Temperature Measurements with the Spider-80Ti using RTD Sensors
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 the same Spider system is achieved by combining front-ends configured as RTDs and thermocouples.
This blog is part one of a two-part series. The next blog will discuss in detail about temperature measurements with the Spider-80Ti using thermocouples.
Selection of Temperature Sensors
The Spider-80Ti front-end supports two widely used sensors, the PT100 RTD sensor and K-type thermocouples, which are ideal for temperature measurements. Some of the key differences that aid in selecting a sensor for an application are listed in the table 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 1s | |
| Susceptibility to Electrical Noise | Less Susceptible | More Susceptible |
| Cost | High (Relative) | Low |
A majority of RTDs are typically designed for measurements up to 500°C, whereas thermocouples can operate at significantly higher and wider temperature ranges. Based on the table and measurement conditions, users can select the appropriate sensor to use. Systems with two or more front-end modules can use both types of sensors (with one sensor type assigned per module).
3-Wire RTD Sensors
An RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its temperature changes. This change follows a known, precise, and repeatable path. The most popular RTD is the PT100. The PT is abbreviated for platinum, the principal resistance element and the 100 represents its default resistance in Ohms at 0°C. RTD sensing elements are made from a length of fine coiled pure metal wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it.
RTDs are built to several standardized curves and tolerances. The most used standard is the DIN / IEC 60751 (shortened to IEC 751) standardized curve. The curve describes the resistance vs temperature characteristics of a platinum 100-ohm sensor, the standardized tolerances, and the measurable temperature range.
In the 3-wire RTD sensor, two wires link the sensing element to the Spider-80Ti on one side of the sensing element, and one links it on its other side as shown in the picture below.
The Spider then calculates the compensation based on the following principle, if three identical wires are used and their lengths are equal, then R1 = R2 = R3. By measuring the resistance through leads 1, 2 and the resistance element, a total system resistance is measured (R1 + R2 + RE).
If the resistance is also measured through leads 2 and 3 (R2 + R3), the resistance of only the lead wires is measured, and since all lead wire resistances are equal, subtracting this value (R2 + R3) from the total system resistance (R1 + R2 + RE) which eliminates any lead wire error, providing an accurate temperature measurement.
Each RTD channel is passed through a programable gain amplifier and then sampled by a 24-bit analog-to-digital converter (ADC). Finally, a user customizable moving linear average is applied to the measurements.
Hardware Connection and Setting up RTD Measurements on EDM
As seen in the diagram below, we can connect either a 2-wire or a 3-wire RTD sensor. When a 2-wire sensor is used - the black lead is connected to Pin 2, the red lead is connected to Pin 3, and Pin 1 is left open.
Once the sensor is connected, the measurement can be set up on EDM.
In Setup->Input Channels we can enable the channels on the temperature module and select RTD PT 100 as the input type. The non–linear correction is set by default to the IEC 751 standard, which is a popularly used RTD standard. Other standards or custom values can be selected from the drop-down menu and custom values can be added.
Once this is done, we can set up the test parameters in the Config Menu, Setup->Test Configurations.
The data rate, measurement range and measurement averages can be set as per 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. All temperature measurements allow users to review the complete data collected on the test while new data is actively added to the graph. This feature allows users to review past events while collecting live data. Users can also simultaneously setup an easy-to-view numeric display of the current live measurement.
The new Spider-80Ti combines 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 project.
For any additional questions or support please contact info@go-ci.com.