Answer / carl ellis

I disagree with answer #1

>other metals create junction (screw, conducting wire of
another metal)

This does NOT create an error according the Thermocouple Law
of Intermediate Junctions, which states that a 3rd metal
inserted between the two dissimilar metals of T/C junction
will have no effect upon the output voltage as long as the
connection junctions are isothermal, that is, both
connection junctions are at the same temperature.

So compensation is not needed for a 3rd metal because the
error in the positive and negative legs are of opposite
polarity, so the error is self correcting.

There is no practical method of compensating for
non-isothermal conditions so T/C compensation does not refer
to that.

>using ckt that may contain RTD at cold junction, eliminate
known emf at cold junction using electronic ckt

The original question is poorly worded because "thermocouple
compensation" refers to correction of non-linearities of
thermocouple's EMF with respect to the accepted temperature
value. That is done mathematically with a polynomial or in
some cases, with a look-up table, neither of which is an
electronic circuit.

The electronic circuit used with ALL thermocouples is a cold
junction circuit. ALL temperature measurements done with
thermocouples require cold junction compensation. The cold
junction measurement does not 'eliminate' anything, it
provides the required difference between the freezing point
of water and the temperature of the terminal block,
necessary to use standardized tables, all of which are
referenced to the freezing point of water. Without cold
junction compensation, the error of a thermocouple
measurement is the cold junction temperature difference.
That's not uncertainty, that's ERROR.

Industrial applications typically use a thermistor to
measure the temperature of the connection block at the
analog input. That temperature or its EMF equivalent is
'added' to the EMF measurement of the thermocouple.

Cold junction compensation can be done with a 2nd
thermocouple in series inserted into an icebath which still
done for laboratory and high accuracy measurements. A 2nd
T/C qualifies as a circuit, too.

Answer #2 is correct insofar as it refers to cold junction
compensation.

Carl Ellis

Answer / yashwant

thermocouple is what an emf generated at junction of two
dissimilar metal. while measuring / recording emf other metals create junction(screw, conducting wire of another metal) emf
will get added to it which is not correct reading. to
eliminate this error we use to compensate error. using ckt
that may contain RTD at cold junction, eliminate known emf at
cold junction using electronic ckt

Answer / john peter

compansation cicuit for thermocouple either tempreture
independent metal or electronic element[transistor ckt] to
produce EMF that adding or sutracting from the other end of
thermocouple

plz tell me clearly about PID.."when v have to change I value,D value,GAIN??

0 Answers  

---

why we use 24v dc in instrument rahter than 230vac????

4 Answers  

---

there are two candidates p and q in an election.During the campaign,40% of the voters promised to vote for P ,and rest for Q.however, on the day of election 15% of the voter went back on their promise to vote for P and instead voted for Q. 25% of the voters went back on their promise to vote for Q and instead voted for P.suppose,P lost by two votes,then what was the total no. of voters.

4 Answers   GATE,

---

can any one give me the cost price of yokogawa & rosemount transmitters

4 Answers  

---

WHAT IS ANNUBAR?

7 Answers  

---

---

What is open loop checking and close loop checking

1 Answers   BARC, NTPC,

---

if we put any thermocouple on a table in a room without any connection,what temperature should it show.

3 Answers  

---

Example of dry contacts & wet contacts?

3 Answers  

---

ORP sensor working principle?

0 Answers   Tata Steel Limited,

---

How do you calibration D.P transmitter? ?

2 Answers   Power Plant,

---

Explain plc level and its type?

0 Answers  

---

can u please tell me the different types of valve uses and wt purposes it is used ~!(controlling the flow other than that which valve should be used in what place)

2 Answers  

---