Introduction to the most commonly used temperature sensor

Companies and households rely on heat sensors to function properly. It is employed in the manufacturing industry and laboratories across the nation. Even household appliances and auto parts use these sensor types to remain operative and accurate. Most objects that rely on heat are probably using this device to manage heat convection, regulate temperature, and remain safe.It can measure in almost all type of environment.Ex-Hot,cold,harsh,smooth,high pressure,low pressure etc.There are many types but based on use they come for industrial and home use.Otherwise there are standard grade available.

At home they are used in devices like ovens,heaters,refrigerators etc.In industries they come in the form of probes to insert in the temperature measurement region.They are used in power plants,Metal ore smelting,glass industries,nuclear furnace and any other machines or region where monitoring the temperature is necessary for safety or controlling purposes.

Intro

Working

Laws

Colour chart

Temperature table

Temperature to emf 

 

Basic questions on tcs

What are tcs?

They are the most common type of temperature sensors using plain metallic wires.Their wires are chosen so as to produce a large electromotive force that varies linearly with temperature. There is no known metal or alloy that has all of these desirable features, although some are very close to it. Since there are no product with

common industrial probes

this perfect behaviour, all electromotive force curves deviate from a straight line or linear response to some degree. In industrial applications, the choice of materials used to manufacture a range is dependent on the temperature to be measured, the type of atmosphere exposed to the material and the required accuracy of measurement.

 

Internal structure of tc

 

Why they are called so?

Tcs are formed by coupling together a pair of metal and works  on the application of thermal energy.Hence the name.

How were they discovered and what is their principle of operation .

There discovery date back to  1822-23 when Thomas seebeck  observed a deflection in a compass needle kept near a closed circuit of two metallic wires whose two junction are at different temperature.He also discovered that the Potential  difference generated was almost directly proportional to the difference in the two temperatures for particular range depending on the type of material of which it is made.But  he thought  it to be magnetic nature.Anyhow The effect was termed seebeck effect.Later in same century Leopoldo Nobili and Macedonio Melloni continued  his worked and made a device to measure heat radiation.It was simple group of metal junction in series like a thermopile combined with galvanometer..It may be the first tc made for any measuring purpose.

Working principle

Thermocouple working principle is based on the seebeck Effect which was discovered by T.J seebeck in 1831.He discovered that when two metals of different wires are joint together and if temperature of the junctions are different then a current flows through the wires.He also discovered that the Potential difference generated was almost directly proportional to the difference in the two temperatures.

For small changes in temperature within the operating region the Seebeck voltage or the  EMF (electromotive force) is directly proportional to temperature.Voltage temperature relationship can be approximated as

Delta T= a1(V1)+a2(V2)^2+a3(V3)^3+…..up to N terms.

However a1 is quite large as compared to other coefficient so the relationship is approximately linear for quite long range of temperature.Lot of maths here huh!

 

MC6000Setiz1

 

 

 

 

 

 

 

 

 

However Measuring the Seebeck voltage directly is not possible because we must first connect a voltmeter to the lead wires ,and the voltmeter leads themselves create a new thermoelectric circuit at the contact point of voltmeter and tc leads.

Thermocouple-300x206

 

 

 

 

 

 

 

Determining the Temperature of by measuring the voltage output.

Reference Junction

How to nullify the effect of extra junction produce

Measuring voltage is an indirect measure of temperature .But it is not possible directly.Consider the diagram above as an example.To measure the temperature we have to connect two wires of say constantan to copper (Red) and Iron to copper (Blue). And then the two ends to a voltmeter.But Since Constantan to copper and iron to copper will itself generate some voltage it forms another junction as shown above.So we cannot determine exact temperature unless we subtract that the extra voltage difference generated at constantan- copper(Red) and Iron-copper(Blue) junction.This is only possible if we know the temperature of reference.One way to employ this is to put the junction at reference temperature and thus know in advance the voltage which will be generated at two junctions and thus subtract it eventually to get voltage generated at target surface ie, Iron-Copper junction and measure temperature.

 

What causes sebeck effect ?

Here is a demo

Suppose a metal wire is heated at one end then electron which are free to move will tend to be denser at cold junction and less denser at hot

An electron gradient will be created.And so a net potential difference but since no current can flow in open loop the The potential difference is maintained by  balancing  two currents inside the metal wire  thermal gradient current  and current induced by potential difference(ohms law) caused by difference in density of electrons.Now when two wire of different metal but at identical temperatures at hot and cold junction are joined current flows.Voltages of hot and cold junction of different metals may not be same since it depend the type of metal,ie how much charge difference is there between two ends.So a current is established at two ends.

But the flow of electron at hot and cold junction are opposite since metal with higher hot junction potential will be having lower colder junction potential and vise versa.So the polarity of  voltage difference will be opposite at two ends.This is because wire at higher voltage means higher positive charge which implies more  negative  charge at opposite end than the other wire..

Now when it comes to connecting wires in tc circuits there are three rules or laws to keep in mind.They are
Law of homogeneous material
Law-of-homogeneous-material.-300x164

 

 

 

 

 

 

A current cannot flow in a single wire loop made of same material by

application of heat alone.So if there is a temperature difference between two points along the length of a homogenous wire there will not be a voltage difference.
Law of intermediate materials

Law-of-intermediate-materials-300x172

 

 

 

 

 

 

At uniform temperature the algebraic sum of the thermoelectric emfs in a circuit composed of any number of dissimilar materials is zero .So if a third metal is inserted in either of two wires and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.
>Law of successive or intermediate temperatures
Law-of-successive-or-intermediate-temperatures-300x69
Provided T1<T2<T3 then if a thermal emf1 is produced by tc made by two dissimilar homogeneous materials whose junctions are at T1 and T2 and emf2 is produced when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2.

Advantage of using 

The advantages include wide temperature ranges, inexpensive costs, variety, rugged exterior, and simple, self-powered design. The cons, however, are low voltage measurements, non-linear voltage-to-temperature conversion chart, reference requirements, low stability, and low sensitivity.

Industrial temperature measurements ranging between -200°C (-328°F) and over 1450°C (2462°F) is normally achieved with them. They are the only sensors that can be used at very low temperatures, particularly in applications where the accuracy is adequate.

Here is comparison of various sensors most common is use

Advantage and disadvantage of various temperature sensors

Most popular temperature sensors compared

General Home appliances which uses tcs

 Gas space heaters,Room heaters,Boilers

These type of appliances uses screw in,clamp or push in pilot burners.Choose the required model from the available probes but refer your manual for replacement and model type before making a purchase.

Patio heaters

These are different type of space heaters which require a specially made probes to fit in the stove.

Gas/electric oven or stove used in household cooking

They are located under the burner.You can locate it after opening the lower panels given in ovens.Most modern oven come with electronic temperature control.They are required to cook your food at required temperature So a thermostat is used to control the temperature.Their design may be different depending upon the company or model of oven.Ovens or stoves have special type of probe which is thin and long.They are connected with a thermostat which is responsible for the controlling action.Complete set of probe and thermostat are available.

Pellet stove

These type of stove uses pellet or wood pieces for lighting.Another tcs are used for its temperature control.

For pyrometer or kilns.

These types have high temperature applications.

For exhaust gas temperature monitors

Generally used to monitor the temperature the exhaust gas of automobile or internal combustion engine so that driver have an indication of air-fuel ratio which should not be higher than required. If it is so then the exhaust temperature will be higher then normal.

For refrigerators(Lagering and brewing)

Generally to replace the tc of your refrigerator you need the model number and company of the fridge.But many times you need to control the temperature of your freezer for certain application like brewer making wine making etc.For that certain general purpose probes with thermostat are available.

Surface tc probes

These types are used to measure the surface temperature of hot plates griddles,central heating radiators etc.They are also helpful in plumbing applications like measuring the temperature of pipes,radiators,boilers etc to maintain an efficient heating system.

For meat (Food processing/catering)

Your food items may require to measure its temperature for culinary purpose.Correct temperature improves its taste and prevent from damage.

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