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4 Wire Pt 100 Sensor Resistance Not Matching

11/22/2014 8:38 AM

Hey all,

We have a four terminal pt 100 sensor. When I try to measure the resistance with my fluke at the pt 100 terminals, it reads 107 ohms which corresponds to 19*C on the pt 100 table ,whereas the actual temperature is 4.9*C Is the Pt 100 faulty. I have seen many situations like these where the resistance does not match. Also is there any other way to verify if the sensor is working perfectly fine

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#1

Re: 4 wire Pt 100 Sensor Resistance not matching

11/22/2014 9:25 AM

The flaw could be in the 1) meter, 2) sensor, 3) wires and connections between, 4) independent device you are trying to use for calibration, 5) typos in the data table, 6) other confusions, 7) etc.

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#2
In reply to #1

Re: 4 wire Pt 100 Sensor Resistance not matching

11/22/2014 9:42 AM

I have checked the pt 100 table many times.

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#3

Re: 4 wire Pt 100 Sensor Resistance not matching

11/22/2014 12:03 PM

Fluke makes a lot of different meters with different levels of precision and accuracy. When was the last time your meter was calibrated?

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#4

Re: 4 wire Pt 100 Sensor Resistance not matching

11/22/2014 1:35 PM

1a) What were you really measuring A?

Did you disconnect the RTD lead wires from the analog input circuit when you made the resistance measurement?

Or did you attempt to measure RTD resistance while the RTD was still in the analog input device's active circuit?

A resistance measurement is made by pumping a known, constant current through the probes/circuit, measuring the resulting voltage drop and interpreting the voltage drop as resistance.

If you did not disconnect the RTD lead wires then you were measuring the resistance of the circuit that the RTD was connected to.

If you attempted to measure the RTD resistance while the RTD circuit was powered and active, then your ohm meter was mixing its excitation current with the RTD circuit's excitation current.

Either would result in a measurement that is not solely the RTD element resistance, but other circuit factors interpreted as resistance.

1b) What were you really measuring B?

For a 385 alpha Pt 100,
19.0 Deg C = 107.405 ohms
5.0 Deg C = 101.953
The difference is ~ 5.4 ohms

The RTD element is embedded in the sheath, you can't measure its resistance without also measuring some lead wire resistance.

You don't say how long the lead wires are to the point you were measuring. A head mounted transmitter uses short lead wires. A remote mounted transmitter or receiver could have long lead wire.

100 feet of 24AWG lead wire has >2.5 ohms resistance, which doubled (out and back) would account for the resistance difference you cited.

2) >whereas the actual temperature is 4.9*C
How do you know that's true?
If you're checking the RTD resistance, you suspect something is wrong with the temperature measurement.
What is your reference standard and how does it measure exactly the same location as the RTD?
3) >verify if the sensor is working
Calibration equipment that is presumably certified and standards traceable like an oil bath, ice bath or dry block calibrator is used to heat or cool a sensor to one or more temperatures. The sensor's output is measured with a certified/traceable measuring instrument, the results compared to published performance tables and analyzed for its range of uncertainty and linearity.

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#5

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/22/2014 11:14 PM

Yes the wires are already disconnected from the circuit.

"The RTD element is embedded in the sheath, you can't measure its resistance without also measuring some lead wire resistance" . You mean to say that we cant measure the resistance at the rtd terminal

Lead wire are are about 5m in length .I am measuring the resistance at the head of the rtd .

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#6

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/22/2014 11:50 PM

Just disconnect all the lead wires , power supply and measure the resistance. You will get the correct reading.

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#7

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/23/2014 8:51 AM

Just to illustrate some of what Iris already said:

You might be measuring the resistance of the wiring and the RTD sensor instead of just the sensor itself. Wire resistance is dependent on length and diameter. Long wires and small diameter wires increase the resistance.

That 107Ω you measured probably includes the resistance of the wires.

Tip:

If your wires are all of the same length and size, you can measure the resistance of the first wire pair (first and second from the top) or the second pair (third and fourth from the top). Next, subtract that value from the 107Ω that you measured initially. That will give you the resistance of the sensor only. Use that figure to determine the temperature.

Better tip:

Measure the resistance at the sensor itself and eliminate the contribution of the wires.

regards,

Vulcan

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#8

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/23/2014 11:02 AM

Is the RTD in a well?

If so, is the well the correct length and size?

Is thermal compound used to improve thermal conductivity between the well and RTD?

First thing I would do is replace the RTD with a known good one, then return the RTD

to the calibration shop for testing with NIST certified equipment and qualified

personnel.

If you get an error with the new RTD,check the well for contamination or damage.

In very high moisture areas,it is a good practice to coat the terminals in the well

head with a thin layer of silicon grease to prevent corrosion of the terminals.

Even if the well has a good gasket,moisture can still get in via flexible conduit or

around conduit threads.

Conduit threads are straight, not tapered as water pipe threads are,

(3/4 inch per foot taper),so there is not a very reliable seal on the fittings.

A good seat-of-the pants- GO_NO_GO- test is get a styrofoam cup,fill it with crushed

ice,with just enough water to fill the gaps between the ice,and measure the

resistance while stirring the ice with the RTD.

The resistance should be very close to the chart for 0 C.

If you have boiling water available, when immersed in boiling water it should be very

close to 100 C.

You can also use this to compare it with a known good RTD.

Oh yeah! Almost forgot:

Mare sure you are referencing the right table for the RTD you are using.

The data should be printed on the RTD itself.(Hard to see sometimes,but it should be there).

Could actually be a mix up when they were ordered.

Good luck!

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#9
In reply to #5

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/23/2014 12:48 PM

Also, please confirm the lead resistance of your Multi Meter and take it into account.

Good luck :)

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#10

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/23/2014 1:34 PM

GA's to Vulcan and HiTekRedNek

One note on using boiling water as a temperature check - water boils at a lower temperature at higher altitudes. I was amazed to discover that El Paso, Tx was at a much higher elevation than I suspected until I saw water boil at about 205 deg F. Google will find a chart with boiling temperature of water at altitude.

So you have a resistance measurement of an RTD isolated from its electronics that shows a resistance that corresponds to a temperature that is higher than what you think the temperature really is.

I can think of 3 scenarios:

1) The RTD is faulty, producing too much resistance for a given temperature.

2) The resistance measurement is incorrect, the Fluke is reading the wrong value.

3) The RTD and resistance measurements are correct, but the temperature that the RTD is seeing is not what you think the temperature should be.

To troubleshoot scenarios 1 or 2 you need some standard to check against. Others mentioned testing with a new RTD or ice bath/boiling water. A 2nd or 3rd DVM can be a rough check against the Fluke meter's resistance reading.

All sorts of process and/or operating conditions, like

- corrosion on terminal blocks where the RTD wiring connects
- RTD is shorter than the bore length of the thermowell|
- air pocket in thermowell (RTD not bottomed out)
- added or missing insulation on well-to-head lagging- air pocket in heating/cooling jackets
- a valve opened or closed that shouldn't be
- a clogged drain
- a bias applied to a controller's setpoint or a PV
- steam leaks

to name a few (half of them mentioned previously), can produce an unexpected change in temperature.

If you replace the RTD and it produces the same higher temperature result that you see now, then you'll have to try to figure why the real temperature is higher than expected.

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#11
In reply to #2

Re: 4 wire Pt 100 Sensor Resistance not matching

11/23/2014 6:00 PM

Well, check the lead resistance, then.

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#12
In reply to #10

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/23/2014 11:28 PM

I am measuring the resistance at the head of the rtd with all wires removed. I am getting a resistance of 106.2 Ohms which corresponds to 16 c,When I connect the wires to the measuring circuit . I am getting a reading of 5.2 C

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#13
In reply to #12

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/24/2014 12:15 AM

That sounds like the very definition of a measurement technique failure. I understand that you believe that you are doing the exact same thing with two different instruments. Obviously your belief is wrong somewhere. You might have an out of calibration instrument. You might have a subtle miswire. You might have a self heating problem. You might have an unexpected secondary power source. I do not know and you do not respond to questions.

Repeating that its broken and you don't know how to fix it gets boring.

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#14
In reply to #12

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/24/2014 5:22 AM

Okay, we're getting clearer information but not by much. We'll need more.

  • Check your multimeter with a 100Ω resistance with a low tolerance value (1% or better).
  • It's possible your meter probes could be adding their own resistance to your reading. Short your probes together to test.
  • Use the same 100Ω resistor to test your measuring instrument. It should read approximately 0°C depending on the tolerance of your resistor.

regards,

Vulcan

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#15

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/24/2014 11:11 AM

A panel mount temperature display with RTD input costs less than your Fluke. That will let you see if your RTD is near to correct. Manufacurers include Precision Digital, Red Lion, Moore, and so on. There are more manufacturers of these than manufacturers of hair dryers, and some are very low cost.

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#16
In reply to #12

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/24/2014 12:28 PM

This is why all standard RTD probes have at least the three wire setup or four wire is most common. The instrument reads all the terminals, and gains information on (1) lead resistance compensation needed, (2)integrity check of the probe itself - i.e. continuity on all points. Use a traceable thermometer to cross check the entire RTD circuit, and calibrate it, and/or use traceable primary standards of temperature.

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#17

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/30/2014 11:15 AM

You did not state the type or model number of the meter you are using. If you are using a low impedance (20kΩ input impedance) VOM then it is possible that the current from the internal battery is providing sufficient current to heat the RTD element and distort the readings.

Try using a high impedance (10MΩ) VTVM and see if the readings are identical. Fluke even makes some that allow you to switch between high and low impedance inputs. Digital meters are great but they can give us a false sense of precision and accuracy when we blindly look at that 4 digit display.

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#18
In reply to #17

Re: 4 Wire Pt 100 Sensor Resistance Not Matching

11/30/2014 8:53 PM

He did say it was a Fluke meter, no model was mentioned, but I doubt a Fluke meter would have an excessive resistance or RTD excitation current. (Did Fluke ever make or sell a Simpson 260 style analog VOM?).

The industry rule of thumb is excitation current of 1mA or less to minimize the self-heating effect.

Here are some representative values for excitation currents:

- the Fluke 787 and 789 process calibrators use RTD excitation curnts of 220uA
- the 712 RTD calibrator uses 1mA.
- the Fluke 80 Series DVMs use resistance scale excitation of <1mA
- the Fluke 114 uses a resistance scale excitation of 350uA

- dedicated RTD measuring integrated circuits like Burr Brown XTR 112 or 114 use either 100uA or 250uA.

Process temperature transmitters do not always publish excitation current, but
- the ABB TH02 uses 300uA
- Siemens TW uses 180uA.

When it is spec'd, which is actually quite rare, the self heating error for a Rosemount 248 RTD is 0.15 deg C/mW, or the dissipation constant (in ~1m/sec flowing water) is 16mW/Deg C for a 385 alpha Pt 100's.

I^2R for 250uA is 1000mW/W * 0.000250^2A* 101.59 ohms = 0.00635mW = 6.35uW

I^2R for 1000uA is 1000mW/W * 0.001^2A*101.59 = 0.102mW

Using the dissipation constant for 1mA excitation, the highest cited Fluke value, 0.1mW/16mW/Deg C = 0.006 DegC error.

The cited error was several orders of magnitude higher. Even if we give 100x for still air dissipation (reduced heat transfer compared to the spec for flowing water), self-heating is still not a big enough factor.

If it were a home/shop made calibrator, I'd be real suspicious, but a Fluke? I doubt it's self-heating.

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