rtd reading

Hello, so I'm having some issues with a few rtds reading in occupied spaces. Several local pocket temp sensors are reading 7 deg higher than the readout on our distech controller. These are 10k type II @77F, it says on the bags of all of these. So to troubleshoot, I took a 10k resistor and put it across the input on the controller and got 70F solid. I then went to location and removed he rtd, and replaced it with the same 10k resistor, also got 70F again. Now i've run into this before, and I need a few more opinions here, should I see the 70F or 77F on the controller readout? The controller is configured for 10k type II. (in fact all of them are here). This has happened at other locations here. What is right? Right now I put in a 7deg offset in the controller.
 
Thank you. I guess my basic question is should the controller readout be 70 (as it is) or should it be 77, with the 10k resistor connected?
 
Hello

Did you hear aboout RTD lead wire compenstion
*
here some notes about it:
Lead Wire Resistance

RTDs are resistive devices, so lead wire resistance directly affects its accuracy.The error can be quite large, depending on the lead wire resistance (measured in ohms / foot).For example, an uncompensated 2-wire circuit using 30 gauge wires can have an error as high as 1.2°F per foot!Fortunately, there is a method to compensate for the lead wire resistance.

2-wire

One lead wire is connected to each end of the element.This arrangement is suitable for uses where the lead wire resistance may be considered as a constant in the circuit, or where changes in the lead wire resistance due to ambient temperature changes can be ignored.

3-wire

This is the most common of RTD configurations.One lead wire is connected to one end of the element and two lead wires are connected to the other end.The purpose of the third lead is to compensate for the lead wire resistance, thereby increasing accuracy.An instrument capable of utilizing a 3- wire RTD must be used to benefit from this configuration.

4-wire

The most accurate of the RTD configurations, this element uses two wires for each end of the element. Building on the 3-wire concept, compensation is made for the resistance in each lead wire, creating a highly-accurate temperature-measurement device for critical applications.An instrument capable of utilizing a 4-wire RTD must be used to benefit from this configuration.

Have a good search& good read on it..

James
 
Since i have connected the resistor directly to the controller, with no extension wire, and have connected it at the location and got the same reading both times, I dont think the wire guage or length is an issue. It reads the same. Should a 10k resistor read 77 deg?
 
Whitout be able to get the scheamtic showing RTD interface board also ...I cannot state more on what happening on your RTDs reading values...

May we see any drawing/sketch of the mentionned subject...
 
So looks like it is a Thermistor that you got ? Right or RTD s Pleae clarify by adding more details...

Here some notes for the Thermistor:
How Accurate is the Reading?

You may notice that above, the temperature reading is 28.16°C - does that mean we have 0.01°C accuracy? Unfortunately no! The thermistor has error and the analog reading circuitry has error.


We can approximate the expected error by first taking into account the thermistor resistance error. The thermistor is correct to 1%, which means that at 25°C it can read 10,100 to 9900 ohms. At around 25°C a difference of 450 ohms represents 1°C so 1% error means about +-0.25°C (you may be able to calibrate this away by determining the resistance of the thermistor in a 0°C ice bath and removing any offset). You can also spring for a 0.1% thermistor which will reduce the possible resistance error down to +-0.03°C


Then there is the error of the ADC, for every bit that it is wrong the resistance (around 25°C) can be off by about 50 ohms. This isn't too bad, and is a smaller error than the thermistor error itself +-(0.1°C) but there is no way to calibrate it 'away' - a higher precision ADC (12-16 bits instead of 10) will give you more precise readings


In general, we think thermistors are higher precision than thermocouples, or most low cost digital sensors, but you will not get better than +-0.1°C accuracy on an Arduino with a 1% thermistor and we would suggest assuming no better than +-0.5°C.

James
 
As others have pointed out, the sensor is a thermsistor, not an RTD.

77°F = 25°C, reference sheets are frequently in °C.

Yes, 10,000 ohms should read 77°F, if your ID statement is correct.

I suspect that your 5% 10k Resistor is not a very good means of checking the controller's input accuracy.

Even a couple ohms of copper wire resistance is not going to affect a reading of a 10K ohm input significantly, whereas a couple ohms in 2-wire 100 ohm 385 Pt RTD circuit would be a significant 9°F error.
 
The resistor is a 1% (gold) and has been tested on several meters to read 9.99-10.01k ohms. its accurate. the wire isnt an issue since we got the same temp reading at the controller and at location. The length is about 100 feet and its 18 guage, will check. i dont believe its the issue, its something in the controller not right. im going to look more today and update. thanks. Any other ideas are welcome.
 
The resistor is a 1% (gold) and has been tested on several meters to read 9.99-10.01k ohms. its accurate. the wire isnt an issue since we got the same temp reading at the controller and at location. The length is about 100 feet and its 18 guage, will check. i dont believe its the issue, its something in the controller not right. im going to look more today and update. thanks. Any other ideas are welcome.
First you mentionned RTD & i ADDED NOTES ON RTDS , then I discorvered that was more a Thermistor than a RTD...

Plz Clarify by adding at least a scheamtic/diagram/drawing if you need a better assistance...
 
You are right, wire is not an issue.

18g copper wire has a resistance of 6.5 Ohms/1000 feet.

A 100' run out and back is 200' total with a resistance of 2.1 Ohms

2 Ohms is only 0.02% of the nominal 10K Ohm input, insignificant.

And a 'increased resistance due to wire resistance' issue would manifest as a positive offset, increasing the reading, not decreasing the reading.
 
Yes I agree. Getting a schematic of the bridge system reading this will be difficult. The packages say "duct sensor 10k type II", we have used this type alot here, possibly 300. I have in the past tried this on several other locations, some have 70 some 77 and some way out at 60s. So I am finally digging into this. I have read that most popular types (these are most likely rtds) should all read 10k at 77f. So a 10k resistance should read 77F right?
 
In answer to your question, should these 10K Type 2 thermistors read 77°F at 77°F, the answer is yes. Here's a chart and statement the differences between Type 2 and Type 3 thermistors as the temperature moves away from 77°F:

Difference between Type 2 and Type 3 thermistors 2.JPG

Bapi's statement on no international standards 2.JPG
and a table of resistances:

resistance vs temperature table.png

The go-to overview of RTDs vs Thermistors, by BAPI, is here:

http://www.bapihvac.com/wp-content/uploads/Therm_vs_RTD.pdf

If you put 10K ohms (± some minor deviation) on the controller input and the controller does not read 77°F, it's a controller/input issue.
 
Yes, I agree. I've done the same test on another controller and the reading is 77f. It is a controller issue, as I suspected all along. The second controller is slightly newer, may have something to do with it. I'll check a few others as time/project allows.
 
When you want accurate readings with an RTD, measure 4 wire or -at least- 3 wire.

Test the RTDs directly on a calibrator (or temperature controller)

Switch over to thermocouples, is also something you could consider. I'm personally not a fan of measurements with RTDs......I prefer the ruggedness and flexiblity of thermocouples. The output signal is low-impedance so you can extend it with very long wires and you only need 2 (but of the correct thermocouple compensation wire).
 
Top