Why Bi-Metallic Strip Can't Produce Millivolt As TC

Try measuring it. Maybe it does....

The Seebeck effect says that a bimetallic strip will produce a millivolt EMF between the different materials but only over a thermal gradient.

If the bimetallic section is the 'hot end' then there would have to be a 'cold end' to form a gradient and the mV EMF would be developed across the temperature gradient between the hot and cold ends.

If the bimetallic is hotter or colder than the temperature of a voltmeter, the voltmeter would become the 'cold end' or 'hot end' respectively and you'd get a non-cold junction compensated voltage measurement of the bimetallic element's thermoelectric effect.

Normally TC measuring junction are connected to process at one end .from there compensating cable runs and connects to thermocouple module in DCS ,there we are doing cold jn compensation by thermistor .my doubt is, here where we are forming cold junction .

http://instrumentation-central.com/pages/thermocouple_reference_table.htm

read the three laws of thermocouples, it will help you understand them better.

Agree with Snel, Bi Metallic strips are fuzed together the length of the strip and are designed to perform a mechanical function in relation to variations in temperature.

ya i agree that . but my doubt is we can make bimettalic strip to sense temp like thermocouple if we do slight modification in its structure...

This is already being done efficiently in a thermocouple designed to measure a temperature! A thermocoupe is a bi-metallic device.

Your idea is not new or original. The two intentions are different and each device is built for the purpose intended but efficiently.

One very important fact you are leaving out, how is the bi metallic strip joined.

If they are no welded or heat fused, then the juncture will not generate any signal as such.

As Iris so correctly pointed out, if you have a hot joint, meaning the fused end, you will also need a cold joint, where the strip is connected to the cables, else you will not be able to sense any difference as such.

The cold welded strips of a bimetal strip produce the exactly correct voltage on the hot end, which gets shortcircuited in the rest of the strip. The shortcircuited "maybe cold" end then produces exactly 0V and 0mA output. As it is Short Circuited.

On the other hand, bimetall strips make use of the different thermal expansion factor of the metals making it up.

never thought of it before but you must be right: bimetal strips must be cold welded or else they would be born in a warped form like a samurai sword!

It is not practical to use a bimetallic strip as a temperature sensor.

A bimetallic strip will produce a thermoelectric EMF (with respect to a cold junction), but that does not mean that it is an EMF from which you can (readily) interpret a temperature.

Commercial thermocouples produce a useable EMF because the materials meet published standards, which also provide the standardized EMF vs temp tables necessary to interpret the EMF as a temperature.

It is highly unlikely that a bimetallic strip is made from commercial grade thermocouple materials, because its intended use is not as a signal sensor, but as a motion source to activate a switch or linkage.

Nor is there commercially available compensating wire to run the signal from the element to an analog input (DCS/PLC/controller/indicator). Even if there were, the bimetallic strip has no convenient means of terminating connecting wires.

In the absence of commercial thermocouple grade material you have no practical means of converting EMF to temperature. You'd have to research the change in EMF over a change in temperature just to build one-time use reference tables.

Because a thermocouple produces an electrical signal, whereas a bimetallic strip produces a mechanical one.

YeP!, Simply put and to the point! HMMM

This is an excellent question, because reason would indicate that any two dissimilar metals joined together should generate a Seebeck voltage.

The problem with trying this on a bimetallic strip is that the strip is so thin. Each "wire" (one ferrous, one brass) is only as "long" as it is thick. Thus, a bi-metal strip is actually a very wide but extremely short thermocouple.

When you attempt to connect a voltmeter to the bimetallic junction to measure the Seebeck voltage, you create "reference junctions" where the meter probes contact the ferrous the brass halves of the strip. Since these additional junctions are so close to the ferrous/brass junction of the strip itself, all the junctions will be at the same temperature; therefore the net voltage in the measuring loop will be zero no matter what temperature the system is at.

Remember: the tricky thing about thermocouple circuits is that they always contain multiple dissimilar-metal junctions, and it is the temperature gradient between those junctions that produces the Seebeck voltage. No temperature difference = no temperature gradient = no Seebeck voltage.

Theoratically TC works on Temperature gradient , pratically if i connect TC measuring jn to one point and voltmeter in other end , then also it will give emf based on masuring jn temp

If the bimetallic strip is the TC junction, then connecting a voltmeter to the strip will form reference junctions at the point of contact between the bi-metal strip and the voltmeter's leads. Since those reference junctions will be at (very nearly) the same temperature as the strip itself, there will be no temperature gradient between the TC junction and the reference junctions, and so there will be no net EMF.

I sincerely hope you have more important things to do than to MacGyver a Bi Metallic strip (Thermometer) to make a Thermocouple. I have the best suggestion here, "Buy a thermocouple" It works fine and lasts a long time! http://www.automationdirect.com/adc/Overview/Catalog/Process_Control_-a-_Measurement/Temperature_Sensors_-a-_Transmitters/Thermocouple_Sensors