Temperature Measurements with the Spider-80Ti using Thermocouples
The Spider-80Ti is a front-end from the Spider-80Xi hardware platform that enables temperature measurement. Spider-80Ti supports temperature measurements from thermocouples and RTD (resistance temperature detector) sensors.
Each Spider-80Ti front-end adds 16 temperature measurement channels to the Spider-80Xi system. A Spider-80Ti front-end is user configurable to support either a PT100 RTD sensor or a K-type thermocouple. A combination of RTDs and thermocouples within a same Spider system can also be achieved by combining the front-ends configured as RTDs and thermocouples.
This blog is part one of a two-part series. The next blog will discuss temperature measurements with the Spider-80Ti using RTD Sensors.
Selection of Temperature Sensors
The Spider-80Ti front-end supports the two widely used sensors, the PT100 RTD sensor and K-type thermocouples making it ideal for temperature measurements. Some of the key differences that aid in selecting a sensor for an application is listed below:
| Sensor Type | RTD | Thermocouple |
|---|---|---|
| Temperature Range (typical) | -200 to 650°C | 200 to 1750°C |
| Accuracy (typical) | 0.1 to 1°C |
0.5 to 5°C |
| Long-term Stability | Good | Variable |
| Stable and repeatable | Drifts because of oxidation | |
| Linearity | Fairly linear | Non-linear |
| Power Required | Constant voltage or current | Self-powered |
| Reference Junction | Not Required | Required |
| Response Time | Generally slow | Fast |
| 1 to 10s | 0.10 to 1 s | |
| Susceptibility to electrical noise | Less Susceptible | More Susceptible |
| Cost | High (Relative) | Low |
A majority of RTDs are designed for measurements up to 500o C whereas thermocouples can operate at significantly higher and wider temperature ranges. Users can choose the appropriate sensor to use based on the preceding table and measurement conditions. Two or more front-end modules allow both type of sensors to be used (one sensor type per module).
Spider-80Ti with Thermocouples
Thermocouple measurement circuitry
Each Thermocouple channel is passed through a programable gain amplifier and then sampled by a 24-bit analog-to-digital converter (ADC). Then the cold junction compensation and a user customizable moving linear average is applied to the measurements.
Cold Junction Compensation
Thermocouples have two junctions, namely the Hot and Cold Junction. The hot junction is the measurement junction and is attached to the measurement point. In theory, the cold junction should be maintained at 00 C. This is not a practical solution since the cold junction is connected to an isothermal block. The temperature of this block is measured by a high precision internal temperature sensor.
The thermoelectric voltage across the sensor is measured. The internal sensor measures the isothermal temperature which is then converted to its corresponding voltage value. This value is used to offset the cold junction temperature to provide an accurate measurement.
Hardware Connection and Setting up Thermocouple Measurements on EDM
As seen in the diagram below, users can connect a thermocouple to the front-end by connecting the red lead to pin 2 and connecting the yellow lead to Pin 3 while leaving Pin 1 open.
Once the sensor is connected, users can set up the measurement on EDM.
In Setup-> Input Channels users can enable the channels on the temperature module and select thermocouple K as the input type. The thermocouple compensation and correction is built into the sensor and are automatically applied based on the input type.
Once this is complete, users can set up the test parameters in the Config Menu, Setup->Test Configurations.
Users can set the data rate, measurement range and measurement averages based on the application requirements. Once this is set the system is now ready to collect data. All temperature measurements are saved as time histories and are saved directly to the PC. For all temperature measurements users can review the complete data collected on the test while new data is actively added to the graph. With this feature users can review past events while collecting live data. Users can also simultaneously set up an easy-to-view numeric display of the current live measurement.
The new Spider-80Ti combined with other Spider platforms such as the Spider-80SG (Strain gauge) and Spider-80Xi (vibration and DAQ) modules to create a combined DAQ system that can measure a wide range of quantities and is an essential instrument for any DAQ projects.
For any additional questions or support please contact info@go-ci.com.