Temperature Measuring DevicesTemperature is a measure of how hot or cold something is. Following devices used to measure temperature.

1. Thermometer2. Pyrometer3. Thermometer 4. Thermister

Thermometer

Thermal expansion thermometers

1. Liquid-in-glass thermometers 2. Bimetallic thermometers

Thermal expansion thermometer:-

A liquid-in-glass thermometer is the simplest and most commonly employed type of temperature measurement device. It is one of the oldest thermometers available in the industry. It gives fairly accurate results within the temperature range of -200 to 600°C. No special means are needed to measure temperature via these thermometers. One can read temperature readings easily with human eyes. They find their use in variety of applications such as medicine, metrology and industry. The foremost liquid-in-glass thermometer was introduced in the year 1650 in which the liquid filled in was spirit from wine. Later on, more linear thermometers were developed with the use of mercury as a liquid inside the thermometer. The principle used to measure temperature is that of the apparent thermal expansion of the liquid. It is the difference between the volumetric reversible thermal expansion of the liquid and its glass container that makes it possible to measure temperature

It mainly comprises:

1. A bulb which acts as a container for the functioning liquid where it can easily expand or contract in capacity.

2. A stem, “a glass tube containing a tiny capillary connected to the bulb and enlarged at the bottom into a bulb that is partially filled with a working liquid”. [2]

3. A temperature scale which is basically preset or imprinted on the stem for displaying temperature readings.

4. Point of reference i.e. a calibration point which is most commonly the ice point.5. A working liquid which is generally either mercury or alcohol.6. An inert gas, mainly argon or nitrogen which is filled inside the thermometer above mercury to

trim down its volatilization.

Advantages:-Following are the major advantages associated with the use of liquid-in-glass thermometers.

1. They are comparatively cheaper than other temperature measurement devices.2. They are handy and convenient to use.3. Unlike electrical thermometers, they do not necessitate power supply or batteries for charging.4. They can be frequently applied in areas where there is problem of electricity.5. They provide very good repeatability and their calibration remains unaffected.

Disadvantages:-

1. They are considered inapt for applications involving extremely high or low temperatures.2. They can not be applied in regions where highly accurate results are desirable.3. As compared to electrical thermometers, they are very weak and delicate. Therefore, they

must be handled with extra care because they are likely to break.4. Besides, they can not provide digital and automated results. Hence, their use is limited to

areas where only manual reading is adequate, for example, a household thermometer.5. “Temperature readings should be noted immediately after removal because a glass

thermometer can be affected by the environmental temperature, heat produced by the hand holding it, cleaning, etc. This temperature should be recorded because a glass thermometer does not offer a recall of the measured temperature.

Bimetallic thermometer:-

Bimetallic thermometers are made up of bimetallic strips formed by joining two different metals having different thermal expansion coefficients. Basically, bimetallic strip is a mechanical element which can sense temperature and transform it into a mechanical displacement. This mechanical action from the bimetallic strip can be used to activate a switching mechanism for getting electronic output. Also it can be attached to the pointer of a measuring instrument or a position indicator. Various techniques such as riveting, bolting, fastening can be used to bond two layers of diverse metals in a bimetallic strip. However the most commonly used method is welding. Since two metals are employed to construct a bimetallic strip, hence they are named so

Working

The working of a bimetallic strip thermometer is based upon the fact that two dissimilar metals behave in a different manner when exposed to temperature variations owing to their different thermal expansion rates. One layer of metal expands or contracts more than the other layer of metal in a bimetallic strip arrangement which results in bending or curvature change of the strip. The working

principle of a bimetallic thermometer is illustrated in figure below. “One end of a straight bimetallic strip is fixed in place. As the strip is heated, the other end tends to curve away from the side that has the greater coefficient of linear expans

Applications:-Bimetallic strips are one of the oldest techniques to measure temperature. They can be designed to work at quite high temperatures i.e. upto 500°F or 260°C. Major application areas of a bimetallic strip thermometer include:

1. For various household appliances such as ovens etc2. Thermostat switches3. Wall thermometers4. Circuit breakers for electrical heating devices

Thermocouple:- A Thermocouple is a sensor used to measure temperature. 

Working principle of thermocouple.

A thermocouple is a device made by two different wires joined at one end, called junction end or measuring end. The two wires are called thermoelements or legs of the thermocouple: the two thermoelements are distinguished as positive and negative ones. The other end of the thermocouple is called tail end or reference end (Figure1). The junction end is immersed in the enviroment whose temperature T2 has to be measured, which can be for instance the temperature of a furnace at about 500°C, while the tail end is held at a different temperature T 1, e.g. at ambient temperature.

   

Figure1:Schematic drawing of a thermocouple

 

Because of the temperature difference between junction end and tail end a voltage difference can be measured between the two thermoelements at the tail end: so the thermocouple is a temperature-voltage transducer.

Types of thermocouple

Advantages

• Temperature range: Most practical temperature ranges, from cryogenics to jet-engine exhaust, can be served using thermocouples. Depending on the metal wires used, a thermocouple is capable of measuring temperature in the range –200°C to +2500°C.

• Robust: Thermocouples are rugged devices that are immune to shock and vibration and are suitable for use in hazardous environments.

• Rapid response: Because they are small and have low thermal capacity, thermocouples respond rapidly to temperature changes, especially if the sensing junction is exposed. They can respond to rapidly changing temperatures within a few hundred milliseconds.

• No self heating: Because thermocouples require no excitation power, they are not prone to self heating and are intrinsically safe.

Disadvantages

• Complex signal conditioning: Substantial signal conditioning is necessary to convert the thermocouple voltage into a usable temperature reading. Traditionally, signal conditioning has required a large investment in design time to avoid introducing errors that degrade accuracy.

• Accuracy: In addition to the inherent inaccuracies in thermocouples due to their metallurgical properties, a thermocouple measurement is only as accurate as the reference junction temperature can be measured, traditionally within 1°C to 2°C.

• Susceptibility to corrosion: Because thermocouples consist of two dissimilar metals, in some environments corrosion over time may result in deteriorating accuracy. Hence, they may need protection; and care and maintenance are essential.

• Susceptibility to noise: When measuring microvolt-level signal changes, noise from stray electrical and magnetic fields can be a problem. Twisting the thermocouple wire pair can greatly reduce magnetic field pickup. Using a shielded cable or running wires in metal conduit and guarding can reduce electric field pickup. The measuring device should provide signal filtering, either in hardware or by software, with strong rejection of the line frequency (50 Hz/60 Hz) and its harmonics.

Applications

Pyrometer:-

Pyrometry measures the temperature of objects without touching them. It is standard procedure in many industries today. Due to its accuracy, speed, economy and specific advantages, pyrometry is steadily gaining acceptance in new fields. But how is it possible to measure temperatures without physical contact? Every object whose temperature is above absolute zero (-273.15 °C) emits radiation. This emission is heat radiation and is dependent upon temperature. The term infrared radiation is also in use because the wavelengths of the majority of this radiation lie in the electro-magnetic spectrum above the visible red light, in the infrared domain. Temperature is the determining factor of radiation and energy. Infrared radiation transports energy. This radiated energy is used to help determine the temperature of a body being measured

Similar to radio broadcasting where emitted energy from a transmitter is captured by a receiver via an antenna and then transformed into sound waves, the emitted heat radiation of an object is received by a detecting device and transformed into electric signals. Thus, the energy emitted by an object is utilized by remote (i.e. non-contact) temperature measuring devices. The instruments which determine an object's temperature in this fashion are called radiation thermometers, radiation pyrometers, or simply pyrometers.

Types of pyrometer:-

Optical pyrometer Radiation pyrometer Digital pyrometer Infrared pyrometer

Advantages of pyrometer

1. It doesn't need any physical contact between the target and the pyrometer to measure temperature.This is one of the main advantages of pyrometer

2. Pyrometers are very accurate with +/-5 degree Celsius.3. Temperature can be measured from a large distance.4. As there is no contact, pyrometer can measure the average temperature of the whole surface.5. Can measure temperature of moving objects instantly.6. Can measure very high temperature without getting damaged.7. Easy to use as no contact is needed.

Disadvantages of pyrometer:-

1. They have a non-linear scale.2. Intervening gases and vapours that absorb radiation and other such obstructions can cause

error in the measurement.3. Sensitive to changes in the emissivity of the target material.4. Target must be hot enough to radiate visibly. Thus it is only applicable for low

temperature(minimum 800 degree Celcius).5. Pyrometers are very expensive.