Thermistors and the various world applications. Comtest (Pty) Ltd. By: Gavin van Rooy

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1 Thermistors and the various world applications Comtest (Pty) Ltd By: Gavin van Rooy

2 Table of contents Abstract. History of the Thermistor. What is a thermistor made of? Different Types of thermistors - NTC or PTC? What industries are covered by thermistors Comparison between the different types of Measuring devices Advantages and disadvantages Conclusion

3 Abstract There is always a lot of talk about thermocouples and Platinum resistance thermometers, especially in the calibration lab. Not a lot is however known about thermistors. They are extremely widely used and are often overlooked for laboratory work.

4 History of The Thermistor Michael Faraday an English scientist, first discovered the concept of thermistors in 1833 while doing research on Silver sulfide semiconductor behaviour. What he came to find was that the resistance of Silver sulfide resistance decreased as the temperature increased. Samuel Ruben would later invent the first commercial thermistor in Technology has improved over the years with improved manufacturing paving the road to improved manufacturing processes; along with the availability of higher quality material.

5 What is a thermistor made of Many NTC thermistors are made from a pressed disc, rod, plate, bead or cast chip of semiconducting material such as sintered metal oxides. They work because raising the temperature of a semiconductor increases the number of active charge carriers - it promotes them into the conduction band.

6 What types of thermistors-ntc/ptc NTC thermistor definition NTC stands for Negative Temperature Coefficient. An NTC thermistor is a thermally sensitive resistor whose resistance exhibits a large, precise and predictable decrease as the core temperature of the resistor increases over the operating temperature range. PTC thermistors definition PTC stands for Positive Temperature Coefficient. PTC thermistors are resistors with a positive temperature coefficient, which means that the resistance increases with increasing temperature. Thermistors are devices whose resistance changes with temperature variation. They are referred as temperature dependent resistors. They are available in temperature range from -100 degree C to +150 degree C. We normally deal with NTC for our applications in the Lab and industrial arenas.

7 Characteristics of NTC Thermistors Steinhart-Hart equation The best approximation known to date is the Steinhart-Hart formula, published in 1968: The Steinhart equation for a precise approximation: 1/T = A + B*(ln(R)) + C*(ln(R))^3 Where ln R is the natural logarithm of the resistance at temperature T in Kelvin, and A, B and C are coefficients derived from experimental measurements. These coefficients are usually published by thermistor vendors as part of the datasheet. The Steinhart-Hart formula is typically accurate to around ±0.15 C over the range of -50 C to +150 C, which is plenty for most applications. If superior accuracy is required, the temperature range must be reduced and accuracy of better than ±0.01 C over the range of 0 C to +100 C is achievable

8 + Applications- Motor Industry Automatic climate control Coolant sensors Electric coolant fan temp. control Emission controls Engine block temperature sensors Engine oil temperature sensors Intake air temperature sensors Oil level sensors Outside air temperature sensor Transmission oil temp. sensors Water level sensors

9 + Applications- Aerospace and Military Aircraft temperature Bathythermograph Fire control equipment Satellites Missiles and spacecraft temperature Physiological monitoring

10 + Applications-Medical Blood analysis equipment Blood dialysis equipment Blood oxygenator equipment Clinical fever thermometers Esophageal tubes Infant incubators Internal body temperature monitors Internal temperature sensors Intravenous injection temp. regulators Myocardial probes Respiration rate measurement equipment Skin temperature monitors Thermodilution catheter probes

11 + Applications-Industrial Fluid flow measurement Gas flow indicators HVAC equipment Industrial process controls Liquid level indicators Solar energy equipment Thermal conductivity measurement (diamond testers etc.) Thermocouple compensation Thermoplastic moulding equipment Water purification equipment Welding equipment

12 + Applications-Consumer Home weather stations Washing machines Oven temperature control Pool and spa controls Rechargeable battery packs Refrigerator and freezer temp. control Small appliance controls Solar collector controls Thermostats Toasters Air conditioners Audio amplifiers Cellular telephones Clothes dryers Computer power supplies Dishwashers Electric blanket controls Electric water heaters Electronic thermometers Fire detectors

13 Advantages and Disadvantages of Thermistors Advantages: They are inexpensive. They are available in small packages. They provide high accuracy They have a fast response time. They need two wire ohms measurement. Disadvantages: They achieve higher precision but in limited temperature range. They are nonlinear and their nonlinearity can be addressed using software or hardware circuits. Thermistors are resistive devices and hence requires excitation current to read the voltage across their terminals. This results into effect known as self heating. In order to limit the self heating error, care must be taken to limit the sensing current to lower value. Some are fragile.

14 Thermistor specs Thermistor Specifications Model Diameter x Length x 229 mm (0.25 x 9 in) Range Drift C /Year 0 C 60 C ± C ± C Accuracy (Mfr.) 0 60 C C Wires Nominal Resistance at 25 C n/a kω x 114 mm (0.125 x 4.5 in) x 229 mm (0.125 x 9 in) 0 C 60 C ± C ± C n/a 4 5 kω 0 C 60 C ± C ± C n/a 4 4 kω x 114 mm (0.125 x 4.5 in) x 229 mm (0.125 x 9 in) 0 C 100 C ± C ± C 0 C 100 C ± C ± C ± C kω ± C kω Does not include long-term drift, resistance traceability adds additional ± %.

15 Advantages and Disadvantages of Thermocouples Advantages: They support wide temperature range from -200 degrees C to degrees C depending upon metal wires used in the construction. They can be used in hazardous environments as they are rugged devices and also are immune to shock and vibration. They do not require any external power. They are simple. They are available in wide variety of physical forms. Disadvantages: They are least stable. The measurement is only as accurate as reference junction temperature measurement, and is approx. within 1 degrees C to 2 degrees C. As it is made of dissimilar metals, corrosion results in certain environments and hence this results into deterioration in the accuracy of measurement. Due to this thermocouples need care, maintenance and protection Thermocouple measurement at microvolt level will be a problem as noise due to stray electric and magnetic fields will create problem.

16 Advantages and Disadvantages of RTD s Advantages: It has reasonable linearity. Higher accuracy Lower drift. They are most stable among all the temperature sensor devices. Suitable for precision applications. They are more linear compare to thermocouples. Disadvantages: It requires signal conditioning. Cost of RTDs are higher. They are slow in operation. Need current source for operation. The effects known as self heating and lead wire resistance can cause errors in the measurements carried out by RTDs.

17 Conclusion Compared to RTDs and Thermocouples, Thermistors have a smaller size, faster response, greater resistance to shock and vibration at a lower cost. They are slightly less precise than RTDs in certain application however NTC thermistors provide greater sensitivity, stability and accuracy than thermocouples at lower temperatures and are used with less additional circuitry and therefore at a lower total cost. Note: No other sensors can match the accuracy and price combination of certain high-accuracy thermistor probes. Not even a SPRT in the range of Degrees C. Thank you

18 References Wikipedia Fluke US Sensor.com Google RF-wireless.com Resistorguide.com