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Cryogenic Thermometers – An Introduction
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A number of different devices have been used to measure temperatures in cryogenics. Resistance thermometers, including carbon resistors and platinum resistors, have been widely used. Diodes are also useful as thermometers because the characteristic or "I-V" curve of a diode is strongly temperature dependent. Thermocouples are less widely used at low temperatures. In some applications vapor pressure bulbs are useful. In comparing these different thermometers there are several features to consider:
Temperature Range—Most thermometers are useful over a limited range of temperatures. The electrical resistivity of a very pure metal, for example, becomes so small at low temperatures that it may be difficult to measure. The usable temperature range for a given type of sensor is not necessarily covered by a single device. A carbon resistor selected for good sensitivity in the 4K to 70K range will probably not be useful below 1K and vice versa. |
A typical Cernox (R) Temperatures Sensors
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Interchangeability—How well does this thermometer follow a "standard" calibration curve? Some thermometers must be individually calibrated for measurements of any accuracy. Others, such as the vapor pressure bulb, rely on a physical property that is universal. Many thermometers fall somewhere in between; they can be used interchangeably without individual calibrations if high accuracy is not required.
Size and Specific Heat—These usually go hand-in-hand, but may affect the choice of thermometers for unrelated reasons. Space constraints may limit the use of some devices. A larger thermometer will tend to have a larger specific heat and therefore a slower response time.
Environmental Considerations—Many potential low temperature thermometers are sensitive to magnetic fields. This may limit their accuracy in and around superconducting magnets. Radiation damage may cause the calibration of a thermometer to change. Sensitivity to radiation may be an issue in and around particle accelerators and in spaceflight applications. Exposure to high temperatures, as might occur when baking out a vacuum system or soldering leads, can cause a sensor's calibration to shift.
Stability—Many sensors show some shift in calibration with time and with temperature cycling. Some sensors require periodic recalibration to maintain accuracy.
For all thermometers, careful installation may be crucial to obtaining accurate temperature readings. The thermometer must be in good thermal contact with the object to be measured. The thermal conductivities of almost all materials fall off rapidly at very low temperatures, so the difficulty of making good thermal contact with a thermometer is considerably more difficult at or below 1K than at 4K. In an electrical measurement, it is often important to insure that the leads are heat sunk as well as the case of the device. Depending on how it is constructed, the sensing element may be in better thermal contact with its surroundings through the leads than through the case. Highly skilled technicians proficient in proper thermometer installation techniques are key to cryogenic thermometry. As specialists in cryogenic, vacuum and pressure technology, Meyer Tool & Mfg., Inc. employs a knowledgeable and experienced staff that has installed a wide range of cryogenic sensors in projects ranging from helium liquefiers to superconducting magnet dewars. This range of experience ensures the correct installation techniques, proper heat sinking and redundant testing points essential to proper installation.
Size and Specific Heat—These usually go hand-in-hand, but may affect the choice of thermometers for unrelated reasons. Space constraints may limit the use of some devices. A larger thermometer will tend to have a larger specific heat and therefore a slower response time.
Environmental Considerations—Many potential low temperature thermometers are sensitive to magnetic fields. This may limit their accuracy in and around superconducting magnets. Radiation damage may cause the calibration of a thermometer to change. Sensitivity to radiation may be an issue in and around particle accelerators and in spaceflight applications. Exposure to high temperatures, as might occur when baking out a vacuum system or soldering leads, can cause a sensor's calibration to shift.
Stability—Many sensors show some shift in calibration with time and with temperature cycling. Some sensors require periodic recalibration to maintain accuracy.
For all thermometers, careful installation may be crucial to obtaining accurate temperature readings. The thermometer must be in good thermal contact with the object to be measured. The thermal conductivities of almost all materials fall off rapidly at very low temperatures, so the difficulty of making good thermal contact with a thermometer is considerably more difficult at or below 1K than at 4K. In an electrical measurement, it is often important to insure that the leads are heat sunk as well as the case of the device. Depending on how it is constructed, the sensing element may be in better thermal contact with its surroundings through the leads than through the case. Highly skilled technicians proficient in proper thermometer installation techniques are key to cryogenic thermometry. As specialists in cryogenic, vacuum and pressure technology, Meyer Tool & Mfg., Inc. employs a knowledgeable and experienced staff that has installed a wide range of cryogenic sensors in projects ranging from helium liquefiers to superconducting magnet dewars. This range of experience ensures the correct installation techniques, proper heat sinking and redundant testing points essential to proper installation.
Installation of cryogenic sensor to achieve proper heat transfer is critical
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The fine wires that connect the cryogenic sensors to to the outside world must be properly strain relieved to prevent damage.
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Resistance thermometers come in several types: Carbon resistors have been used as thermometers at very low temperatures for many years. Carbon resistors are used at temperatures of around 80K to below 1K. Typically a resistor with a room temperature resistance of 200 to 500 Ohms is used. Carbon resistors have a negative temperature coefficient; the resistance increases as temperature decreases. Carbon resistors are not interchangeable; individual calibrations are required for accurate temperature measurements. Because of their low cost, uncalibrated carbon resistors are used as temperature monitors in applications where high accuracy is not required. When commercial carbon resistors are used as thermometers, the outer epoxy shell may be removed for improved thermal contact. Carbon resistors show a relatively small shift in calibration in magnetic fields. Germanium resistance thermometers offer improved stability and are also sensitive at temperatures below 1K; however, they are more sensitive to magnetic fields. Zirconium Nitride resistors, available under the tradename Cernox™ from Lakeshore Cryotronics, are useful from below 1K to 420K and are relatively insensitive to magnetic fields. Resistors which have been specially made as temperature sensors are considerably more expensive than commercial carbon resistors. However, in addition to improved stability, they are offered in packaging that makes achieving good thermal contact easier. Thick film ruthenium oxide resistors are useful as thermometers from around 40K to below 1K. These chip resistors are relatively easy to mount with good thermal contact. Ruthenium Oxide resistors of the same type, particularly if purchased from the same batch, are interchangeable with accuracy on the 1% level. They are also relatively insensitive to magnetic fields and can be obtained as a commercial resistor at relatively low cost.
For all of these sensors, the effects of self-heating by the resistance measurement must be evaluated, particularly at low temperatures. Because the sensor is connected to its environment through some thermal resistance, the electrical power dissipated in the sensing element will cause it to be warmer than its surroundings. If the I/V curve of the sensor is measured with the surroundings at a fixed temperature, then self-heating will appear as a deviation from Ohms law. Such a measurement allows the thermal resistance of the sensor to its surroundings to be estimated. For example, suppose we have a sensor with a resistance R=1000Ω at 4.2K with a measured thermal resistance of 10,000 K/W to its surroundings at 4.2K. The temperature rise due to self heating is ΔT = P (10,000 K/W) where P is the power dissipated in the sensor. If we require self heating effects to be less than 1% of the reading then P < 0.04 K/ (10,000 K/W) or P < 4 x 10-6 W. With a sensor resistance of 1000 Ω then our excitation voltage should be no more than V2/(1000 Ω) < 4 x 10-6 W or V < 0.06 V. Using an excitation voltage that is unnecessarily small will result in a small signal and reduced sensitivity of the temperature measurement. In very low temperature applications, heating by extraneous (usually high frequency) signals must also be guarded against.
For all of these sensors, the effects of self-heating by the resistance measurement must be evaluated, particularly at low temperatures. Because the sensor is connected to its environment through some thermal resistance, the electrical power dissipated in the sensing element will cause it to be warmer than its surroundings. If the I/V curve of the sensor is measured with the surroundings at a fixed temperature, then self-heating will appear as a deviation from Ohms law. Such a measurement allows the thermal resistance of the sensor to its surroundings to be estimated. For example, suppose we have a sensor with a resistance R=1000Ω at 4.2K with a measured thermal resistance of 10,000 K/W to its surroundings at 4.2K. The temperature rise due to self heating is ΔT = P (10,000 K/W) where P is the power dissipated in the sensor. If we require self heating effects to be less than 1% of the reading then P < 0.04 K/ (10,000 K/W) or P < 4 x 10-6 W. With a sensor resistance of 1000 Ω then our excitation voltage should be no more than V2/(1000 Ω) < 4 x 10-6 W or V < 0.06 V. Using an excitation voltage that is unnecessarily small will result in a small signal and reduced sensitivity of the temperature measurement. In very low temperature applications, heating by extraneous (usually high frequency) signals must also be guarded against.
Installation of cryogenic sensors can be quite complex, as shown here on the cryogen piping of this 'cold box'.
Commercial resistance temperature sensors installed on a pipe. Two sensors are installed for redundancy. The sensors are clamped to the pipe. A high conductivity vacuum grease is used as a heat sinking compound. The leads are attached to the pipe to provide for heat sinking. There are four leads for each sensor because the resistance is measured by a four-wire measurement.
Platinum resistors employ high purity platinum metal to achieve a high degree of consistency in their calibration and good interchangeability. Platinum resistors can be used at temperatures as low as 20K but are more frequently used at 70K and above. The interchangeability is better than 1.3K over the range 70K to 273K (standard EN 60751, class B) making these popular sensors in both laboratory and industrial applications. The resistivity of pure metals decreases with temperature, giving platinum resistance thermometers a positive temperature coefficient. Platinum resistors are sensitive to magnetic fields, particularly at lower temperatures.
Rhodium-iron resistors are typically used at temperatures to 1.4K and have been used as low as 0.5K . They are sometimes selected for their stability and reproducibility. Rhodium-iron resistors are seldom used outside of cryogenic applications and are not interchangeable.
Diodes are also widely used as temperature sensors, with silicon diodes being the most common. Diode temperature sensors are based on the fact that the voltage drop across a forward biased pn junction is a function of temperature. This voltage drop is determined by the nature of the semiconductor, so in principle diodes should be interchangeable. In practice, commercial diodes are not generally used as thermometers at cryogenic temperatures. Diodes for cryogenic thermometry are manufactured for that purpose. Even so, the interchangeability of silicon diode thermometers is somewhat limited, with some manufactures offering interchangeability with large error bands or interchangeability over restricted temperature ranges. This comes about because in addition to the "diode" term, describing the voltage drop across a pn junction, there is a resistance term which varies considerably from one unit to the next. Diodes are usable from 1.4K to 325K, but are more frequently used at 4.2K and above. This temperature range can be covered by a single device. Because diodes follow a standard calibration curve with reasonable accuracy, and because a single device can cover this broad temperature range, diodes are widely used in instrumentation and control systems for helium liquefiers, cryogenic distribution systems and similar equipment. Diodes are sensitive to magnetic fields and are more sensitive to radiation than other sensors.
Thermocouples are occasionally, but not frequently, used at cryogenic temperatures. The need to maintain a reference junction is normally inconvenient but may be desirable if a direct reading of a temperature difference is desired. The small size of the thermocouple junction may be advantageous in some applications, but that advantage is somewhat offset by the need to heat sink the leads. Care must be taken in bringing thermocouple leads out of a cryostat to avoid setting up additional junctions.
Vapor pressure bulb thermometers rely on a pressure measurement rather than an electrical measurement. When a pure substance in liquid form is brought to a boil and the resulting liquid and vapor are allowed to reach thermal equilibrium, a unique pressure/temperature relationship is established. Increasing or decreasing the pressure will increase or decrease the boiling point. When liquid and vapor of a substance coexist we are said to be "on the vapor pressure curve", which is the line separating liquid and vapor phases on a phase diagram. The vapor pressure curve depends only on the properties of the substance. If we enclose a pure substance in a container, and maintain it at a temperature where the liquid and vapor coexist, then measuring the pressure provides us with the temperature.
Rhodium-iron resistors are typically used at temperatures to 1.4K and have been used as low as 0.5K . They are sometimes selected for their stability and reproducibility. Rhodium-iron resistors are seldom used outside of cryogenic applications and are not interchangeable.
Diodes are also widely used as temperature sensors, with silicon diodes being the most common. Diode temperature sensors are based on the fact that the voltage drop across a forward biased pn junction is a function of temperature. This voltage drop is determined by the nature of the semiconductor, so in principle diodes should be interchangeable. In practice, commercial diodes are not generally used as thermometers at cryogenic temperatures. Diodes for cryogenic thermometry are manufactured for that purpose. Even so, the interchangeability of silicon diode thermometers is somewhat limited, with some manufactures offering interchangeability with large error bands or interchangeability over restricted temperature ranges. This comes about because in addition to the "diode" term, describing the voltage drop across a pn junction, there is a resistance term which varies considerably from one unit to the next. Diodes are usable from 1.4K to 325K, but are more frequently used at 4.2K and above. This temperature range can be covered by a single device. Because diodes follow a standard calibration curve with reasonable accuracy, and because a single device can cover this broad temperature range, diodes are widely used in instrumentation and control systems for helium liquefiers, cryogenic distribution systems and similar equipment. Diodes are sensitive to magnetic fields and are more sensitive to radiation than other sensors.
Thermocouples are occasionally, but not frequently, used at cryogenic temperatures. The need to maintain a reference junction is normally inconvenient but may be desirable if a direct reading of a temperature difference is desired. The small size of the thermocouple junction may be advantageous in some applications, but that advantage is somewhat offset by the need to heat sink the leads. Care must be taken in bringing thermocouple leads out of a cryostat to avoid setting up additional junctions.
Vapor pressure bulb thermometers rely on a pressure measurement rather than an electrical measurement. When a pure substance in liquid form is brought to a boil and the resulting liquid and vapor are allowed to reach thermal equilibrium, a unique pressure/temperature relationship is established. Increasing or decreasing the pressure will increase or decrease the boiling point. When liquid and vapor of a substance coexist we are said to be "on the vapor pressure curve", which is the line separating liquid and vapor phases on a phase diagram. The vapor pressure curve depends only on the properties of the substance. If we enclose a pure substance in a container, and maintain it at a temperature where the liquid and vapor coexist, then measuring the pressure provides us with the temperature.
At very low temperatures, 3He and 4He are the working fluids of choice. Practical lower limits for these substances occur when the vapor pressures become too small to measure. 3He vapor pressure bulbs are useful from 0.5K to the critical point of 3He, 3.3K and 4He vapor pressure bulbs cover 1K to 5.2K. The range covered by hydrogen begins at its triple point at 13.8K and extends to its critical temperature of 33K. The range for neon overlaps hydrogen and extends to the neon critical temperature of 44K. We then have another gap before nitrogen becomes available at its triple point of 63K. Beyond this point substances exist to provide vapor pressure curves at all temperatures. Vapor pressure thermometry is capable of very high accuracy. It is practically immune from the effects of radiation and magnetic fields as long as electronic pressure transducers are located in a region of low radiation and low field. To obtain the highest possible accuracy with a vapor pressure bulb, it is necessary to insure that the working fluid is of high purity. It is also necessary to insure that the pressure measurement is accurate. If the bulb and the pressure transducer are at different heights, it may be necessary to correct for the weight of the column of vapor. At very low temperatures and pressures, and with small diameter tubes, corrections for the thermomolecular effect may be required. The thermomolecular effect is responsible for the difference in pressures between the cold vapor bulb and the warm pressure transducer. It is due to the fact that it is the collision rate of molecules with the connecting orifice, and not the pressures in the two volumes, that equalize.
A variety of other devices have been used as thermometers. Capacitance sensors rely on the change in dielectric constant of the insulator with temperature. The change in resonant frequency of a crystal with temperature has also been used as a thermometer. The melting curve thermometer, analogous to the vapor pressure bulb, follows a substance along the liquid/solid coexistence line. At the very lowest temperatures, below 0.04K, temperature measurements are often based on changes in magnetic susceptibility with temperature.
Selecting the best temperature sensor for a given application will depend on a number of factors. A balance between accuracy and cost must be found. In some cases, a single accurate sensor to measure the temperature of interest may be combined with less expensive and less accurate sensors for process control and monitoring. Environmental factors such as a magnetic field may greatly limit the selection. Corrections for the presence of a magnetic field can be made accurately if the effect is small, but sensors that are very sensitive to magnetic fields will probably not be useful in high field regions. The need for interchangeability or standardization throughout a large installation may also be a consideration.
Meyer Tool has the experienced engineering staff that can assist you in selecting the best temperature sensor for your application. Meyer Tool’s commitment to providing our customers with the lowest total cost of ownership means our staff is available to assist you throughout your project, from concept to fabrication. This type of partnering, typical of Meyer Tool, significantly impacts the reliability in both the selection and installation processes.”
Let us apply our Reduce Project Risk Process to your application so you too can experience receipt of ‘plug-and-play’ vacuum chambers or components. Established in 1969, Meyer Tool has over 50 years of experience manufacturing custom and OEM cryogenic equipment and components. Our experience spans a wide range of materials, processes, and project sizes. Whether you’re building a single prototype or need production manufacturing support for your unique cryogenic application, Meyer Tool’s engineering and manufacturing team approaches each challenge using our Reduce Project Risk Process to support your needs.
Further Reading:
Most books on cryogenics or low temperature physics devote at least one chapter to thermometry. Manufacturers' catalogs also provide detailed data on their sensors. The report Techniques for Approximating the International Temperature scale of 1990 may be found on the Bureau International des Poids et Mesures (BIPM) website. This report includes a great deal of background information dealing with thermometry, particularly at low temperatures.
A variety of other devices have been used as thermometers. Capacitance sensors rely on the change in dielectric constant of the insulator with temperature. The change in resonant frequency of a crystal with temperature has also been used as a thermometer. The melting curve thermometer, analogous to the vapor pressure bulb, follows a substance along the liquid/solid coexistence line. At the very lowest temperatures, below 0.04K, temperature measurements are often based on changes in magnetic susceptibility with temperature.
Selecting the best temperature sensor for a given application will depend on a number of factors. A balance between accuracy and cost must be found. In some cases, a single accurate sensor to measure the temperature of interest may be combined with less expensive and less accurate sensors for process control and monitoring. Environmental factors such as a magnetic field may greatly limit the selection. Corrections for the presence of a magnetic field can be made accurately if the effect is small, but sensors that are very sensitive to magnetic fields will probably not be useful in high field regions. The need for interchangeability or standardization throughout a large installation may also be a consideration.
Meyer Tool has the experienced engineering staff that can assist you in selecting the best temperature sensor for your application. Meyer Tool’s commitment to providing our customers with the lowest total cost of ownership means our staff is available to assist you throughout your project, from concept to fabrication. This type of partnering, typical of Meyer Tool, significantly impacts the reliability in both the selection and installation processes.”
Let us apply our Reduce Project Risk Process to your application so you too can experience receipt of ‘plug-and-play’ vacuum chambers or components. Established in 1969, Meyer Tool has over 50 years of experience manufacturing custom and OEM cryogenic equipment and components. Our experience spans a wide range of materials, processes, and project sizes. Whether you’re building a single prototype or need production manufacturing support for your unique cryogenic application, Meyer Tool’s engineering and manufacturing team approaches each challenge using our Reduce Project Risk Process to support your needs.
Further Reading:
Most books on cryogenics or low temperature physics devote at least one chapter to thermometry. Manufacturers' catalogs also provide detailed data on their sensors. The report Techniques for Approximating the International Temperature scale of 1990 may be found on the Bureau International des Poids et Mesures (BIPM) website. This report includes a great deal of background information dealing with thermometry, particularly at low temperatures.
