The NTC Thermistor is an alternative to the Platinum resistance thermometer; the name derives from “thermal resistor” and defines a metallic oxide which displays a high negative temperature coefficient of resistance. This compares with the small positive coefficient of say Platinum used for the Pt100 sensor. The temperature-resistance characteristic of the thermistor is up to 100 times greater than that of the alternative resistance thermometer and provides high sensitivity over a limited temperature range.
PTC (Positive Temperature Coefficient) versions are also available but their use is much less common than the popular NTC types.
High resistance thermistors, greater than 100kOhms are used for high temperatures (150 to 300°C); devices up to 100kOhms are used for the range 75 to 150°C. Devices below 1kOhm are suitable for lower temperatures, -75 to +75°C.
Thermistors provide a low cost alternative to the Pt100 although the temperature range is limited; interchangeability and accuracy place them between Pt100 and thermocouple alternatives. Since their resistance value is relatively high, a simple 2 wire connection is used.
RESISTANCE / TEMPERATURE CHARACTERISTIC
The electrical resistance of a NTC (Negative Temperature Coefficient) Thermistor, decreases non-linearly with increasing temperature.
The amount of change per degree Celcius (C) is defined by either the BETA VALUE (material constant), or the ALPHA COEFFICIENT ( resistance temperature coefficient).
The Beta Value is defined by:
where T1 and T2 are two specified temperatures, usually 273.15K (0°C) and 323.15K (50°C), and R1 over R2 is the ratio of the measured resistance at the two specified temperatures. Beta is expressed in degrees Kelvin.
The Alpha Coefficient is defined by:
where T is specified temperature in degrees K, R is resistance at specified temperature T. Alpha value is usually expressed in % per °C. There is a direct relationship between the Alpha Coefficient and the Beta Value.
The larger the Alpha or Beta Value, the greater the change in resistance per °C, (the greater the sensitivity). Within the thermistor industry, a thermistor material system is usually identified by specifying the Alpha coefficient, Beta Value, or the ratio between the resistance at two specified temperatures (typically, RO/R50, R25/R125, RO/R25, R70/R25, or RO/R70).
Electrical Resistivity (Ohm-cm) is one electrical characteristic of different materials. It is equal to the resistance to current flow of a centimetre cube of a particular material, when the current is applied to two parallel faces. It is defined by the following equation:
where R is resistance, l is length of a uniform conductor, A is cross-sectional area, and p is resistivity .
When comparing different thermistor materials, the material with the larger Alpha or Beta value will generally have the larger resistivity.
Material resistivity is an important consideration when choosing the proper thermistor for an application. The material must be chosen such that a thermistor chip of a specified resistance value will not be too large or too small for a particular application. Thermistor materials are available with a variety of resistivity values. The resistance of an NTC thermistor is determined by material resistivity and physical dimensions. Required resistance value is usually specified at 25°C.
At low measuring current levels, the power dissipated by a thermistor is small and is of little consequence to measurement accuracy. Increased current results in increased dissipation causing the sensor to heat up; an increased temperature is indicated resulting in measurement errors.
Probe construction and connection to instruments are as for resistance thermometers but only a 2 wire arrangement is used (lead resistances will be very small compared with sensor resistance).
INFRARED TEMPERATURE MEASUREMENT
Principles of Infrared Sensing
Energy is radiated by all objects having a temperature greater than absolute zero (-273°C). The energy level increases as the temperature of the object rises.
Therefore by measuring the level of the energy radiated by any object, the temperature of that object can be obtained. For this purpose, energy in the infrared band is used (wavelengths of between 0.5 micron and 20 micron are observed in practice). Emissivity has to be taken in to account when evaluating the temperature using infra-red radiation (described below).
Methods of Measurement
The two most common methods of sensing and measuring temperature on a non-contact, infrared basis are:
a) Optical pyrometry
b) Non-contact thermocouple
Optical pyrometry uses comparison techniques to measure temperature ; non-contact thermocouple techniques provide an accurate, convenient and relatively inexpensive alternative.
Infrared thermocouples are passive devices which provide a “true” thermocouple output signal appropriate to the type specified (usually type J or type K). Such sensors can therefore be directly connected to the thermocouple input of an instrument but, unlike the standard thermocouple provide convenient, non-intrusive, remote temperature sensing. This approach is usually inexpensive, especially when compared with optical systems. The compact dimensions of these devices makes them as convenient as a thermocouple to install in industrial processes or to use in experiments; hand held sensors are also available.
The detection method used by many infrared thermocouples is similar in principle to that of optical systems, the thermopile. A thermopile consists of an array of thermocouple junctions arranged in a high density series matrix; heat energy radiated from the object results in an “amplified” output from the sensor (i.e. a multi-thermojunction signal as opposed to that of a single junction).
The output is scaled to correspond to that of the specified thermocouple type (e.g. approx. 40µV/°C for type K over a limited and reasonably linear range).
Since the sensor receives only infrared radiation energy, the rules of thermal radiation apply and such things as non-linearity and emissivity must be considered.
Over a restricted temperature range, the sensor output is sufficiently linear to produce a signal which emulates that of the thermocouple with reasonable accuracy; an accuracy of around 2% can be achieved for a type K non-contact sensor over the range 50°C to 650°C for example.
Emissivity is a parameter which defines how much radiation an object emits at a given temperature compared with that of a black body at the same temperature. A black body has an emissivity of 1.0; there is no surface reflection and 100% surface emission.
The emissivity of a surface is the percentage of the surface which emits; the remaining percentage of the surface reflects. The percentage though, is expressed as a coefficient hence 100% equivalent to 1.0. All values of emissivity fall between 0.0 and 1.0.
For accurate measurement of different materials, ideally, the emissivity should be taken into account and correction applied. Simple instruments may not allow for this but more sophisticated alternatives incorporate emissivity adjustment.
Other considerations include sensor to object distance / target area considerations and the possible need for sensor cooling in high temperature applications.