Why three wire is using with a RTD?

Answers were Sorted based on User's Feedback

Why three wire is using with a RTD?..

Answer / guest

Resistanc Temperature Detector(RTD)encorporates two basic

1)Variation of temperature varies the resistance of Pt wire.
2)Variation in resistance affects the balancing of DC
bridge circuit.

For measurement purposes lead wires are brought out to make
external connections.These lead wires have some resistances
which affects the bridge balancing.

Since we can not reduces the resistances of lead wire to
zero.So, either 3 wire or 4 wire connection is employed to
resistances of lead wires so that an accurate measurement
can be made.These wires(3 or 4) are called compensating


Is This Answer Correct ?    29 Yes 4 No

Why three wire is using with a RTD?..

Answer / sam

You are right, and the temperature indication does not
increase so significantly that it is worth the effort to use
a 3 or 4 wire RTD.
It might increase with something like 0,001Deg C so that is
so small you might as well say there is no difference in the
accuracy of a 2,3 and 4 wire RTD, using a local or smart
In the old days we use to use a 2 wire RTD in the field and
then run a cable say 200m to the temperature indicator. By
the time it gets to the indicator the temperature is
completely different from what it was in the field due to
the cable resistance that add itself the the RTD resistance.
We compensated for that by installing a 3de wire for the
sole propose to measure the resistance of the cable itself
and deduct that from the total resistance measured at the
temperature indicator.
So the actual resistance of the RTD as measured at the
temperature indicator is RTD - RLine1.(or [(Rline1/2)x2]if
you want)
With the 4 wire it makes it more accurate in that you can
now measure line one and line 2. The theory is that the one
line might have a small difference compare to the other line.
So the actual RTD resistance at the temperature indicator is
measured RTD - [(RLine1/2)+(RLine2/2)]
But since we all use the small compact local and smart
temperature transmitters these days, 3 and 4 wires are no
longer needed since the distance from the RTD to the
transmitter is only from about 50 to 500mm and but it seems
it have stayed due to some design engineer always saying,
why buy a 2 wire if you can get the 3 and 4 wire for just
about the same price. It will make the indication just more
accurate, but they never say by how much (0,001Deg C)
Good luck

Is This Answer Correct ?    14 Yes 0 No

Why three wire is using with a RTD?..

Answer / maswood alam

Actuly there is little diffrence b/w 3 wire nd 4wire, 3wire
has 95% accurcy nd 4 wire has 100% but 4 wire more costly nd
both are use as long distance so we genarally prefared 3
wired RTD System......

Is This Answer Correct ?    0 Yes 1 No

Post New Answer

More Instrumentation Interview Questions

what do you know about Foundation Fieldbus?

1 Answers   CCIE,

How to do calibration of magnetic flowmeter

1 Answers   Kharafi National,

who discoverd ohms law?

6 Answers   Patni, HPCL,

i have a dp tx range is 5000mmwc. in which calibration i use in a open tank.1.5*3.5*2.5 mtr.for level msmnt. what is the calculation.

2 Answers   Bajaj Hindusthan,

How we can do the linearity test for Bently Naveda Vibration Probe?

1 Answers   Sabic,

Why we are measuring 4-20ma? Why we are not taking 0-20ma? Plz answer this

1 Answers  

How to zero trim the level transmitter by using foundation fieldbus communicator.

2 Answers   Emerson,

what is the difference between 3,4 wire RTD sensor?

6 Answers   HTA Instrumentation,

Explain about relays.. Can we give both AC and DC to the same relay... How it is energising.. Explain its working..

7 Answers   IOCL,

Any one can help me to identified, how can we interchange the device in FF

0 Answers   Ion Exchange,

generally transmitter power supply having some range(12v to 35v) but the out put(4-20 mA) must same for given input.If we vary the power supply voltage then output mA also to be change according to V=IR. Is there any voltage regulator in the transmitter ?

1 Answers   Indian Steel,

how do i tune dearator pr control valve using PID. we set the set point is 0.6kg/cm2 now i want 0.5 to 0.7kg/cm2 limits so what should i give p,i,d values. then pls explain how do set pid values for control valve tuning?

0 Answers   Sharjah Electricity and Water Authority SEWA, SEWA,

  • Civil Engineering Interview Questions Civil Engineering (4646)
  • Mechanical Engineering Interview Questions Mechanical Engineering (3704)
  • Electrical Engineering Interview Questions Electrical Engineering (15793)
  • Electronics Communications Interview Questions Electronics Communications (2638)
  • Chemical Engineering Interview Questions Chemical Engineering (762)
  • Aeronautical Engineering Interview Questions Aeronautical Engineering (79)
  • Bio Engineering Interview Questions Bio Engineering (30)
  • Metallurgy Interview Questions Metallurgy (95)
  • Industrial Engineering Interview Questions Industrial Engineering (199)
  • Instrumentation Interview Questions Instrumentation (2884)
  • Automobile Engineering Interview Questions Automobile Engineering (171)
  • Mechatronics Engineering Interview Questions Mechatronics Engineering (41)
  • Marine Engineering Interview Questions Marine Engineering (62)
  • Power Plant Engineering Interview Questions Power Plant Engineering (170)
  • Engineering AllOther Interview Questions Engineering AllOther (1336)