NTC Thermistor Basics
Q: How fast do NTCs respond?
A: Response time is defined as the time it takes to reach 62% or a new temperature, and is a function of mass. The smaller the sensor the faster the response. A discrete sensor will respond faster than the same sensor packaged inside a metal housing. Typical response times for a Series I NTC thermistor sensor is <15 seconds.
Q: Are NTCs available in a smaller size?
A: Typical size for an epoxy coated discrete in an OD of 0.95" Max. Miniature glass sensors have a max. OD of 0.15".
Q: How stable are NTC sensors?
A: Different sensor families have different stability ratings. Epoxy coated NTCs are less stable than a hermetically sealed glass NTC sensors.
Q: How do you select a resistance value for an application?
A: The rule of thumb is to use a low resistance sensor for a low temperature application and a high resistance sensor in a high temperature application. The goal is to have a working resistance value in your temperature range of interest.
Q: Can NTCs be used in cryogenic applications?
A: Yes, but accuracy at -200°C would be based on mathematical modeling.
Q: What is the price range for NTCs?
A: Pricing is based on cost, which is related to yield. The tighter the accuracy the lower the yield.
Q: What is the difference between a thermistor and an RTD?
A: TE manufactures 5 distinctly different technologies within our temperature products. Each technology has its advantages and disadvantages and which one is best for a specific application will depend upon a number of factors including temperature range, accuracy required, time response, cost and many other factors. Its best to understand as much as possible about the application in order to determine which product or technology is best suited for the customer.
View the infographic defining the performance and application differences across TE’s various temperature sensor technologies.
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Q: Can you show the math behind conversion from % tolerance to actual temperature tolerance?
A: To determine the temperature accuracy simply divide the total deviation (resistance tolerance) by the Alpha for the temperature of interest.
Example: A sensor that has a 2% resistance at 0°C following TE curve #3 which has a 0°C Alpha of 5.2%/°C: 2/5.2= ± 0.38°C
Q: Does the thermistor accuracy specifications include any long term resistance change over time (resistance stability)?
A: No, the accuracy is specified is the accuracy of the sensor shipped. We have no control of the application or the environmental conditions the sensor will be subjected to in the field.
Q: What does '%' mean when talking about temperature accuracies?
A: Sensors accuracy can be specified as resistance tolerance (see question 9) or to a temperature accuracy at either a single point or over a span. Example: ± 0.2°C from 0°C to 70°C
Q: Can you explain the sensitivity resolution in more detail? Why a higher value is better?
A: A high sensitivity eliminates any lead wire resistance. It also simplifies the ancillary electronic circuit. A 1°C change for a 10,000 ohm thermistor is 4.4%, or 440 ohms. A 1°C change for a 100 ohm platinum sensor would be 1/3 of an ohm.
Q: What does the Y-Axis division in the graph on stability represent?
A: The Y axis was intentionally done without actual numbers on the scale. The rate of aging will vary by formulation and form factor.
Q: Can you provide additional details on your calibration method? For high accuracy medical applications, what equipment/technique is used for calibration? What is considered best practice?
A: See application note NTC Thermistor Testing Considerations for details.
Q: Do you have any recommandations for electronics circuits for optimal accuracy and speed? (opamps, ADCs, etc.)
A: The main concern when designing a measuring circuit for accuracy is limiting the current through the part. NTC resistance specifications are referred to as zero power resistance values. It is not possible to have a truly zero power circuit, but current should be low enough as to not cause significant self heating of the sensor element. The dissipation constant can be used to estimate the amount of self heat error for a given power input.
Q: If using a voltage divider conditioning for a 10K or 20K NTC, is there any special considerations for 20ft to 60ft length of wire for electrical noise?
A: You can employ either a cable shield or a ferrite filter on the long wires to mitigate noise effects. Averaging is another method.
Q: What advice can you give for glueing a thermistor to a metallic surface have?
A: Adhesives are used in many applications to attach a thermistor for surface temperature measurement. A thermally conductive adhesive, usually epoxy, will give optimal results.
Q: Is there a standard NTC for lithium battery?
A: There is no standard for battery packs. The selection usually depends on available space, maximum temperature, and assembly method. I have seen insulated lead epoxy coated discrete thermistors, SMD thermistors, and DO35 glass axial thermistors all used for this application.
Q: Do you have any white paper or technical papers on resistive welding your thermistor leads?
A: Not at this time. The lead wires alloys used are Alloy 180 (Cu:Ni), Copper, Nickel, or Dumet (Fe:Ni). Welding methods will vary by alloy type.
Q: What type of NTC thermistor would you recommend for a medical thermometer application?
A: The 400 series is a legacy industry standard from the days of analog. This part is 1355 Ohms at 37°C with Beta 25/85 of 3976. The medical thermometer standards usually call out accuracy of +/-0.1 from 32 to 42°C and +/-0.2 from 25-50°C or 0-50°C for the measurement system, with 1/2 of that tolerance allocated to the thermistor and 1/2 to the measurement circuit.