Temperature Measurement and Data Acquisition

Overview

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 often measured with sensors such as RTD’s (Resistance Temperature Detectors), thermistors, or thermocouples.

Each of these temperature sensors include 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 the sensor are all important considerations. The physical packaging style of the sensor may also be a consideration. For example, will a thermocouple with twisted wires 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 contain 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, generally 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 also be impacted by electrical noise. Electrical noise can be generated from sources around the sensor, or along the length of the sensor wiring. Noise can be generated from items such as pumps turning on and off, generators running, and more. 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 they are susceptible to the length of their wires, commonly referred to as lead resistance. Thermistors with a negative temperature coefficient have such high sensor resistance that they are often immune to both electrical noise and lead resistance.

The electronic circuitry used to measure each of these sensors contains different requirements. Each data acquisition hardware manufacturer designs their own unique circuits and then implements them within their 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 as low as possible to prevent internal self-heating of the temperature sensor. Self-heating can lead to accuracy errors within the system. Each DGH RTD and thermistor products uses 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 are usually the highest cost, thermocouples and thermistors are usually much less. Thermistors can be purchased at very economical prices. Sensor packaging styles will 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, RTD’s and thermistors. Additional information about each temperature sensor, their intended applications, and some DGH measurement solutions are included below.

DGH Products

DGH offers high-quality temperature measurement solutions for thermocouples, RTD’s and thermistors. Each product contains precision measurement circuitry to make accurate temperature measurements without the need for external components or circuitry. Each product includes isolated electronic signal conditioning circuitry that is optimized for the specific type of sensor being measured, a high-resolution 15-bit analog to digital (A/D) converter to perform precise temperature measurements, and internal software digital filtering algorithms to convert each measurement to a usable temperature reading.

These products include additional features such as varying amounts of discrete digital inputs and outputs for localized on/off control. The discrete outputs in some products can be configured as on/off alarms based on user-selected temperature values. They are ideal for controlling solid-state relays and annunciators. The discrete inputs may be used for sensing contact closures such as cabinet doors opening and closing, or other detecting other control signals.

Each product contains a removable screw terminal plug for connecting the temperature sensor directly to the device. The removable plugs are also beneficial for making configuration changes or for troubleshooting purposes. Simply remove the plug and connect another plug from a computer into the module, perform the changes and restore the original plug.

These cost-effective temperature measurement products are easy to use, small in size, they support industry standard communications protocols and interfaces, are very accurate, and operate over a wide temperature range. Making them ideal temperature measurement solutions in applications ranging from laboratories to distributed data acquisition systems. DGH temperature products are available in single channel, four channel and seven channel versions. Specifications for each temperature product family are listed in detail below. Further product information is available, and you can call our offices to discuss your application with our technical sales team.