Contact temperature measurement

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Temperature Measurement

Abstract

In contrast to the non-contact temperature measurement (infrared (IR) thermometer) the sensor needs to be in contact with the object to be measured. The sensor has to be in thermal equilibrium with the measured object. For this purpose it will either move towards energy gain or energy loss. The sensor should have a very small thermal mass. It needs some time to adjust to this equilibrium [Bernhard:2003]Title: Technische Temperaturmessung. Physikalische und meßtechnische Grundlagen, Sensoren und Meßverfahren, Meßfehler und Kalibrierung
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Among other measuring methods thermocouples and resistance thermometers are most suitable for the contact measurement of objects. They are applied in large numbers. Selection of a sensor are: accuracy, response, temperature range and chemical properties

Liquid thermometers

Today, Liquid thermometer are rarely used. Their functioning bases on the change in volume of the liquid inside the thermometer. When the liquid is heated, it expands up into a capillary, which is mounted onto a measurement scale. The narrower the capillary, the further apart the units and the better the accuracy of the reading. An accuracy of 1/10 th degree is easily reached with precision thermometers. The following liquids have been used as thermometer liquids. The selection of the liquid depends on the scope.

Table 1: Liquids used for temperature measurement

Liquid	Solidification temperature	Boiling pointoC at 1013 hPa
Alcohol	-114,50	+78,50
Aniline	-20,00	+184,00
Creosol	-20,00	+220,00
Pentane	-200,00	+36,00
Petroleum dyed		+150,00 to +250,00
Petrol ether????	-100,00	+40,00 to +60,00
Mercury	-38,87	+356,70
Thermal liquid	-50,00	+150,00

Spirits or mercury thermometer liquids are rarely used in households, nowadays. Mercury has a more linear expansion, but is drawn from the market dew to its toxicity and disposal problems. Due to its low solidifying temperature alcohol (ethanol) is most suitable for outdoor thermometers. Alcohol thermometers are easier to read than mercury thermometers, because the blue or red dye is more visible than the fine silvery line of mercury.

Resistance thermometer or resistance temperature detectors (RTD)

Resistance thermometers use the fact that electrical resistance of an electrical conductor varies with different temperatures. A differentiation is made between cold and hot conductors. While the cold conductors resistance increases due to rising temperature, the resistance of the hot conductor decreases.

Resistance thermometers use the property of electrical conducters. Their electrical resistance changes with temperature.

Some cold conductors are metal conductors. The metals used for this purpose are mainly platinum (Pt), nickel (Ni), iridium (Ir), non-doped silicon, or nickel-iron (NiFe). The platinum resistance thermometer is most widely used. One of the advantages is the chemical resistance of the metal, this reduces the risk of contamination from oxidants and other chemical elements.

Principle: The crystal lattice of the metals consists of positively charged atomic cores, that have lost the outer shell electrons. They move freely, in random thermal motion within the lattice interstices, in the form of electron gas. When a current is applied, the motion progresses to a preferential direction. This directional movement is disrupted be the random thermal motion (swinging lattice atoms) and it increases the higher the temperature becomes. Metals have a positive temperature coefficient, i.e. the resistance increases when temperature rises.

Platinum-temperature sensors : The resistance of platinum temperature sensors is defined by R₀ bei T₀ = 0 °C. A value of 100 (Pt 1000) is most common. More unusual are values from 200 (Pt 200), 500 (Pt 500) and 1000 (Pt 1000). The application range of the Pt 100 ranges from -200 . . . +850°C.

Thermistor sensors are made of special metal oxides. Their resistance decreases when temperature rises. If temperature is very low, there are no free charge carriers. The electrons are bonded inside the crystal lattice. The bonds are not very strong, therefore the low energy supply from warming is sufficient to release the charge carriers. The material becomes more conductive, because the amount of free charge carriers increases, and eventually displays a /-typical behavior for metals.

The term hot conductor?? or thermistor is due to the sensors properties to have good electrical conductivity at higher temperatures. NTC stands for Negativ Temperature Coefficient, which is due to the temperature/ resistance characteristics falling.

Because of the nature of the underlying processes the number of conductor electrons increases exponentially with increasing temperature, so that the curve is characterized by a sharply rising trend.

This strong non-linearity is a major shortcoming of the NTC-resistance and limits the temperature range. Their area of application is simple monitoring and displaying tasks, in areas where temperatures up to 200 °C occur and an accuracy of a few Kelvin is sufficient. For such simple applications they are, however, superior to the expensive thermocouples and metal resistance thermometers, because they are cheap and they can be supplied with simple supplementary electronics. Also very small fabrications with short response times and low thermal mass are available. The material is a sintered semiconductor, usually these are mixtures of metal oxides (Ni- Co-Mn mixed with Li) or iron oxide with a spinel structure. Application range is from -100 - +300 ºC.

Thermocouples

The basis of Thermocouples is the effect, that the bond between to different metals causes a current that increases when temperature rises [Nau:2007]Title: Elektrische Temperaturmessung mit Thermoelementen und Widerstandsthermometern
Author: Nau, Matthias
. In comparison to the resistance thermometers, they have the clear advantage of a higher upper limit temperature of up to several thousand degrees Celsius. However, their long-term stability is not as good and the measuring accuracy slightly lower (average ±0.75% measuring range).

The thermocouple is an active sensor, it operates without supplemental energy. It does not measure absolute temperature, but a temperature difference.

Basic principle: The lattice electrons are in continuous thermal motion, therefore some are able to leave the surface. In order to counter act the forces of the bond, there must be a work function. This has to be brought on by thermal energy. If different kinds of metals or alloys come into contact, the electrons from the lower work function move to the metal with the higher work function. The first metal is therefore charged positively in contrast to the second metal and a contact current emerges, also known as the Seebeck effect. If the contacts have different temperatures, a current flows that is dependent on the resistance in the circuit and the difference of contact stresses (called thermal voltage). ??????

By combining different metals or alloys, a series of sensors for a very high temperature range are made possible. For thermocouples the IEC standard 584 applies. Most common is type K.

Table 2: Different types of thermocouples

Type	Metal 1 Pluspole	Metal 2 Minuspole	Temp. coeff. average	Application
T	Cu	Cu-Ni	42.8 mV/°C	-200 . . .  +600 °C
J	Fe	Cu-Ni	51.7 mV/°C	-200 . . .  +900 °C
E	Cr-Ni	Cu-Ni	60.9 mV/°C	-200 . . . +1000 °C
K	Cr-Ni	Ni	40.5 mV/°C	-200 . . . +1300 °C
S	Pt	Pt-10%Rh	6.4 mV/°C	0  . . . +1500 °C
R	Pt	Pt-13%Rh	6.4 mV/°C	0 . . .  +1600 °C
B	Pt-6%Rh	Pt-30%Rh		0 . . .  +1800 °C
G	Tu	Tu-26%Re		0 . . .  +2800 °C
C	Tu-5%Re	Tu-26%Re	15.0 mV/°C	0 . . .  +2800 °C

Thermocouple measurement is basically a differential temperature measurement between one measuring point Tm and a reference point Tv. For measuring the absolute temperature of a specific measuring point:

1. The temperature of the reference point is kept constant. A certain voltage has to be subtracted inside electronic evaluation system. The reference point also needs a temperature control.

2. The temperature of the reference point is measured be a second temperature sensor. Then the evaluation electronic has to subtract the appropriate value.

Bimetal thermometer

The principle of the Bimetal thermometer is based on the difference in thermal expansion behavior of two metals that are soldered together. When temperature rises, the bimetal strips bend in proportion to the temperature increase. After calibration the movement of the metal must be transferred to a needle and scale for displaying the temperature measurement. If the change in length of the bimetal strip is transferred onto the recording device by means of a lever mechanism it is called 'thermograph Advantage of the bimetal thermometer over the liquid thermometer is, that it is not susceptible to shock and vibration and virtually unbreakable. If the scale is large enough, it can also be read from a distance. Bimetal thermometers are unsuitable for precision measurements.

Quartz-thermometer

Quartz thermometers are suitable for very precise measurements. Their oscillators are made to swing for example at 0°C and f=28.2 MHz, where f is a function of temperature at 1kHz. f is compared with the oscillation frequency of a reference crystal, which is practically independent of the temperature, due to the manufacturing process. The difference frequency is measured with an electronic counter. The quartz thermometer is very accurate, its measurement error does not exceed 0.04 K. Temperature differences can be determined more accurately, because the resolution is best with the longest possible the counting time. At 10 sec counting tim the resolution is 0,0001 K.

Literatur

[Bernhard:2003]	Bernhard, Frank (eds.) (2003): Technische Temperaturmessung. Physikalische und meßtechnische Grundlagen, Sensoren und Meßverfahren, Meßfehler und Kalibrierung, Springer, Berlin
[Nau:2007]	Nau, Matthias (2007): Elektrische Temperaturmessung mit Thermoelementen und Widerstandsthermometern, JUMO

an official verification of measurement accuracy of instruments.