Are Thermistors the Right Choice for Your Temperature Measurement Application? A Primer on Thermistor Technology Hampton VA., June 8, 2008 There are numerous technologies available to measure temperature and with many overlapping characteristics, choosing the ideal methodology can be challenging. Thermistors are gaining popularity - often replacing thermocouples or electromechanical thermostats. Among the benefits that they offer are, simplified circuits, elimination of system calibration requirements, and low power designs. But are they right for your application? Here s how to tell if you can benefit from using thermistors. Thermistors derive their name from the device s major characteristics they are thermally sensitive resistors. Thermistor based temperature sensors exhibit a change in their electrical resistance in response to a change in ambient temperature conditions. They are highly-sensitive and have very reproducible resistance-versus-temperature properties. Thermistors can be further classified as either Positive Temperature Co- Efficient (PTC) or Negative Temperature Co-Efficient (NTC). PTC thermistors demonstrate a rapid increase in resistance near a fixed temperature point, called the Curie temperature, which is typically in the range of 100 to 150 C. PTC thermistors are useful for self regulating heaters and simple thermal switch applications. NTC thermistors exhibit a steep drop in resistance as temperature increases. The resistance changes approximately three orders of magnitude in a 100 C range. This high sensitivity to temperature changes allows NTC thermistors to measure very small temperature variations accurately. NTC thermistors are commonly used for precision temperature measurement in environments where the cost of failure is high. For the remainder of our discussion, we ll focus on NTC thermistors. This accuracy makes them unique in their functionality in many applications compared to temperature sensors/switches such as thermocouples or electromechanical thermostats. Additional factors that come into play when evaluating temperature sensing methodologies are interchangeability, ease and accuracy of 2-wire connection,
ruggedness, hermetic seal capability, and flexibility. We ll look at each one separately. Interchangeability Through process control and precision manufacturing practices thermistor sensors can be produced with very accurate tracking of a specified Resistance vs. Temperature curve. These precision thermistors can be dropped in to a circuit to measure temperature very accurately over a wide range. They are referred to in the industry as interchangeable thermistors, and they are available with tolerances as tight as ±0.05 C not just at a single point, but over a temperature range from 0 to 70 C. This interchangeability eliminates the need for individual sensor calibrations saving time and cost. Thermocouples while producing a repeatable voltage output based on temperature, have a much wider tolerance on this output, and thus are less accurate in indicating temperature for critical thermal applications. Typical thermocouple accuracy is +/-2.2 C (Type K standard) and while the best ones can attain +/-0.5 C over a short span, they are 10 times less sensitive than thermistors. (Type T. ANSI MC96.1 special limits of error.) Thermocouples are relative temperature measuring devices, which can only sense temperature differences. To be useful their measuring circuits require an absolute temperature sensor (a thermistor!) to determine the cold junction temperature. Electromechanical switches by definition switch at a designated temperature level, so are not useful in applications where temperature indication over a range is required. Two-Wire Connection Because thermistors operate with just 2-wire connections, there is no reference junction compensation necessary as is required for thermocouples. The inherently higher resistance and higher temperature coefficient for thermistors allows longer lead length without introducing significant errors as compared to platinum RTDs which must operate in a 3- or 4-wire mode. For instance, 10 feet of 26AWG two conductor wire has a resistance of 0.82 Ohms. This resistance represents an error of 2.1 C for a 100 Ohm platinum RTD, but is insignificant (0.002 C) for a 10KOhm thermistor. Thermocouples, while two wire connections, are limited by the necessity to carefully match metal types in connectors and extension cables to avoid temperature error from spurious thermocouple effects. Ruggedness The more severe the application environment for the temperature sensor, the more rugged the device must be. Monitoring of office or manufacturing plant ambient temperature is far less demanding than monitoring of temperature on space station batteries the latter being an NTC thermistor application. NASA and ESA (European Space Agency) qualifications programs include numerous tests of ruggedness which NTC thermistors are able to pass.
Durability is also a deciding factor, which is one reason that NASA has specified thermistors qualified for extended space flight. Thermocouples could be used from a ruggedness standpoint, but their relative inaccuracy compared to thermistors makes them less attractive. Switches are only useful when a temperature set-point is involved, and depending upon their construction may not be suitable for use in space because of mechanical constraints. Hermetic Seal Glass-encapsulated thermistors achieve a hermetic seal between the environment and the thermistor element. This permits measurement in severe moisture environments without concern for silver migration and provides stability over a wide range of operating temperatures. Size Size parameters are another concern. Thermistors can be miniaturized with construction as small as 0.3mm OD allowing them to be using in very tight quarters. This is critical in aerospace applications where small footprint and light weight are vital. Thermocouples are available in very small sizes, down below 0.2mm but if high accuracy is required, thermistors are a better choice. Electro mechanical thermostats because of their mechanical nature are more difficult to miniaturize. Flexibility Thermistors come in a great variety of resistance, slope characteristics, lead configurations and encapsulation materials which allow them to be tailored to many temperature sensing applications within industrial, commercial, medical, military and aerospace applications. Thermocouples and switches cannot compete with the high level of accuracy of thermistors. Within the service range of thermistors there is no better solution when comparing the accuracy and cost. This flexibility and a wide variety of configurations and protective coatings allow NTC thermistors to be successfully used in a wide variety of applications including those within Medical, Aerospace, Industrial, HVAC & R, and instrumentation. Medical applications: Patient monitoring and diagnostic and treatment are the two primary medical applications for NTC thermistors. Probes can be configured as disposable devices or as reusable ones the latter being able to withstand steam sterilization by autoclave. Patient monitoring applications typically feature operating ranges from 0 to 50 C with accuracy of ± 0.1 C. FDA, CE and MDD approvals are required and attainable for use with core temperature monitoring, multi-parameter monitoring, incubators, infant warmers, and respirators.
Diagnostic and treatment applications typically have an operating range of 0 to 100 C with an accuracy of ± 0.1 C. Specific applications include catheters, ablation thermodilution, urology, oncology, thermography, and blood oxygenation. Aerospace Applications ESA and NASA approved NTC thermistors are available for operating temperature ranges of -55 C to 115 C with an accuracy of ± 0.1 C available. Since footprint is critical, probes are less than 2.4 mm in diameter. Uses include monitoring of the temperature of space station batteries, precise monitoring of the temperature of the joints in the robot arms of the space station, as well as monitoring of solar panels and instrumentation control in aerospace applications. HVAC & R Applications Standard glass coated or miniature glass coated thermistors are commonly used for HVAC & R applications. Standard glass coated thermistors are used for operating temperature from of -55 C to 300 C with accuracies of 5%, 10% and 20% interchangeable or 1% to 5% point matched. Uses include thermistor components are used in standard thermostat applications; specialized single use probes are used for balancing air flow in HVAC ducts in modern efficient office buildings; overmolded parts are used in grocery stores and food vending machines; and stainless steel probes are used for monitoring food and drug during refrigerated transport just to name a few. Miniature glass coated thermistors attain comparable accuracy over a range of - 80 C to 325 C. Common uses include sensor for engine temperature control, hot water boiling systems, air conditioning systems, and refrigeration control. The low mass, fast time response, high stability, and high temperature capabilities of the miniature glass parts make them ideal for measuring gas flow and fluid properties. One customer is measuring gas flow in residential gas meters, another is measuring mix ratios in soft drinks, another is measuring specific gravity, or the fat content of milk. Thermistors available are also available as surface mount devices for general temperature sensing and compensation network applications. Thermistor Chips are suitable for laser diode compensation, IR measurement compensation, battery packs and temperature sensing at board level. Determining whether a specific application will benefit from the use of a thermistor depends upon the unique parameters of the project. A better understanding of the attributes of the various temperature sensing technologies is a good starting point to help you determine which sensor type is best suited for your particular requirements. -xxx-
About Measurement Specialties Measurement Specialties, Inc. designs and manufactures sensors and sensorbased systems. The Company produces a wide variety of sensors and transducers to measure precise ranges of physical characteristics including pressure, temperature, position, force, vibration, humidity and photo optics. Measurement Specialties uses multiple advanced technologies including piezoresistive, electro-optic, electro-magnetic, capacitive, application specific integrated circuits (ASICs), micro-electromechanical systems (MEMS), piezoelectric polymers and strain gages to engineer sensors that operate precisely and cost effectively. www.meas-spec.com or www.betatherm.com Product Information: If you require any further information regarding our thermistors, please contact sales@betatherm.com Marketing/Communication Information Cathy Tetrick MEAS Temperature Products Group Marketing (937) 427-1231 ext 388 catherine.tetrick@meas-spec.com