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Member

Join Date: May 2008
Posts: 8

WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

10/04/2008 4:20 AM

CAN ANYBODY EXPLAIN TO ME THE REASON WE USE A 4-20mA CONVERTER ON A THERMOCOPLE.

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Power-User

Join Date: Mar 2008
Location: Mumbai
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#1

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

10/05/2008 1:48 AM

Coz T/C Output is in MV and may b u r DI Module accepts 4-20 ma

Regards

jose

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Active Contributor

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#2

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

10/06/2008 5:12 AM

I can think of 3 reasons

a) Distance. If cable length is too long, you may have problems with a mV signal

b) Receiving device only has mA inputs (or it will be cheaper to use one)

c) To avoid laying special compensating cables or to use exisiting spare instrument cables

Steve

MTL Instruments
www.mtlblog.com

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#3

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

10/10/2008 1:27 AM

because of the thermocouple is working under the principle of " seeback effect " and peltier effect,thomsan effect

thermoucouple generates the milli volt output only but DCS expectation is in terms of voltage

V=i*r

24 = (4-20mA)*250 -----------> gives the 1 - 5 v it is happening at the DCS by converting the ( mA to v by A/D converter )

DCS accepts 1 - 5 v only

thats while we are converting the mV to mA by ( voltage to current converter )

at TT

u know how the RTD output is converted to the 4 to 20 mA by wheatstone bridge circuit if the RTD type is PT 100 type .

the unbalance in the wheatstone is converted into millivolt that is converted into current mA

in wheatstone bridge ressistance at three side is connected equal value 100 ohms and remaining sensor side will connected

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Anonymous Poster
#4

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

11/23/2008 12:21 AM

For me,

The T/C output is in mv.

and we may get loss of signal over long length of cable.

It also depends upon I/P module use for indication or control.

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Anonymous Poster
#5
In reply to #4

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

12/15/2009 1:02 AM

i dont understand I/P module

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Anonymous Poster
#6
In reply to #5

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

12/15/2009 3:28 AM

I/P = input.

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Commentator

Join Date: Aug 2011
Posts: 76
#9
In reply to #4

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

08/22/2011 5:44 AM

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Anonymous Poster
#7

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

03/03/2010 5:22 PM

hi,

I think it is because T/C is always used to measure very high tempratures and so we must put the receiver of the signal of it far away from this hazardous area and so usually we use a long cable to connect between them and for the mv signal that T/C produces, it will be vanished before it arrives to the receiver so we must convert it to a mA signal.

Maged Anter

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Anonymous Poster
#8

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

01/03/2011 7:35 AM

the situation is follows:

if any problem happens on plant , espicellay temperature value is zero thermocouple indicates the value of 4mA, where as the the thermocouple is fault it indicates the zero mA. Mainly used for differntiating d temperature o/p and thermocouple o/p

if its wrong kindly reply...

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Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#10

Re: WHY DO WE USE A mA CONVERTER ON A THERMOCOUPLE?

08/29/2023 7:24 AM

The rest of the <...WE...> might help, though PLEASE STOP SHOUTING, because many here are trying to get some sleep.

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