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Thermocouples


 


Temperature Sensor - The Thermocouple

J,K,E,TS,R,B type & RTD, PRAJAPATI INDUSTRIES of- fer a wide range of THERMOCOPLE, RTDS, CABLES & ACCESSORIES in various application like, PLASTIC, CHEMICAL PLANTS & OTHERS.

  1. HEAD TYPE THERMOCOUPLES
    Ceramic Insulated wire-in-bead construction with hand-filled Mgo powder, available in various sheath like S.S Inconel etc. & fitting arrangement like fixed male fitting below head, threaded nipple, Nipple union nipple assemble adjustable flange in various type like FLAMPROOF T/C, GASPROBE TEFLONE OR REFRACTORY COATED PRAB. PLANTINUM T/C & MI PROBE

    THERMOCOUPLES / RTDS FOR PLASTIC INDUSTRIES
    Pencil type , bayonet type, nut or washer type in various size & shapes are available in thermocouples / RTDs in ANSI or DIN Standard.
  2. HANDLE TYPE THERMOCOUPLES
    Various Types of T/c / RTDs generally used with hand – held Digital Temp indicator. For Various application like

    GANERAL PURPOSE IMMERSION PROBE
    Used for air & liguid immersion M. Temp. 400* C, High Temp probe INCONEL 600 sheath & MI Construction Measu. Max Temp. 1150* C

    AIR/GAS PROBE
    For Air or gas in ducts, chambers, etc. perforated shield prevents radiation heating. Very sensitive due to exposed junction. M Temp. 800* C

    LEAF SURFACE PROBE
    For surface measurement temp. Up to 800*C .

    PRICK PROBE
    A chit like Sharp tip allows easy penetration into semisolids. Max Temp. up to 400* C

The Thermocouple

Why Use thermocouples To Measure Temperature?
They are inexpensive.
They are rugged and reliable.
They can be used over a wide temperature range.

What Does A Thermocouple Look Like?
Here it is. Note the two wires (of two different metals) joined in the junction.

What does a thermocouple do? How does it work?
The junction of two dissimilar metals produces a temperature dependent voltage.
For a better description of how it works.

How Do You Use A Thermocouple?
You measure the voltage the thermocouple produces, and convert that voltage to a temperature reading.
It may be best to do the conversion digitally because the conversion can be fairly nonlinear.

Things You Need To Know About Thermocouples
A junction between two dissimilar metals produces a voltage.
In the thermocouple, the sensing junction - produces a voltage that depends upon temperature.
Where the thermocouple connects to instrumentation - copper wires? - you have two more junctions and they also produce a temperature dependent voltage. Those junctions are shown inside the yellow oval.
  • When you use a thermocouple, you need to ensure that the connections are at some standard temperature, or you need to use an electronically compensated system that takes those voltages into account. If your thermocouple is connected to a data acquisition system, then chances are good that you have an electronically compensated system.

  • Once we obtain a reading from a voltmeter, the measured voltage has to be converted to temperature. The temperature is usually expressed as a polynomial function of the measured voltage. Sometimes it is possible to get a decent linear approximation over a limited temperature range.

  • There are two ways to convert the measured voltage to a temperature reading.
    • Measure the voltage and let the operator do the calculations.
    • Use the measured voltage as an input to a conversion circuit - either analog or digital.

Let us look at some other types of base-metal thermocouples. Type T thermocouples are widely used as are type K and Type N.

  • Type K (Ni-Cr/Ni-Al) thermocouples are also widely used in the industry. It has high thermopower and good resistance to oxidation. The operating temperature range of a Type K thermocouple is from -269 oC to +1260 oC. However, this thermocouple performs rather poorly in reducing atmospheres.

  • Type T (Cu/Cu-Ni) thermocouples can be used in oxidizing of inert atmospheres over the temperature range of -250 oC to +850 oC. In reducing or mildly oxidizing environments, it is possible to use the thermocouple up to nearly +1000 oC.

  • Type N (Nicrosil/Nisil) thermocouples are designed to be used in industrial environments of temperatures up to +1200 oC.

More Facts On Various Thermocouple Types

  • A variety of thermocouples today cover a range of temperature from -250 oC to +3000 oC. The different types of thermocouple are given letter designations: B, E, J, K, R, S, T and N

  • Types R,S and B are noble metal thermocouples that are used to measure high temperature. Within their temperature range, they can operate for a longer period of time under an oxidizing environment.
    • Type S and type R thermocouples are made up of platinum (Pt) and rhodium (Rh) mixed in different ratios. A specific Pt/Rh ratio is used because it leads to more stable and reproducible measurements. Types S and R have an upper temperature limit of +1200 oC in oxidizing atmospheres, assuming a wire diameter of 0.5mm.
    • Type S and type R thermocouples are made up of platinum (Pt) and rhodium (Rh) mixed in different ratios. A specific Pt/Rh ratio is used because it leads to more stable and reproducible measurements. Types S and R have an upper temperature limit of +1200 oC in oxidizing atmospheres, assuming a wire diameter of 0.5mm.
    • Type B thermocouples have a different Pt/Rh ratio than Type S and R. It has an upper temperature limit of +1750 oC in oxidizing atmospheres. Due to an increased amount of rhodium content, type B thermocouples are no quite so stable as either the Type R or Type S.
    • Types E, J, K, T, and N are base-metal thermocouples that are used for sensing lower temperatures. They cannot be used for sensing high temperatures because of their relatively low melting point and slower failure due to oxidation.
    • Type B thermocouples have a different Pt/Rh ratio than Type S and R. It has an upper temperature limit of +1750 oC in oxidizing atmospheres. Due to an increased amount of rhodium content, type B thermocouples are no quite so stable as either the Type R or Type S. we will look into some differences between different base-metal thermocouples.
    • Type E (Ni-Cr/Cu-Ni) thermocouples have an operating temperature range from -250 oC to +800 oC. Their use is less widespread than other base-metal thermocouples due to its low operating temperature. However, measurements made by a Type E have a smaller margin of error. 1000 hours of operation in air of a Type E thermocouple at +760 oC, having 3mm wires, shold not lead to a change in EMF equivalent to more than +1 oC.
    • Type J (Fe/Cu-Ni) thermocouples are widely used in industry due to their high thermopower and low cost. This type of thermocouple has an operating temperature range from 0 oC to +760 oC.
 
 
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