Different Types of Temperature Sensors

Temperature is most widely measured physical variable in the environment. An ambient temperature condition is a vital aspect in pharmaceutical industry because it may directly or indirectly influence the physical, chemical or biological properties of a formulation. Therefore device called temperature sensors are installed that provide the measurement of temperature through electrical signals. Different types of temperature sensors are available which are described as follows:

Based on Physical Nature:

• CONTACT TYPE:  In this type the temperature sensor is directly connected to the device being sensed and fluctuations are monitored through conduction. These are generally used when:

1. The thermal contact with object is to an acceptable extent.
2. If the temperature range is between 40°C-1700°C.

• NON-CONTACT TYPE: They work on convection and radiations senses emitted from the devices. For example a device that emits infra red radiations the radiant energy is converted into electrical signals into readable form of temperature measurement. They are used when:

1. Object is moving.
2. Contact to the object is difficult.
3. Contact may damage the object because of it’s too much heat.
4. Area desired to be measured is very big.

• THERMOSTAT:  It is an electro- mechanical temperature sensor that is usually built up by using two thermally different metallic strips which are bolted together. This bimetallic strip works as a bridge, normally when it is cold the current passes down the electric circuit but once one of strips gets heated it expands and break the bridge leading to cut of circuit and hence preventing the overheating. Once the temperature again retain back to normal the metal strip regains its position back and completes the circuit again. These bimetallic strip thermostats are of two types: Snap Action and Creep Action. The simple on and off switches are snap action that provides instant electrical contacts at set temperatures whereas creep action works by gradually changing the position in relation to temperature changes. The unwinding capacity is slow in creep action type but they are more sensitive to the temperature changes.

• RTD (Resistive Temperature Detectors): They are electrical resistance temperature sensors in which resistance created is directly proportional to the temperature but the sensitivity is poor as it produces a change of just 1Ω/^oC. The make is of high purity metals such as platinum or could be of low cost copper and nickel wounded into a coil or thin films. They have positive temperature coefficients but are widely linear output over the temperature curves which are quite accurate. For example: the platinum RTD’s provide fair linearity to a wide range of (0.1 to 1 °C) across -200 to 600 °C.  As they are resistive devices therefore the need is to pass the current through them and obtain the resulting voltage. However due to external current providence generates heat that may result in errors in measurement of temperature. This error is calculated by formula: delta T= P*S (Where T= Temperature, P= I squared power produced, S= degree C/ mill watt.

• THERMOCOUPLE: They are most widely used temperature sensors. They offer certain advantages that make them popular choice among various temperature sensors. They are:

1. Simplicity and ease of use.
2. Have small sizes that result in speedy response in temperature changes.
3. Extremely rugged and low cost
4. Have the wide range from-200^oC to 2000^oC or over.

Thermocouple consists of two junction’s one cold and another hot of different metals such as copper and constantan. As the temperature gradient is developed it produces electromotive force resulting in voltage difference between two junctions which is called as seebeck effect.

Same temperature at both junctions= zero potential difference (V1=V2)

When the junction is connected in the circuit,

Temperature difference = difference in voltage relative to temperature difference (V1-V2)

As thermocouples have a wide range of materials that are allow extreme temperatures the choice to select the metals is an important aspect. The British has developed colour coding system to select the appropriate thermocouple for the specific applications.

• THERMISTOR: It is generally a thermally sensitive resistor whose resistance changes with the temperature. They are made up of ceramic materials (oxides of nickel, manganese, cobalt) which are packed airtight into balls or small pressed discs. Thermistors have higher sensitivity than the RTD’s and thus provide highly non linear resistance vs temperature curves. They usually have Negative Temperature Coefficient (NTC) which means the resistance decreases with the increase in the temperature.  They also require passage of current first in order to produce measurable voltage therefore called as passive resistive devices.

• IR SENSORS: These are non- contact electronic instrument which senses characteristics in the surrounding by IR radiations (emitting or detecting). They are classified as :

1. Thermal infrared sensors
2. Quantum infrared sensors.

They are expensive devices and the cost depends upon the type of the work it is intended for. They are            built with a semiconductor chip inside the sensor powered to 3-5 V.

Picture Courtesy:

Thermostat: www.explainthatstuff.com

Thermocouple: www.electronics-tutorials.ws