique):

The following was tested using T7 firmware 1.0126, LJM 1.0705, and Kipling 3.0.2.  The AIN-EF technique is supported in command-response mode only ... not stream mode.

To use the thermocouple analog input extended feature (AIN-EF) system, you write to some configuration registers, and then can directly read thermocouple temperature from a special read register.

Typical steps for a Type K thermocouple using the default units of Kelvin are:

  • Set AIN#_EF_INDEX = 22 to specify type K thermocouple, where # is the channel for the positive lead of the thermocouple.
  • Set AIN#_EF_CONFIG_B to specify the modbus address that will be used for CJC.  The address 60052 (TEMPERATURE_DEVICE_K) uses the internal temperature sensor to get the device temperature, which is a good match for the AIN0-AIN3 terminals right on the T7.
  • Set AIN#_EF_CONFIG_D and AIN#_EF_CONFIG_E to specify slope and offset for the CJC reading.  For the internal sensor registers these are typically slope(D) = 1.0 and offset(E) = 0.0.
  • The thermocouple feature will use the normal analog input settings for negative channel, resolution-index, and settling.  If, for example, you are using a differential connection (AINeven-AINodd), set AIN#_NEGATIVE_CH = #+1 (and don't forget your resistor from the negative AINx to GND).
  • If the range of the applicable channel (AIN#_RANGE) is set to the default ±10 volts, the thermocouple feature will automatically use the ±0.1 volt range, otherwise the specified range will be used.
  • Read the register AIN#_EF_READ_A in any software to get a new reading from the thermocouple.  It should read about 298 degrees K for room temp.

 

Kipling is a very easy way to configure AIN-EF.  In Kipling, go to the Analog Inputs tab and click the "+" at the right end of the row for the channel of interest, and then you will have controls to configure as desired.  To save these settings, use the "Power-Up Defaults" screen.

To configure analog inputs beyond AIN0-AIN13, you can use the Register Matrix in Kipling, or on Windows you can use "AINEFConfigTool.exe" from the Additional Utility Applications page.  This tool is useful any time you want to configure AIN-EF for lots of channels.