Temperature Sensor Types
The most commonly used type of all the sensors are those which detect Temperature or heat.
These types of temperature sensor vary from simple ON/OFF thermostatic devices which control a domestic hot water system to
highly sensitive semiconductor types that can control complex process control plants.
We remember from our school science classes that the movement of molecules and atoms produces heat
(kinetic energy) and the greater the movement, the more heat that is generated. Temperature Sensors measure
the amount of heat energy or even coldness that is generated by an object or system, allowing us to "sense" or detect any
physical change to that temperature producing either an analogue or digital output.
There are many different types of Temperature Sensor available and all have different
characteristics depending upon their actual application. Temperature sensors consist of two basic physical types:
- Contact Temperature Sensor Types - These types of temperature sensor are required to be in physical contact with the object being sensed and use conduction to monitor changes in temperature. They can be used to detect solids, liquids or gases over a wide range of temperatures.
- Non-contact Temperature Sensor Types - These types of temperature sensor use convection and radiation to monitor changes in temperature. They can be used to detect liquids and gases that emit radiant energy as heat rises and cold settles to the bottom in convection currents or detect the radiant energy being transmitted from an object in the form of infra-red radiation (the sun).
The two basic types of contact or even non-contact temperature sensors can also be sub-divided into the following three groups of sensors, Electro-mechanical, Resistive and Electronic and all three types are discussed below.
The Thermostat
The Thermostat is a contact type electro-mechanical temperature sensor or switch, that
basically consists of two different metals such as nickel, copper, tungsten or aluminium etc, that are bonded together to
form a Bi-metallic strip. The different linear expansion rates of the two dissimilar metals produces a mechanical
bending movement when the strip is subjected to heat. The bi-metallic strip is used as a switch in the thermostat and are
used extensively to control hot water heating elements in boilers, furnaces, hot water storage tanks as well as in vehicle
radiator cooling systems.
The Bi-metallic Thermostat
The thermostat consists of two thermally different metals stuck together back to back. When it is cold the contacts are closed and current passes through the thermostat. When it gets hot, one metal expands more than the other and the bonded bi-metallic strip bends up (or down) opening the contacts preventing the current from flowing.
On/Off Thermostat
There are two main types of bi-metallic strips based mainly upon their movement when subjected to
temperature changes. There are the "snap-action" types that produce an instantaneous "ON/OFF" or "OFF/ON" type action on
the electrical contacts at a set temperature point, and the slower "creep-action" types that gradually change their position
as the temperature changes.
Snap-action type thermostats are commonly used in our homes for controlling the temperature
set point of ovens, irons, immersion hot water tanks and they can also be found on walls to control the domestic heating
system.
Creeper types generally consist of a bi-metallic coil or spiral that slowly unwinds or coils-up as the
temperature changes. Generally, creeper type bi-metallic strips are more sensitive to temperature changes than the standard
snap ON/OFF types as the strip is longer and thinner making them ideal for use in temperature gauges and dials etc.
Although very cheap and are available over a wide operating range, one main disadvantage of the standard
snap-action type thermostats when used as a temperature sensor, is that they have a large hysteresis range from when the
electrical contacts open until when they close again. For example, it may be set to 20oC but may not open until
22oC or close again until 18oC. So the range of temperature swing can be quite high. Commercially
available bi-metallic thermostats for home use do have temperature adjustment screws that allow for a more precise desired
temperature set-point and hysteresis level to be pre-set.
The Thermistor
The Thermistor is another type of temperature sensor, whose name is a combination of
the words THERM-ally sensitive res-ISTOR. A thermistor is a type of
resistor which changes its physical resistance with changes in temperature.
Thermistor
Thermistors are generally made from ceramic materials such as oxides of nickel, manganese or cobalt
coated in glass which makes them easily damaged. Their main advantage over snap-action types is their speed of response
to any changes in temperature, accuracy and repeatability.
Most types of thermistor's have a Negative Temperature Coefficient of resistance or (NTC),
that is their resistance value goes DOWN with an increase in the temperature but some with a Positive Temperature Coefficient,
(PTC), their resistance value goes UP with an increase in temperature are also available.
Thermistors are constructed from a ceramic type semiconductor material using metal oxide technology such
as manganese, cobalt and nickel, etc. The semiconductor material is generally formed into small pressed discs or balls which
are hermetically sealed to give a relatively fast response to any changes in temperature.
Thermistors are rated by their resistive value at room temperature (usually at 25oC), their
time constant (the time to react to the temperature change) and their power rating with respect to the current flowing
through them. Like resistors, thermistors are available with resistance values at room temperature from 10's of MΩ down
to just a few Ohms, but for sensing purposes those types with values in the kilo-ohms are generally used.
Thermistors are passive resistive devices which means
we need to pass a current through it to produce a measurable
voltage output. Then thermistors are generally connected in series with a
suitable biasing resistor to form a potential divider network
and the choice of resistor gives a voltage output at some pre-determined
temperature point or value for example:
Example No1
The following thermistor has a resistance value of 10KΩ at 25oC and a resistance value of
100Ω at 100oC. Calculate the voltage drop across the thermistor and hence its output voltage (Vout) for both
temperatures when connected in series with a 1kΩ resistor across a 12v power supply.
 |
At 25oC |
 |
At 100oC |
 |
by changing the fixed resistor value of R2 (in our example 1kΩ) to a potentiometer or preset, a voltage output can be obtained at a predetermined temperature set point for example, 5v output at 60oC and by varying the potentiometer a particular output voltage level can be obtained over a wider temperature range.
It needs to be noted however, that thermistor's are non-linear devices and their standard resistance
values at room temperature is different between different thermistor's, which is due mainly to the semiconductor materials
they are made from. The Thermistor, have an exponential change with temperature and therefore have a Beta
temperature constant ( β ) which can be used to calculate its resistance for any
given temperature point.
However, when used with a series resistor such as in a voltage divider network or Whetstone Bridge type
arrangement, the current obtained in response to a voltage applied to the divider/bridge network is linear with temperature.
Then, the output voltage across the resistor becomes linear with temperature.
Resistive Temperature Detectors (RTD).
Another type of electrical resistance temperature sensor is the Resistance Temperature Detector or
RTD. RTD's are precision temperature sensors made from high-purity conducting metals such as platinum, copper or nickel
wound into a coil and whose electrical resistance changes as a function of temperature, similar to that of the thermistor. Also
available are thin-film RTD's. These devices have a thin film of platinum paste is deposited onto a white ceramic substrate.
RTD
Resistive temperature detectors have positive temperature coefficients (PTC) but unlike the thermistor
their output is extremely linear producing very accurate measurements of temperature. However, they have poor sensitivity,
that is a change in temperature only produces a very small output change for example, 1Ω/oC. The more common
types of RTD's are made from platinum and are called Platinum Resistance Thermometer or PRT's with the most
commonly available of them all the Pt100 sensor, which has a standard resistance value of 100Ω at 0oC.
The downside is that Platinum is expensive and one of the main disadvantages of this type of device is its cost.
Like the thermistor, RTD's are passive resistive devices and by passing a constant current through the
temperature sensor it is possible to obtain an output voltage that increases linearly with temperature. A typical RTD has
a base resistance of about 100Ω at 0oC, increasing to about 140Ω at 100oC with an operating
temperature range of between -200 to +600oC.
Because the RTD is a resistive device, we need to pass a current through them and monitor the resulting
voltage. However, any variation in resistance due to self heat of the resistive wires as the current flows through it,
I2R ,
(Ohms Law) causes an error in the readings.
To avoid this, the RTD is usually connected into a Whetstone Bridge network which has additional connecting wires for
lead-compensation and/or connection to a constant current source.
The Thermocouple
The Thermocouple is by far the most commonly used type of all the temperature sensing
devices due to its simplicity, ease of use and their speed of response to changes in temperature, due mainly to their small
size. Thermocouples also have the widest temperature range of all the temperature sensors from below -200oC to
well over 2000oC.
Thermocouples are thermoelectric sensors that
basically consists of two junctions of dissimilar metals, such as
copper and constantan that are welded or crimped together. One junction
is kept at a constant temperature called the reference (Cold)
junction, while the other the measuring (Hot) junction. When the two
junctions are at different temperatures, a voltage is developed
across the junction which is used to measure the temperature sensor as
shown below.
Thermocouple Construction
The operating principal of a thermocouple is very simple and basic. When fused together the junction of the two dissimilar metals such as copper and constantan produces a "thermo-electric" effect which gives a constant potential difference of only a few millivolts (mV) between them. The voltage difference between the two junctions is called the "Seebeck effect" as a temperature gradient is generated along the conducting wires producing an emf. Then the output voltage from a thermocouple is a function of the temperature changes.
If both the junctions are at the same temperature the potential difference across the two junctions is
zero in other words, no voltage output as V1 = V2. However, when
the junctions are connected within a circuit and are both at different temperatures a voltage output will be detected
relative to the difference in temperature between the two junctions, V1 - V2.
This difference in voltage will increase with temperature until the junctions peak voltage level is reached and this is
determined by the characteristics of the two dissimilar metals used.
Thermocouples can be made from a variety of different materials enabling extreme temperatures of
between -200oC to over +2000oC to be measured. With such a large choice of materials and temperature
range, internationally recognised standards have been developed complete with thermocouple colour codes to allow the user
to choose the correct thermocouple sensor for a particular application. The British colour code for standard thermocouples
is given below.
Thermocouple Colour Codes
| Thermocouple Sensor Colour Codes
Extension and Compensating Leads | |||
| Code Type |
Conductors (+/-) | Sensitivity | British BS 1843:1952 |
| E | Nickel Chromium / Constantan | -200 to 900oC |  |
| J | Iron / Constantan | 0 to 750oC |  |
| K | Nickel Chromium / Nickel Aluminium | -200 to 1250oC |  |
| N | Nicrosil / Nisil | 0 to 1250oC |  |
| T | Copper / Constantan | -200 to 350oC |  |
| U | Copper / Copper Nickel Compensating for "S" and "R" | 0 to 1450oC |  |
The three most common thermocouple materials used above for general temperature measurement are Iron-Constantan (Type J), Copper-Constantan (Type T), and Nickel-Chromium (Type K). The output voltage from a thermocouple is very small, only a few millivolts (mV) for a 10oC change in temperature difference and because of this small voltage output some form of amplification is generally required.
Thermocouple Amplification
The type of amplifier, either discrete or in the form of an Operational Amplifier needs to be carefully selected, because good drift stability is required to prevent recalibration of the thermocouple at frequent intervals. This makes the chopper and instrumentation type of amplifier preferable for most temperature sensing applications.
Other types of Temperature Sensor not mentioned here include, Semiconductor Junction
Sensors, Infra-red and Thermal Radiation Sensors, Medical type Thermometers, Indicators and Colour Changing Inks or Dyes.
In this tutorial about Temperature Sensor Types, we have looked at several examples of
sensors that can be used to measure changes in temperature. In the next tutorial we will look at sensors that are used
to measure light quantity, such as Photodiodes, Phototransistors, Photovoltaic Cells and the Light Dependant Resistor.
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