3-Wire RTD

[PhaseShift]

Can someone tell me what the difference between a 3-wire and 2-wire RTD is? What does the third wire do?

Also I have seen a case where one of the wires (maybe the 3rd wire) is connected to a common bus with wires from other multiple rtd's on the same motor. The third wire going back to the instrument was also connected to this bus. Is this correct? In one case this common bus was grounded.

 

[SG-1]

If I remember correctly the third wire is used to compensate for distance.

I am used to seeing one wire common on three wire systems.

 

[ELA]

We also used 4 wire RTDs.
Think of it like a power supply with remote sense capability.
Two wires supply the current to excite the device and the other two are only to make the measurement of voltage directly at the device.

The extra wires increase accuracy by eliminating the voltage drop caused by the excitation current through the wire leads. This becomes more important on long lengths of lead-in wire.

In a three wire system they "assume" the wires are all the same guage and length.

 

[millerdtm]

I believe the 3rd and 4th wire eliminate the effects of lead wire resistance, therford making the measurment more accurate.

 

[KentAT]

I believe the 3rd and 4th wire eliminate the effects of lead wire resistance, therfore making the measurment more accurate.

This is correct. The RTD (Resistance Temperaure Device) sensor uses the fact that the resistance of a material changes with the temperature of the sensor material. The temperature measurement is based on the resistance of the circuit.

2-wire sensors are for short lead wire lengths and where accuracy is not important. The instrument does not know how much of the circuit's total resistance is at the sensor's measurement area and how much is due to the reistance of the lead wire to the sensor.

3-wire sensors add a third wire, attached to one side of the measuring element and running back to the instrument. This wire, ran in parallel with one of the original two wires (and thus the same color), is used to approximate the resistance of the lead wires due to length and the effect of environmental temperatures on the lead wires. Basically, the instrument is measuring the resistance of a loop of lead wire run to the sensing area and back without passing through the sensing element. The instrument assumes all 3 wires are exactly the same, so it uses this value to come up with an approximate or average resistance that is due to the lead wires only, and subtracts it from the total circuit resistance. This means the remaining resistance is due to the sensor element only, and is much more accurate.

4-wire sensors have a parallel wire run to each side of the sensing element (and therefore the same color), resulting in extremely accurate measurements.

For my industrial applications using RTD measurement for input to a PLC, 3-wire RTDs are most common.


Kent

 

[Cold Fusion]

I've never seen a 2-wire RTD, nor a measuring circuit for a 2wire rtd.

Thermistors - yes
Thermocouples - yes

RTDs - no

Does anyone have a reference to manufacturers for a 2W RTD and a measurement module with a 2W RTD input?

 

[KentAT]

To the OP:

Attaching one of the 3 wires to a common bus is not correct for 3-wire RTD measurement. Is it possible that the third wire was in fact a bare drain wire commonly found in shielded instrument cable?



Kent

 

[don_resqcapt19]

I've never seen a 2-wire RTD, nor a measuring circuit for a 2wire rtd.

Thermistors - yes
Thermocouples - yes

RTDs - no

Does anyone have a reference to manufacturers for a 2W RTD and a measurement module with a 2W RTD input?

Omega has both two wire elements and transmitters. I have never installed a two wire stand alone element, but was just hooking up some Rosemount conductivity transmitters the other day and they used a two wire RTD as part of the sensor element, but the wiring between the sensor element and the transmitter was 3 wire.

 

[GeorgeB]

Does anyone have a reference to manufacturers for a 2W RTD and a measurement module with a 2W RTD input?

When I have had short runs and used an RTD for convenience rather than for its superior accuracy, I have jumpered the sense terminal to the appropriate input terminal. We in industrial hydraulics often monitor reservoir temperature to control heat exchangers; the lead may be less than 10 feet and will not be a significant source of error, and no source of (in)stability based on environment temperature.

 

[PhaseShift]

I posted a sketch of the wiring for these rtd's. The are rtd's in a motor and the common for each was pre-wired from the factory to the common bus as shown. The drawing showed also to ground this common bus. Each of these RTD's goes to an input on a motor protection relay.

Does this seem correct?

For a 2-wire RTD then the device (in this case the relay) sends a small current through two wires to the device and then measures the voltage dropped across this device to calculate the resistance?

For a 3-wire RTD the device sends our a current on the + wire and then takes two voltage measurement one between the "+" and "-", and the other between the comm and "-". The meausrement between the comm and "-" will calculate the resistance dropped in the wire leads and this value will be subtracted from the overall resistance value calculated from the measurement between "+" and "-"?

I think maybe that by connecting all the comms to a common bus at the motor then maybe due to some physical characteristics differences in RTD's or wire leads thes comm points could be at different voltages and cause circulating currents? Maybe thats why these points are grounded? But why ground them as opposed to brining the comm directlcy back the the relay input?

 

[KentAT]

This is an RTD installation design that I know nothing about.

Maybe one of the motor guys can shed some light on it. The grounding of the comm lead has a purpose, like maybe for safety reasons.

I have also read of using motor RTDs from the windings to detect PD (Partial Discharge) occurring in the windings by placing a PD instrument in series with the RTD wiring without affecting the temperature monitoring aspect - again I know nothing about that use.

From an electrical standpoint, I can't come up with a way that the common bus would provide an erroneous reading though.

Kent

 

[Cold Fusion]

Omega has both two wire elements and transmitters. ...

Thanks Don -
I looked up the cut sheets you referenced. The 2W RTD is available, but the input device is 3W RTD.

From the cut sheet:​

​

​

Specifications​

Input:​

​

​

​

​

3-wire Pt 100 RTD,​

a = 0.00385,

cf

 

[KentAT]

Here are two Rosemount transmitters that accept 2-, 3-, and 4-wire RTDs.
http://www2.emersonprocess.com/en-US/brands/rosemount/Temperature/Single-Point-Measurement/3144P/Pages/index.aspx
http://www2.emersonprocess.com/en-US/brands/rosemount/Temperature/Single-Point-Measurement/148/Pages/index.aspx

Here is an excerpt from the manual of the Rosemount 148 (with bold emphasis added by me):

RTD or Ohm Inputs
The Rosemount 148 will accept a variety of RTD configurations, including
2-wire, 3-wire, and 4-wire designs. If the transmitter is mounted remotely from
a 3-wire or 4-wire RTD, it will operate within specifications, without
recalibration, for lead wire resistances of up to 60 ohms per lead (equivalent
to 6,000 feet of 20 AWG wire). In this case, the leads between the RTD and
transmitter should be shielded. If using only two leads, both RTD leads are in series with the sensor element, so significant errors can occur if the lead
lengths exceed three feet of 20 AWG wire (approximately 0.05 ?C/ft). For
longer runs, attach a third of fourth lead as described above.

Kent

 

[Cold Fusion]

When I have had short runs and used an RTD for convenience rather than for its superior accuracy, I have jumpered the sense terminal to the appropriate input terminal. We in industrial hydraulics often monitor reservoir temperature to control heat exchangers; the lead may be less than 10 feet and will not be a significant source of error, and no source of (in)stability based on environment temperature.

Interesting. I've never thought of doing that. I guess I've always figured the extra $0.22 of wire, to go three wire, was worth putting in.

Okay, I may be exaggerating - it may be only 13 cents

cf

 

[GeorgeB]

Interesting. I've never thought of doing that. I guess I've always figured the extra $0.22 of wire, to go three wire, was worth putting in.

Okay, I may be exaggerating - it may be only 13 cents

cf

It has not been wire costs, rather the cable the electrician used, usually 16-2 shielded pair they are accustomed to using for 4-20 mA in industrial plants.

 

[Cold Fusion]

It has not been wire costs, rather the cable the electrician used, usually 16-2 shielded pair they are accustomed to using for 4-20 mA in industrial plants.

Of course it isn't the costs.

If it is with in 3 feet you can replace the wire. And if it is 20 feet and they did not put in 3wire because of you or the engineers or the electricians - doesn't matter who, you're screwed anyway. It won't work worth a damn.

Or did I miss something.

cf

 

[PhaseShift]

So do you think the arrangement that I've shown will work? Should I leave the common grounded or ungrounded? Would not grounding it lead to error?

Instead of using this arrangement could I just bring all three wires straight to the multilin relay input? Or does this arrangement more accurate for some reason?

 

[glene77is]

To the OP:
Attaching one of the 3 wires to a common bus is not correct for 3-wire RTD measurement. Is it possible that the third wire was in fact a bare drain wire commonly found in shielded instrument cable?
Kent

Kent,
In my understanding, you are making reference to what is called a 'Faraday' sheild, which has a 'drain' line attached to one end in order to produce an "Inductive Faraday sheild" around signal wires.

This kind of sheild will inhibit static noise from passing along with the signal wires over long distances and being capacitively coupled into the signal wires. This noise on the signal lines can mess up controls for a VFD.

In contrast, the GEC attached to the ground is bonded at both ends,
producing a "Capacitive Faraday Sheild" to conduct high frequency components of lightning to the Ground Rod.

Hope I got my wording right.
At any rate, they are two different approaches,
for two different applications.

Interesting,
However,
I lean towards the others response concerning
equalizing or diminishing the overall resistance
of the long-distance conductors that the sensor
and RTD have to work through.

This is such an interesting thread.
Keep it up!

 

[PhaseShift]

Was I correct with my understanding of 2-wire vs 3-wire rtd's?

 

[Cold Fusion]

So do you think the arrangement that I've shown will work? Should I leave the common grounded or ungrounded? Would not grounding it lead to error?

Instead of using this arrangement could I just bring all three wires straight to the multilin relay input? Or does this arrangement more accurate for some reason?

PS -
I would have to look at the book for your relay and see what it says. With out the relay instruction book, I wouldn't have a clue. You use the term "multilin". If you mean one of the"GE Multilin" familyof relays, all of their instruction books I've seen have specific instructions and specifications concerning how the circuits are to be connect. I could not make any generalized comments about how a specific RTD measurement circuit is affected by grounding one of the terminals.

cf