Senior Principal Engineer
Single Iteration Div.
Watlow Electric Manufacturing Company
St. Louis, Mo.
A relatively simple solution for obtaining accurate temperature measurement embraces three factors: The inherent accuracy of a particular sensor type, the environmental factors that can affect the sensor’s accuracy, and sensor calibration techniques.
Thermocouples come either protected or not. One type has simple, fabric covered leads with the element exposed and unprotected so it can directly contact the surface to be measured. The other type is embedded in a variety of protective sheaths. However, the protection is a mass that displaces the measured temperature from the actual temperature, stretches out the time constant, and requires more time for the sensor to respond to temperature changes.
Thermocouples are the smallest, fastest, and most durable of temperature measurement devices. They can withstand extremely high temperatures and harsh mechanical punishment, and they are simple to operate. Because they are so small, they respond rapidly to temperature changes, and their sensing junction can be placed close to the measurement point. The durability and simplicity of this sensor type make it ideal for embedding into other devices.
However, thermocouples are the type most vulnerable to noise, inaccurate readings, and precision error. When extreme accuracy and precision are required, many of these shortcomings can be compensated with short, insulated, shielded thermocouple lead wires; balanced, low-pass, filtered differential amplifiers; and complex calibration procedures.
Also, differences in metal purity and alloy homogeneity can make thermocouple temperature profiles deviate from the National Institute of Standards and Technology’s (NIST’s) standards. This can be a problem when long lead wires are needed between the sensing element and the signal conditioners. A thermocouple containing a minimum number of elements, such as a type T, J, or G should be used when extreme accuracy is required, but a means for calibration it is not available.
Thermistors are ideal for measurements that require high accuracy and sensitivity over a narrow range of temperatures (typically less than 300º C), but they cannot endure the high temperatures or mechanical stresses that thermocouples can. This makes them unsuitable for applications and assembly operations where these factors are not well controlled. To compensate, the sensors can be encased in a protective metal enclosure, but this slows their response to temperature changes. Because some thermistors can handle temperatures to 1000º C, their slower response is traded off for their higher temperature capability.
Although thermistors are less subject to different types of errors, local signal conditioning is still recommended. Because thermistors tend to be larger than thermocouples, they are slower to respond to temperature changes and can have larger heat transfer errors than thermocouples.
Thermistors produce relatively high but non-linear changes in resistance near their maximum sensitivity point with small changes in temperature. However, padding resistors can be added in a voltage divider circuit to obtain a more linear response. Thermistors can be made fairly uniform within a batch, but batch-to-batch variations can be a problem when high accuracy is needed. Also, NIST does not publish standards for thermistors, so variations may be seen among different manufacturer’s devices.
RTDs are extremely stable, make precise measurements, and maintain high accuracy over long periods. Often, they are more accurate and precise than thermistors and thermocouples. RTDs conform to Deutsche Industrie Normen (DIN) and Joint Information Systems Committee (JISC) national standards. Consequently, off-the-shelf RTDs maintain consistent tolerance specifications regardless of batch number.
Compared to thermistors and thermocouples, RTDs are more fragile, and although the melting temperature of an RTD element is high enough to survive many high-temperature manufacturing operations, they do not survive aggressive mechanical compaction processes. For this reason, they are difficult to embed into custom mechanical devices. Metal-sheathed assemblies can help overcome the fragility factor, but this, coupled with their larger size, often makes their response time slower than thermocouples.
For a typical 100 ohm RTD, wire and termination resistance from long lead lengths and multiple connections can become a significant source of error. Three or four wire RTDs are often used to obtain the highest accuracy. Sensor conditioning electronics can remove errors from lead resistance, but there is usually a tradeoff between cost and number of wires needed for the measurement.
RTDs suffer measurement errors from self-heating, but this can be reduced considerably by applying an extremely low bias or a 10% duty cycle instead of a constant bias. Although this approach helps alleviate one problem, it generates another: noise that can affect the RTD measurements. However, the noise from this and other external sources can often be minimized by using differential, ungrounded, and shielded elements.
General measurement errors
Temperature measurement errors arise from several sources, which include location, transients, and heat transfer. Often, it is difficult to sense the temperature at the exact point where it is needed because the sensor has a finite size or is surrounded by an encasement that displaces the sensing element. Then the sensor’s actual temperature will be different from that at the intended measurement point. However, such location errors can be compensated when surrounding heat sources and sinks are known. A simple solution, which avoids complex calibration techniques, uses the smallest sensor possible, placed as close to the temperature source as possible. Thermistors and RTDs have larger errors than thermocouples located in the same place, due to their larger size.
Thermistors and RTDs can be made extremely small and surrounded with a case to protect them from the environment. Thermistors are a little bigger and a little slower than thermocouples, and they generate a non-linear response, but they have a high resistance to temperature ratio at their maximum sensitivity point. RTDs can be more accurate, but they are more fragile than the other types and do not usually survive excessive vibrations and mechanical abuse.
Transient errors are dynamic errors that are difficult to offset because every material within the thermal system has a unique thermal conductivity and capacity. Of the three most widely used types, thermocouples typically best minimize transient errors due to their small size and short time constant.
Sensors are also susceptible to heat transfer errors and measurement inaccuracies from conduction, convection, and radiation inputs. Higher and lower ambient temperatures can move heat into and out of the sensor through their thermally conductive lead wires. Type E and J thermocouples are alloys with less conductive leads, which make them most suitable for minimizing these kinds of errors. Self-heating errors apply to thermistors as well as RTDs. The same compensation strategies used for thermistors can be used for RTDs.
All three types of sensors can be custom made and packaged for specific applications to deliver exceptionally accurate and repeatable temperature measurements. One interesting configuration is the eyelet lug that looks like a ground strap but contains a temperature-sensing element.
Atmosphere and environmental influences
For all three sensor types, operating or cycling them near their temperature limits can accelerate deterioration and make them deviate from their original profile. Thermistors and RTDs are usually well sealed from the environment, so they may be less susceptible to internal corrosion. However, these sensors are typically connected to copper wires, which increase their risk of lead-wire deterioration.
The lead-wire corrosion problem is reduced for RTDs by using three or four-wire units that measure the resistance of the sensing element instead of the connection wire. This gives RTDs the greatest overall stability of the three sensor types. Thermistors usually drift a little initially, but they generally stabilize after some aging. Thermocouples behave in a more complex way, because the voltages they produce come from their different metals and alloys, which change as the metals age and deteriorate.
When measuring temperature on a surface, forced airflow on and around a sensor can contribute to a false reading because of heat-transfer error. Convective currents add or remove heat from the sensor and measurement surface. When the atmosphere and surface temperatures differ, or the measurement environment is moist, convective heat flow must be considered as if it were another heat source or sink.
Frequently, temperature sensors cannot be located at the ideal sensing spot. It may be displaced because of the sheath in which it resides, dimension B, or it simply cannot be moved closer than dimension A because of some other physical limitation of the device under test.
Other unique effects
Small wire gages and fragile sensors should be avoided where they may be subjected to extreme mechanical motion, vibration, or high-intensity acoustics. The most common wire failures develop near connection points with the greatest amount of flexture. However, mechanical motion or vibration can also stimulate resonances inside the sensor and cause it to fail. Thermocouples are generally the most durable of the three sensor types because many of the alloys used in the wires are more ductile, thus they can tolerate more motion. In addition to fatigue, cables in motion can generate low-voltage triboelectric effects. For microvolt sensors such as thermocouples or RTDs, these triboelectric voltages could be the same magnitude as the thermal voltage intended to be measured.
Thermocouples and RTDs have the highest noise sensitivity of the three sensor types. Shields and proper grounds can reduce noise susceptibility and offset currents due to capacitive coupling and radio frequency interference, but magnetic sources are a little more difficult to shield.
Sensors often operate in the same environment with motors, solenoids, or high-current producing devices that generate high transient currents and magnetic surges. Signal conditioners for thermistors and RTDs should contain power supplies that are immune to these droops and surges when operating from the same power mains. Otherwise, the signal conditioners’ power supply changes could adversely affect the sensors’ temperature readings. Also, large inductive spikes can generate circulating currents that alter ground potentials near the sensors. These spikes can bias the voltage output of the sensor and produce false readings.When thermistors measure temperatures near their lower extremes, their resistance may approach 100k or more. With such high resistance, long thermistor lead wires can act as antennas and add noise to the measurement system. Most of this can be filtered out, but a small dc charge, called the electret effect, can bias and affect the measurement. The best method to protect sensors from outside electrical and magnetic sources is to keep the sensor and lead wires as far away as possible from the interfering source, shield them, and use appropriate electronics isolation and grounding. Also, keep sensor lead wires short and convert the analog signals into a digital format as close to the measurement point as possible.
Sensor calibration techniques
A common method for correcting inherent accuracy errors is to calibrate the sensor in a controlled isothermal liquid bath and compare temperature readings to a standard reference. A point calibration method immerses the sensors in an ice bath or other standard freezing point medium such as a gallium-freeze bath at 29.7647° C.
Accuracy and precision or repeatability are distinctly two different properties of sensors. Accuracy defines the degree to which each measurement falls into the target area, and precision defines how tightly the measurements cluster together. Often, measurements are more valuable for their repeatability than their accuracy.
When only relative accuracy is critical, an array of sensors can be calibrated by immersing them in a common bath at a known temperature (0º C for an ice bath). The bath temperature can be raised slowly while tracking all sensor responses. To obtain the best results, the calibration bath should span the the same temperature range as the intended measurement. The rate of temperature increase should be slow, relative to the sensor’s response time, which reduces 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 require calibration to obtain accuracies down to 0.1° C. Calibrating thermocouples is more difficult than thermistors and RTDs because the procedure must consider both hot and cold junction temperature errors. DW
For more information, see www.watlow.com
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Filed Under: Factory automation, Data acquisition + DAQ modules, Sensors (position + other), Sensors (temperature)
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