Temperature values are one of the most common elements measured in any data acquisition system. Temperature measurements are often required in production test systems, energy management systems, environmental monitoring, aviation systems and more. These temperature values are measured with sensors such as resistance temperature detectors (RTDs), thermistors or thermocouples.
Each of these temperature sensors includes unique measurement characteristics that must be considered when selecting sensors for a data acquisition system. Characteristics such as measurement range, accuracy and the electronic circuitry required to measure are all important considerations. The physical packaging style of the sensor may also be a consideration. For example, will a thermocouple with twisted wires be acceptable, or does the application require a screw-in metal probe? These characteristics all need to be considered.
When specifying temperature sensors for any system, the first consideration is the temperature measurement range. Each of these sensors contains different measurement ranges. Thermistors offer a small measurement range, typically from sub-zero to a few hundred degrees Celsius. Thermocouples offer a much wider measurement range from sub-zero to over 2000 degrees Celsius.
Sensor accuracy is another important consideration when specifying sensors for a data acquisition system. Each of these sensors offers different accuracy specifications from tenths of one degree up to multiple degrees. Thermistors have the best overall accuracy specifications, close to one tenth of one degree Celsius. RTD accuracy specifications are slightly higher than thermistors at a few tenths of one degree Celsius. Thermocouple accuracy on the other hand is generally within a couple of degrees Celsius.
Sensor accuracy can be impacted by electrical noise. Electrical noise can be generated from sources around and along the sensor, such as pumps switching on and off or generators running. While thermocouple measurements are not affected by the length of their wires, they are susceptible to picking up electrical noise. RTD sensors are less immune to electrical noise, but are susceptible along the length of their wires, commonly referred to as lead resistance. Thermistors with a negative temperature coefficient have a high sensor resistance and are often immune to both electrical noise and lead resistance.
The electronic circuitry used to measure each of these sensors has different requirements. Each data acquisition hardware manufacturer designs its own unique circuits and implements them within its products. Typically, each circuit will contain circuitry to measure millivolt DC signals from the sensor. Thermocouples output a very small millivolt DC signal. Additionally, the thermocouple measurement circuitry will include an electronic cold-junction-compensation reference, as found within all DGH thermocouple measurement products.
Thermistors and RTD’s are resistive devices and do not output millivolt signals. The circuitry required to measure these sensors must include a small electronic current source. The current source signal is passed through the resistive sensor to develop a millivolt signal. That signal is then measured and converted to a temperature value.
The amount of current passing through the sensor can be an important consideration. Some current source values can be as high as one milliamp. This current value should be designed and specified to be as low as possible to prevent internal self-heating of the temperature sensor, as self-heating can lead to accuracy errors within the system. DGH, RTD and thermistor products use a much lower current value to prevent sensor overheating.
Finally, the cost of temperature sensors can be a budget concern. Sensor accuracy requirements will usually lead to increased sensor costs. RTD sensors tend to carry a higher pricetag , while thermocouples and thermistors less costly. Sensor packaging styles can also impact the sensor pricing.
While there may be concern for additional sensor parameters in each application, this article attempts to address the primary concerns for measuring temperature with thermocouples, RTDs and thermistors. Additional information about each temperature sensor, its intended applications and DGH measurement solutions is available upon request from Vepac Electronics.
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