How are you?

I'm charge to instrument of GAS plant and have question about RTD sensor. 3 wire RTD with 300 meters lead wire, how many errors we accept for normal? (3 deg C or 5 deg C)
Of caurse, No error is best and I wish it.

second question is why do we use temprature transmitter instead of direct wiring to DCS? Is it not relate accuracy?

Best regards
Michael

 

1) The best solution is to install a rtd/4-20 mA , the one i use is from phoenix contact, with that converter you can substract the cable resistance by setting and offset on the converter. So at the 4-20 mA side you will have no error.

I recommend you to use a fluke 725 or similar as your instrument in order to calibrate the offset due to cable resistance.

2) Probably the reason is the same, due to cable resistance offset.

William Delatorre

 

It depends on the type of lead wire. for every lead resistance of 0.385 ohms you will get one degree error . so know about your lead cable resistance value and you can estimate your error and for better measurements you go for teansmitter to the sensor end and mininmize the lead length and have better accuracy.

all the best

E.M.V.PRASAD

 

Hi,

The 3rd wire on the RTD is to compensate for lead wire resistance. I would recommend possibly going one step further and recommend a 4-wire RTD which fully compensates for lead resitance discrepencies. Rosemount has the capability of going one step further and provides sensor matching using Callandar-van Dusen constants as well as having the xmtr calibrated with the sensor. To get greater accuracy request a Class "A" RTD IEC - 751. You can read more about this and other details on http://www.Rosemount.com

To answer your second question: Temperature Xmtrs are used for many different reasons a few being: greater immunity to any transient EMI or EFI, takes the "low level" resistance signal and provides a stronger more robust analog 4-20mA signal. Basically the use of the Xmtr will improve the measurement accuracy and reliability.

Hope this helps!!

Cheers,
Roley

 

You are wrong: with 3-wire connections does not matter the cable length. This kind of connection compensate the cable resistance.

Also you will find 4 wire connection that also compensate for drifts in the terminal resistance.

Only with 2-wire connection the cable length has influence of 1 degree C for eache 0.385 ohms (if you are using IEC 761 RTD).

Daniel M.

 

Thanks for your answer.

I agreed your opinion theoretically but actually have different opinion. because 300 meter lead wires can not have same resistance and so have unbalance. it caused error.

My question is how many error could you accept with unbalance wires.

Michael

 

Hi

generally an RTD gives an output in voltage. the purpose of using a temp. transmitter is it will give a standardised output in the range of 4 to 20 mA. The input charcteristics of the DCS should be taken into account for this issue.

Hope this one give you an idea.