Besides fatigue, cables in motion can also generate low- voltage triboelectric effects. For microvolt sensors — such as thermocouples or RTD’s — these effects could contribute to inaccuracy. This error can be significant if the motion stimulating the effect is of the same order as the thermal responsiveness you intend to measure.
Magnetic and other issues
Thermocouples and RTDs are susceptibility to noise; thermistors are more immune. By shielding and properly grounding, immunity from potential noise can be improved. These methods work well for noise from capacitance currents, radio frequencies and offset currents. Immunity from magnetic sources is not so easily achieved.
Sensors often operate in areas containing large motors, solenoids, or high current devices. This equipment can cause transient currents or magnetic surges. For sensor types that require stimulating electronics (thermistors and RTDs), these power droops can affect the power supplies and sensing circuits inside the sensor electronics, which subsequently affects temperature readings. Additionally, large inductive spikes can create circulating currents that alter ground potentials near the sensors — biasing the voltage and causing sensor error.
When thermistors are used to measure temperatures near their lower extremes, the resistance may approach 100K or more. When this happens, long runs of thermistor wire can create an antenna that adds noise to the measurement system. While most of this can be filtered out, the potential for biasing the measurement becomes greater because the direct current charge collects noise (known as the electret effect).
The best method to protect from outside electrical and magnetic sources is to keep the sensor and lead wires away from them, shield them, and pay close attention to electronics isolation and grounding — only a few feet can make a big difference! Keeping sensor lead wires short and converting the signals into digital form, as close to the measurement point as possible, can also help minimize noise.
Sensor calibration techniques
There are two ways to calibrate sensors to correct for inherent errors: a controlled isothermal bath (usually water) or a point calibration. The controlled bath is highly accurate and allows for multiple points. However, it is limited to the physical range of the liquid in the bath. In the point method the sensor is immersed in a liquid/solid bath — a standardized melting point — an ice bath (0.01°C) or a gallium bath (29.7646°C) are good examples. This method is only as sound as the ability to extrapolate, or interpolate, from the measurements.
If only relative accuracy is acceptable, an array of sensors can be calibrated against each other by immersing them in a common bath at a known temperature (0˚C for an ice bath). The temperature in the bath can then be slowly raised, while tracking all sensor responses. To achieve the best results, the calibration bath should span the same temperature range as the intended measurement. Additionally, the rate of temperature increase should be slow, relative to sensor responsiveness, which will reduce time-transient errors.
The limiting factor for minimizing inherent sensor error is the uncertainty (including both the accuracy and precision) of the calibration process. Generally, thermistors and RTDs have better inherent accuracy than thermocouples, but all three types of sensors will require calibration to achieve accuracies down to 0.1˚C. It is more challenging to calibrate thermocouples than thermistors and RTDs — calibration must consider both hot and cold junction temperature errors.
Putting it all together
Sensor selection goes beyond having a sound knowledge of the inherent accuracy of particular sensor types. In selecting the best sensor for an application, environmental factors must also be considered for potential sources of error. It is also important to be familiar with the strategies that can be used to minimize environmental influences. Table 1 compares all three sensor types.
Cal Swanson is a senior principle engineer at Watlow Electric Manufacturing Company, St. Louis, MO. E-mail him at firstname.lastname@example.org