Temperature Sensors for Data Loggers
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Temperature measurements are among the most common data logging applications. There are 4 main types of temperature sensors that are used with data loggers for measuring temperature:
Thermocouples which generate a voltage proportional to temperature are the most common type of temperature sensor and are widely used where cost, simplicity and operating range are important and extremely high accuracy is not required. RTD's and thermistors provide a change in resistance that is related to temperature and while to offer higher accuracy than thermocouples the have a more limited operating range. Semiconductor devices provide an output voltage or current proportional to temperature and offer low cost devices with a limited operating range and accuracy comparable to thermocouples.
Thermocouples are the most common and one of the least expensive sensors. A thermocouple is nothing more than 2 dissimilar metal wires of very specific alloys that are fused together at a single point. A thermocouple produces an output voltage (typically at the millivolt level) that is related to the temperature. The data logger measures the voltage and then applies a calibration equation to convert the voltage to temperature. The data logger also incorporate a cold junction reference to compensate for any offset voltage that occur at the connections between the thermocouple wires and the data logger. A thermocouple temperature measurement actually involves multiple voltage measurements of the thermocouple an reference junction as well as scaling the voltage to temperature and compensating for the reference temperature. Because of variations in the composition of the thermocouple wire, typical thermocouple accuracies are on the order of 1-2°F although there are special composition wires with reduced errors.
Because of the very low voltages involved with thermocouple measurements, noise is often an issue particularly in industrial environments or where long cable runs are required. There are 2 methods of reducing measurement noise:
1. Use shielded thermocouple wire - Thermocouple wire is available with a extra layer of electrical shielding around the conductors to reduce the effects of electrical interference from nearby equipment. For optimal performance, this shield should be tied to a good electrical or earth ground at one end only.
2. Use a temperature transmitter - A number of manufacturers provide small temperature transmitters that take a standard thermocouple input and provide a 4-20 mA output than can then be measured with the data logger. These are extremely useful if you have very long cable run between the measurement point and the data logger. The 4-20 mA current signal is much less succeptible to noise than the mV level thermocouple output. It's often not significantly more expensive to use transmitter because of the reduced cost of standard cabling for 4-20 mA signals compared to shielded thermocouple wire.
An RTD sensor is a device whose resistance increases linearly with temperature. The most common RTD consists of a fine platinum wire wound around a cylinder but nickel and copper wire is also used in some cases. The resistance vs. temperature curve has a very specific slope and the RTD is made so that it has a specific resistance at 0°C, with 100 Ω being the most common value. To measure temperature, the data logger will source a known current through the RTD and measure the resulting voltage from which it can calculate the resistance. Finally, using the slope of the resistance vs. temperature curve and the 0°C resistance, the data logger can calculate the temperature. RTD's are typically more stable and accurate than thermocouples, but at the expense of a more limited operating range.
Thermistors are similar to RTD's in that they are devices whose resistance changes with temperature but the resistance change is highly non-linear. Because of this characteristic, thermistors can offer very accurate temperature measurements, down to an accuracy of 0.01°C, but only over a very limited temperature range typically -40°C to 150°C. Like RTD's, thermistors are designed to have a specific resistance at 0°C, 2252 Ω is a common value and each family of thermistors have a specific resistance vs temperature characteristic that the data logger must be able to accommodate.
Semiconductor sensors incorporate a solid state device such as a diode or voltage reference that have a well established voltage vs temperature characteristic along with signal processing electronics to generate a voltage or current output that is proportional to temperature. For example, the Analog Devices AD592 transducer provides an output current of 1µA/K with an output of 248 µA at -25°C. These device only require a simple voltage or current measurement and linear calculation to scale the signal to temperature.