Temperature Measurement

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Temperature Measurements using Data Acquisition Processors

Now review the manual and use the complete Sensors Conditioning Module with xDAP systems

Option: Download the temperature sensor section in a PDF file.

Off-the-shelf temperature measurement and display products are excellent for what they do. They offer accuracy within a couple of percent, sometimes better. And they are ready to go with all necessary functions included. If they do what you want, you should use them. But if they don't have exactly what you need, they offer you few options. See how Data Acquisition Processors can help.

Take a look at some sample multi-channel system drawings.
DAP 4000a System | DAP 5200a System | DAP 5216a System

Choosing Sensors

Sensors detect physical phenomena and convert these to corresponding electrical signals that are relatively easy to measure. Sensors that do not produce an electrical signal naturally are supplemented with electronics to produce the measurable signal.

Several temperature sensor types are available, each with its own strengths and weaknesses. To help you make an appropriate selection, here are brief descriptions of some of the most important temperature sensor types. DAPs are intended to work with any sort of sensor that delivers a measurable electrical signal.

Calibration

The term calibration is often used incorrectly, to mean applying a standardized device curve. That assumes that a real device will conform perfectly to the selected theoretical curve. In practice, any real device will deviate from published theoretical curves, which are only statistical composites. A true calibration process determines how an actual device responds, given inputs established by a reference standard.

Published standard curves are a good start, and sensor manufacturers try to match them as close as they can, but actual devices deviate from the ideal. Check your tolerance requirements. If you don't need best accuracy, keep things simple, and stay with "typical" curves. But for better accuracy, calibrate. Check the calibration and linearization page for more information.

Some DAPL processing commands can help you apply sensor calibration for improved measurement accuracy.

The INTERP command can represent nonlinear response curves of arbitrary sensor devices over an extended range.
The RTD command can apply nominal or calibrated conversion curves for RTD devices.
The THERMISTOR command can apply nominal or calibrated conversion curves for thermistor devices.

Measurement

Thermocouple Example

RTD Example

Thermistor Example

Temperature sensors tend to fall into two categories: those that produce a voltage naturally as an indicator of temperature, and those that produce some other physical response that must be converted to a voltage for measurement.

Natural output potentials.

The signals may be weak or strong. When weak, they are vulnerable to external interference. When strong, they typically require a power source to drive them, with potential dangers of power supply noise leaking into the measurements. For both cases, differential signals help to keep the measurements clean.

When working with natural voltages, you will typically need:
- A SCALE or DAPL expression command to correct for gains and offsets.
- A linearization command like THERMO or INTERP to compensate for nonlinearity and calculate the corresponding temperature.
The article Thermocouple Measurement Applications on this site describes a low-cost, low-speed 8 channel application using thermocouples. It applies averaging for noise reduction, a conventional cold junction compensation device, and standard conversion curves for measurements of moderate accuracy.

Another article on this site covers accurate measurement with thermocouples, using carefully-calibrated measurements and cold junction compensation.
Indirect voltage output.

Excitation power is typically required to convert the response into an electrical signal.

For example, resistive devices are driven by regulated voltage or current sources. You will typically need:
- A SCALE or DAPL expression command to correct for gains and offsets.
- A command like DIVIDER or BRIDGE to determine the device resistance from observed output voltage.
- A command like RTD or THERMISTOR to convert from a resistance measurement to the corresponding operating temperature.
An article on this site describes an applications using a thermistor in conjuction with a thermocouple, for very accurate measurements of cold junction temperature.

Another article on this site discusses an application using an RTD for measuring moderately high temperatures with moderate accuracy, using simple calibration procedures.