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Basic Physical Types of Temperature Sensors
In general, there are two sensing methods:
Contact temperature sensors are in physical contact with the object or substance. They can be used to measure temperature of solids, liquids or gases.
Non-contact temperature sensors detect temperature by intercepting a portion of emitted infrared energy of the object or substance, and sensing its intensity. They can be used to measure temperature of only solids and liquids. It is not possible to use them on gases because of their transparent nature.
Types of Temperature Sensors
There are many different types of temperature sensors available that vary from simple on/off thermostatic devices which control a domestic hot water system to highly sensitive semiconductor types which can control complex process control plants. The two basic types of contact and non-contact temperature sensors are further classified into resistive, voltage, and electromechanical sensors. The three most commonly used temperature sensors include
- Resistance temperature detectors (RTDs), and
These temperature sensors differ from each other in terms of operating parameters. For moderate temperature range applications, solid state sensors are also available which provide the advantage of easy interface and built-in signal conditioning.
Thermistor is a temperature sensitive resistor that changes its physical resistance with the change in temperature. Generally, thermistors are made from ceramic material semiconductor, such as cobalt, manganese or nickel oxides coated in glass. It is formed into small pressed hermetically sealed discs that give relatively fast response to any temperature changes.
NTC Type Thermistor (Epcos)
Due to the semiconductor material properties, thermistors have a negative temperature coefficient (NTC), i.e. the resistance decreases with the increase in temperature. However, there are also thermistors available with positive temperature coefficient (PTC), their resistance increases with the increase in temperature.
NTC Thermistor graph with resistance as a function of temperature (Thermodisc)
Advantages of Thermistors
- Better speed of response to changes in temperature, accuracy and repeatability.
- Inexpensive compared to RTDs
- Higher resistance in the range of 2,000 to 10,000 ohms
- Much higher sensitivity (~200 Ω/°C) within a limited temperature range of up to 300 °C
Resistance vs. Temperature
For a value of resistance, the temperature is found by the following equation:
where A,B,C are formula constants, R is resistance in Ohms and T is temperature in Kelvin.
In the NTC thermistor datasheets, A,B and C constants are generally given and you can easily calculate the temperature from a measured resistance or vice versa. If those constants are not provided, you can use three samples from Resistance-Temperature table and calculate these values.
How to Use a Thermistor?
Thermistors are rated by their resistive value at room temperature (25o°C), time constant, and power rating. Thermistor is a passive resistive device, therefore, it requires current to produce an output voltage. Generally, they are connected in series with a suitable biasing resistor forming a potential divider network.
Consider a thermistor with a resistance value of 2.2KΩ at 25°C and 50Ω at 80°C. Thermistor is connected in series with a 1kΩ resistor across a 5V power supply.
Hence, its output voltage can be calculated as follows:
At 25°C, RNTC = 2200Ω;
At 80°C, RNTC = 50Ω;
By replacing the fixed resistor value with a potentiometer, we can obtain a voltage output at a predetermined temperature.
However, it is important to note that standard resistance values are different at room temperature for different thermistors since they are non-linear. Thermistor has an exponential change with temperature; therefore it has a Beta temperature constant (β) that is used to calculate its resistance for a given temperature. However, in a voltage divider network, the obtained current for the applied voltage is linear with temperature, thus the output voltage across the resistor and temperature are linearly related.
Resistive Temperature Detectors
Resistive Temperature Detectors (RTDs) are electrical resistance temperature sensors made of films or coils of metal, such as platinum whose electrical resistance is a function of temperature.
Resistive Temperature Detector (RTD)
RTDs have positive temperature coefficients (PTC) and unlike thermistors, they provide accurate temperature measurements since they have linear output. However, they have poor sensitivity producing a small output change, for example 1Ω/°C for a change in temperature. Pt100 is the most commonly available sensor with a standard resistance value of 100Ω at 0°C. The main disadvantage is its high cost.
Advantages of RTDs
- Wide temperature range from -200 to 650°C
- Provides a high output for a current drop
- More linear compared to thermocouples and thermistors
How to Use RTDs
RTDs are passive resistor devices like thermistors and current is passed through the sensor to obtain an output voltage that is linearly related to the temperature. However, an error can occur in the reading due to the resistance variation caused by the self heating of the current flowing through the resistive wires. To overcome this problem, RTD is connected in a resistive bridge network with additional connecting wires for lead compensation and/or addition of a constant current source.
The most commonly used temperature sensors are thermocouples because they are accurate, operate over a wide temperature range from below -200°C to over 2000°C, and are relatively inexpensive.
A thermocouple with wire and plug
Construction and Working
Thermocouple is created with two dissimilar metals that are welded together producing a small potential difference (mV) as a function of temperature. One junction is maintained at a constant temperature called the reference (cold) junction, while the other is the measuring (hot) junction. With the difference of temperature between the two junctions, a voltage is developed across the junction which is used to measure the temperature. The voltage difference between the two junctions is called the “Seebeck effect”.
Construction of a Thermocouple
If both junctions are at the same temperature, the potential different across the junctions is zero, i.e. V1 = V2. However, when junctions are at different temperatures connected in a circuit, the output voltage is relative to the temperature difference between the two junctions, i.e. V1 – V2.
Types of Thermocouples
Thermocouples are available in different temperature ranges and materials; therefore there are different types of thermocouples available for specific applications as set by international standards. Type J and K are the most commonly used thermocouples.
|Nickel Chromium / Constantan||
-200 to 900 °C
|Iron / Constantan||
0 to 750 °C
|Nickel Chromium / Nickel Aluminium||
-200 to 1250 °C
|Nicrosil / Nisil||
0 to 1250 °C
|Copper / Constantan||
-200 to 350 °C
|Copper / Copper Nickel Compensating for “S” and “R”||
0 to 1450 °C
Advantages of Thermocouple
- High temperature ranges
- Rugged and withstand shock and vibration
- Provides immediate response to temperature changes
How to Use a Thermocouple?
Thermocouple produces an output voltage in a few milli volts for a temperature change of 10°C. Therefore, it is required to amplify the output voltage.
It is required to carefully select an amplifier to obtain good drift stability for preventing recalibration of thermocouple. This makes an operational amplifier preferable for most applications.
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