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U3 with PT100 RTD sensor


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#1 PantsOn

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Posted 26 April 2006 - 06:56 PM

From the Internet I found the following information about RTDs. How does this apply to the U3? Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from -200 to 850 degC). Standard Sensors are are available from many manufacturers with various accuracy specifications and numerous packaging options to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor. The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 degC and 138.4 ohms at 100 degC. There are also PT1000 sensors that have a resistance of 25 ohms and 1000 ohms respectively at 0 degC. The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 degC range, the error at 50 degC is 0.4 degC. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90). The linearisation equation is: Rt = R0 * (1 + A* t + B*t2 +C*(t-100)* t3) Where: A = 3.9083 E-3 B = -5.775 E-7 C = -4.183 E -12 (below 0 degC), or C = 0 (above 0 degC) For a PT100 sensor, a 1 degC temperature change will cause a 0.384 ohm change in resistance, so even a small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor) can cause a large error in the measurement of the temperature. For precision work, sensors have four wires- two to carry the sense current, and two to measure the voltage across the sensor element. It is also possible to obtain three-wire sensors, although these operate on the (not necessarily valid) assumption that the resistance of each of the three wires is the same. The current through the sensor will cause some heating: for example, a sense current of 1 mA through a 100 ohm resistor will generate 100 mW of heat. If the sensor element is unable to dissipate this heat, it will report an artificially high temperature. This effect can be reduced by either using a large sensor element, or by making sure that it is in good thermal contact with its environment. Using a 1 mA sense current will give a signal of only 100 mV. Because the change in resistance for a degree celsius is very small, even a small error in the measurement of the voltage across the sensor will produce a large error in the temperature measurement. For example, a 100 uV voltage measurement error will give a 0.25 degC error in the temperature reading. Similarly, a 1 uA error in the sense current will give 0.25 degC temperature error. Because of the low signal levels, it is important to keep any cables away from electric cables, motors, switchgear and other devices that may emit electrical noise. Using screened cable, with the screen grounded at one end, may help to reduce interference. When using long cables, it is necessary to check that the measuring equipment is capable of handling the resistance of the cables. Most equipment can cope with up to 100 ohms per core. The type of probe and cable should be chosen carefully to suit the application. The main issues are the temperature range and exposure to fluids (corrosive or conductive) or metals. Clearly, normal solder junctions on cables should not be used at temperatures above about 170 C. Sensor manufacturers offer a wide range of sensors that comply with BS1904 class B (DIN 43760): these sensors offer an accuracy of 0.3 degC at 0 C. For increased accuracy, BS1904 class A (0.15 degC) or tenth-DIN sensors (0.03 degC). Companies like Isotech can provide standards with 0.001 C accuracy. Please note that these accuracy specifications relate to the SENSOR ONLY: it is necessary to add on any error in the measuring system as well. Related standards are IEC751 and JISC1604-1989. IEC751 also defines the colour coding for PRT sensor cables: the one or two wires attached to one end of the sensor are red, and the one or two wires at the other end are white.

#2 LabJack Support

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Posted 27 April 2006 - 08:23 PM

RTDs can seem tempting because of their high accuracy, but as you can tell in the post above it is difficult to ever achieve that accuracy.

The problem with an RTD is that the sensors themselves might be very accurate (resistance versus temperature), but they simply provide a resistance that varies (just slightly) with temperature, and it is often difficult and expensive to determine the resistance of the RTD with similar accuracy. It is much easier to use an accurate probe like the EI-1034 (available at labjack.com) with a high-level voltage output.

The typical way to measure resistance is to put a constant current through the resistor (RTD in this case) and measure the voltage that results. The post above points out some of the easy sources of error than can cause problems. I see 3 major sources of error:

1. Accuracy of the resistance versus temperature for the RTD.
2. Accuracy and stability of the constant current source.
3. Accuracy of the LabJack's analog input.

Assume you want an accuracy of 0.03 degrees C from -200 to +850 degrees C as mentioned for one class of RTD in the above post. That means 1 part in 35000 or an accuracy of 0.003% full scale. All those errors above have to combine to less than 0.003%. It seems that it would be tough to find a constant current source that is sufficienty accurate and stable, and that accuracy is beyond the U3.

Another way to handle RTDs is with a bridge circuit, but this has just as many problems and complications as the constant current source.

An RTD can certainly be used with a LabJack, but just because you are using an RTD that is accurate to 0.03 degrees C, do not expect your overall system to easily achieve that same accuracy.

Beyond creating your own current source or bridge circuit, following are some commercial signal conditioning units that can be used to interface an RTD to a LabJack analog input.

The TX94A from Omega is an option with 4-20 mA output. See Section 2.6.3.7 of the U3 User's Guide to handle the 4-20 mA signal:

http://www.omega.com...X93A_TX94A.html

5B34 or 5B35 from Analog Devices have simple voltage outputs and good accuracy:

http://www.analog.co...odules/fca.html

#3 PantsOn

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Posted 01 May 2006 - 01:20 AM

Thanks!! What is the measurement range if used with U3? I think the upper limit will be lowered, since U3 can read only up to 2.5volts. Please make Unit Conversion from F to C including the PDF file

#4 LabJack Support

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Posted 17 May 2006 - 09:14 AM

I am not sure what you are talking about specifically, but the U3 has a max input voltage of about 2.5 volts (or 3.6 volts with the special range), so that might limit the maximum signal you can measure.

#5 DaviD_H

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Posted 12 January 2009 - 07:57 AM

Hi I have don the 3 100ohm resistor bridge. What is the formule i have to use to obtain the Temp.?? Thanks

#6 LabJack Support

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Posted 12 January 2009 - 10:04 AM

You can find tons of bridge equations around the Internet, but the simplest is at the bottom of this page:

http://www.efunda.co...tone_bridge.cfm

This equation does have a lot of simplifications, but is a good place to start. Later if you need you can back up to the full unsimplified bridge equations.

Since you only have one active element (say dR1), the other 3 delta values are 0 and the equation is just:

dVg = (dR1 * Vin) / 4R

That lets you solve for the resistance of your RTD, and then you can use the resistance-to-temperature relationship provided for your RTD to get temperature.

#7 JJakobsen

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Posted 17 February 2009 - 05:22 AM

Hi Can anyone tell me how I connect a PT100 (3wire censor) to my LabJack U3-HV. If I buy a LJTick-CurrentShunt would that do the trick or do I need it at all. /JJ

#8 LabJack Support

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Posted 17 February 2009 - 08:35 AM

A raw RTD connected directly to a U3-HV will not do anything useful. You need some signal conditioning for the RTD, as discussed above.

 

If the RTD probe already has signal conditioning built-in, and is providing 4-20 mA output, then yes the LJTick-CurrentShunt is good for monitoring a 4-20 mA signal.



#9 LabJack Support

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Posted 18 November 2013 - 08:12 AM

The U6 and T7 have fixed current sources that are useful for RTDs.  See our temperature sensor app note:

 

http://labjack.com/s...erature-sensors




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