PC Based Temperature Measurement System
Basic overview of temperature sensors and using PC based data using data acquisition hardware
There are many different sensors used to measure temperature. Thermocouples, RTDs and thermistors are the most commonly used. Each provide pros and cons that must be evaluated based on the system you are developing. Below is a summary of these commonly used sensors. Please visit the resources listed at the end of the article for more information.
Thermocouples are, by far, the easiest and most straight forward sensors to use. The signal generated from this sensor is usually in the millivolt range, which can present a problem if the signal is not conditioned properly. In other words, the hardware that will be digitizing the signal needs to provide proper filtering and amplification. This is sometimes overlooked, and a big source of error when using thermocouples.
A thermocouple is created when two dissimilar metals are bonded together and a voltage is produced (albeit very small) proportional to temperature. This is known as the Seebeck voltage, named after Thomas Seebeck.
Now, as you are connecting your thermocouple sensor to your measurement device, you will be creating another junction between two different metals, your terminal block connector is one type of metal while your thermocouple is another type. This junction introduces errors in your temperature measurements, and must be accounted for in order to have an accurate temperature measurement at the thermocouple tip. The process of applying this “correction factor” is referred to as cold-junction-compensation (or CJC). The correction factor can be accounted for in the hardware that is performing the data acquisition.
There are several different types of thermocouples. Each is based on the type of metals used, and are designated by a letter. Each thermocouple has a specific linear temperature range and the correct thermocouple must be selected for your application. For example, a K-Type thermocouple is made up of nickel-chromium alloy for the positive conductor and nickel-aluminum alloy for the negative conductor. The voltage produced by a K-Type thermocouple is linear to its temperature range of −200 °C to +1350 °C.
To properly select the appropriate hardware to digitize a thermocouple signal, it should, at minimum, contain the ability to filter out noise, amplify the signal, and perform cold-junction-compensation. Additionally, if your thermocouple is placed on a device that can generate high common mode voltages, or ground loops, then additional isolation would be required.
Unlike a thermocouple, a resistance temperature detector, or RTD, is a sensor that has a resistance of 100 Ω at 0 °C. The resistance value changes proportional to the temperature. Since RTDs are passive devices, an excitation current must be supplied in order to read the resistance value. To minimize self-heating caused by the current flowing through the resistor, the smallest possible current should be used. Ideally, the data acquisition hardware can supply the excitation current and also read the resulting voltage.
There are several ways to connect the RTD sensor to the measurement device. Using the 2 wire method, the wires that provide the excitation current and the wires used to measure the resistance are the same. The potential side-effect of using this method would be if the wires have a high lead-resistance, which would effectively add additional resistance and not provide an accurate temperature reading. On the other hand, using the 4-wire method, a more accurate measurement can be made. The excitation current travels on a different wire than the resistance measurement.
Similar to thermocouples, RTDs can be susceptible to noise, and therefore, it is recommended to have a 50/60 Hz lowpass filter in the data acquisition hardware (i.e. before the signal is digitized).
Thermistors are similar to RTDs, in that, the resistance changes with temperature. The difference is that they are constructed of a metal oxide semiconductor material. Compared to RTDs, thermistors have a smaller temperature range that they can measure and can be very nonlinear. However, unlike RTDs, thermistors are much more sensitive and can respond to temperature changes more quickly.
Thermistors are wired similarly to RTDs, as shown above. An excitation current is required and the measurement device will then measure the voltage. Because thermistors are much more sensitive to temperature changes, faster data acquisition rates can be used.
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