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PT 100 Measurement


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19 replies to this topic

#1 embayweather

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Posted 04 January 2013 - 11:33 AM

I am currently trying to include some PT100 devices into my weather station for temperature measurements. I am using a Rosemount 244R transmitter, coupled with my own voltage divider to obtain a voltage which varies with resistance. I am getting around 1.35 volts out of the device, which of course varies with the temperature. My problem occurs when I connect it to the Labjack U3 in that the voltage measure by teh U3 varies between 0.1 volts to 2.2 volts on a pseudo sine wave pattern. Clearly, as you will know, this is not much use to me in measuring small changes in voltage. Have you any ideas where I might be going wrong please ? I have tried checking the wiring, and polarity, as well as changing teh channels being used on the U3. None of these seems to change the apparent voltage. WHilst measureing teh voltage at the terminals with a voltmeter, around 1.3 volts appears at the terminals. I have tried placing a resistor across the terminals to see if that would help. again, no change. Thank you for your advice. Best wishes Mike

#2 embayweather

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Posted 06 January 2013 - 05:47 AM

I see it has everyone stumped. Perhaps if I ask in a different way then it may be more helpful. I have a device with around a 1.33 V output and I need to msure and record that with Labjack U3. How is this best done? Without any connection at all the channel I am using (11), is showing around 2 Volts, although very variable. I have tried other channels and they are all the same. What can I do to eradicate that voltage and measure the 1.33 Volts? Thanks for your advice Mike

#3 LabJack Support

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Posted 06 January 2013 - 10:57 AM

I was not able to find more details for the 244R (only found 244ER which appears to have 4-20mA current output not voltage output). Can you provide more details about the electrical specs and connections, or post a link to such information? How do you have your 244R connected to the U3?

#4 embayweather

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Posted 07 January 2013 - 02:24 PM

I was not able to find more details for the 244R (only found 244ER which appears to have 4-20mA current output not voltage output). Can you provide more details about the electrical specs and connections, or post a link to such information?

How do you have your 244R connected to the U3?

The details for the 244R are no longer available I believe, I have not found any myself. However, the intererior workings are very similar to the newer model, and does produce an output in the 4 - 20ma range. This range I normally have no problems with and indeed I have a number of sensors already using it with the U3The temperature transmitter is connected to a three wire PT100 attached to teh appropriate input. Using an 18V power supply witht he positive output to the transmitter , the negative pole of the psi is connected via a small home constructed module as can be found at http://labjack.com/s...s-guide/2.6.3.6, figure 2.4. With this arrnagement I am getting around 1.33 volts at the output across R2. This value is quite consistent and varies with temperature. are there some settings I might have missed somewhere , or is it I am plain unlucky?

Best wishes

Mike

#5 LabJack Support

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Posted 07 January 2013 - 03:11 PM

Without any connection at all the channel I am using (11), is showing around 2 Volts, although very variable. I have tried other channels and they are all the same. What can I do to eradicate that voltage and measure the 1.33 Volts?

The reading from a floating low-voltage analog input such as EIO3 is pretty much meaningless:

http://labjack.com/s...s-guide/2.6.3.8


My problem occurs when I connect it to the Labjack U3 in that the voltage measure by teh U3 varies between 0.1 volts to 2.2 volts on a pseudo sine wave pattern ... WHilst measureing teh voltage at the terminals with a voltmeter, around 1.3 volts appears at the terminals.

So if you connect the DVM probes to EIO3/GND on the U3, you measure 1.3 volts, but the reading reported by the U3 is varying from 0 to 2.2 volts? Could be that the voltage is actually varying and the slow DVM is giving you an average.


Using an 18V power supply witht he positive output to the transmitter , the negative pole of the psi is connected via a small home constructed module as can be found at http://labjack.com/s...s-guide/2.6.3.6, figure 2.4.

I don't quite follow this. You don't want the voltage divider, you just want a shunt resistor to measure current:

http://labjack.com/s...s-guide/2.6.3.7

#6 embayweather

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Posted 09 January 2013 - 04:03 PM

I am sorry I did not make things clear enough. The voltage coming out of the PT100 transmitter is around 1.33 volts, steady, except when the temperature changes of course. I have measured that via a voltmeter at the output terminals. I would have hoped that when introducing this voltage to the U3 it wold then be picked up by the U3 and recorded as usual. Instead of being a steady voltage as I mentioned it varies considerably. Using the circuit from the reference I sent you, which has worked admirably with other devices, I had hoped all would be well. As it seems at the moment the input is not being detected by the U3. Is it possible that I need another resistor across the input terminals to the U3 channel in addition to those already in the device? Perhaps I may need a higher value resistor in the out put of the device to sink more of the voltage across the terminals. I was looking at voltage dividers initially as the PT100 transmitter natively outputs 5 volts ot thereabouts, clearly too much for the U3, hence the need to build my own voltage divider to make the output from the PT100 transmitter come within the range of the U3. The PT100 transmitter is powered by an 18 volt Dc, stabilised power supply . The DVM was certainly not giving me an average reading as I have confirmed the voltage by other methods (eg oscilloscope ). Any guidance you could offer would be greatly appreciated as I have a number of these and similar devices to connect to complete my weather station. Thank you for your patience Best wishes Mike

#7 LabJack Support

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Posted 10 January 2013 - 04:03 PM

Did you look through Section 2.6.3.7? From what I understand, you have a current transmitter not a voltage transmitter, so you should be using one of those figures. Perhaps wire up a simple current loop without the labjack, but with your DVM in series, and confirm that the transmitter gives you 4-20 mA. Then you can add a resistor to the loop and measure the voltage across that resistor.

#8 embayweather

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Posted 12 January 2013 - 04:42 PM

Thank you for your helpful suggestions. I did not think to try to measure the curretn directly using my meter. I did that and discovered all was well with the device. However, all was not well with the breadboard as it was faulty and thus causing a major problem. there will be a short break now whilst I get a new one and try the setup again. But I think that that has cracked it. Once again, many thanks. Best wishes Mike

#9 embayweather

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Posted 16 January 2013 - 02:26 PM

I did as you have suggested and found a nice, steady responsive current coming out of the device, around 16.5ma. Varies well with a change in temperature too. i took delivery of an LJTick Current shunt as well. I hooked up the shunt to the channels 12 and 13 on my U3 extender, plugged in my device to channel 12 and , well , nothing. Tried the current measurement at the end of the wire again and all was fine, its just that when I plug it into the LJTick, nothing comes out, I read a steady voltage of around 0.02 v. Clearly I am missing something here. I have consulted the data sheet on line as well as a number of other sources but still not making progress. Have you any more brilliantly simple ideas like the last one please? Measuring separately across 50 Ohm resistor away from the U3 I get around 0.8 Volts, variable with the temperature at the probe. Thanks Mike

#10 LabJack Support

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Posted 16 January 2013 - 02:35 PM

Again, I would refer to Section 2.6.3.7:

http://labjack.com/s...s-guide/2.6.3.7

One side of the shunt should be at GND or SGND, and then you do a single-ended measurement.

#11 embayweather

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Posted 16 January 2013 - 04:36 PM

OK thank you. I have tried that many times, and have tried it five more times since your post, religously following the circuit in Fig. 6.2. Again All I have is the very variable voltage, yet when measured away from the U3 I get the levels I expect. However, I will not trouble you further, I will experiment a lot more and work towards a solution in another way. Thanks for your help Mike

#12 LabJack Support

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Posted 17 January 2013 - 05:25 PM

Sorry to keep repeating, but you need to look at Figure 2.6-4 or Figure 2.6-5 in Section 2.6.3.7:

http://labjack.com/s...s-guide/2.6.3.7

Not Figure 2.6-2 in Section 2.6.3.6.

#13 embayweather

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Posted 24 January 2013 - 04:19 PM

Thank you for your reply. This I have done many times, referring back to the illustrations you linked in to. I still have the same problem. It appears that there is a ground/earth problem as I am getting 1.6 volts across the earth for the temperature transmitter and Labjack ground. I guess that it is this that is causing the difficulty for me, even with the LJTick Current Shunt installed. Best wishes Mike

#14 LabJack Support

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Posted 25 January 2013 - 08:08 AM

If the 244R is similar to the 244EH and244ER, then it is a 2-wire sensor and is connected as shown in Figure 2-11 of it's datasheet:

http://www2.emersonp...9-0100-4737.pdf

Since you now have the LJTick-CurrentShunt, you should use Figure 6 from the LJTCS datasheet:

http://labjack.com/s...shunt/datasheet

Make sure your 18V power supply is isolated. If it is an AC/DC supply, it will typically be isolated if you just use the +/- DC outputs and do not use a ground on the power supply.

Remove all other connections from the U3 except USB. Connect the LJTCS to a low-voltage AIN block such as FIO4/FIO5. Then connect as follows from Figure 6:

PowerSupply+ to Sensor+
Sensor- to INA+
INA- to PowerSupply-
INA- to SGND

Does this work? If it is not working, troubleshoot by putting a DMM in series between Sensor- to INA+ to measure current. Are you not getting the right current, or are you getting the right current but the LJTCS is not producing the right voltage?


If that setup does not work, then disconnect the wires going to the LJTCS that come from Sensor- and PowerSupply-, and instead put a 100 ohm resistor across those wires. You can check if this basic circuit works by using the DMM in series again, and also check by using the DMM to measure the voltage across the 100 ohm resistor.

#15 embayweather

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Posted 30 January 2013 - 05:23 PM

Thank you for your comprehensive reply. I have undertaken all the work you have suggested, and made absolutely sure my system was connected up as per the FIg 6. I managed to get a stable voltage across a resistor at the terminals of the U3, a voltage which varied with temperature, when connected and un connected from the U3. However, I noticed that the U3 was continuing to give me the varying voltage. I did notice something very strange when disconnecting the set up, inasmuch as I was getting the varying voltage even with only the connection to FI06, and no earth connection. AS stable voltage still exists across the resistor which still remains connected to teh temperature transmitter . It remains there even when the power supply to the transmitter is turned off. I guess that the signal is coming from insde the U3 in some way. Perhaps as you suggest from one of the many other device connected. AS it is a very complicated set up I will have to take these out one by one and watch the effect. I will keep you informed. Best wishes Mike

#16 embayweather

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Posted 30 January 2013 - 06:04 PM

I have now systematically disconnected all the devices in turn, including the LJTicks plug ins, again with no change to the out put of the single wire in to FI06. Disconnecting the wire at the transmitter end still keeps the voltages changes going. Connecting a small wire, i.e. about 3" in length gives me a smaller fluctuating voltage. It does seem to suggest that the connection is acting like some form of antenna, rather than a pure connection. It is located near to my computer monitors, i.e.an iMac and a PC monitor. Is this recommended? Thanks Best wishes Mike

#17 LabJack Support

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Posted 31 January 2013 - 09:09 AM

I was getting the varying voltage even with only the connection to FI06, and no earth connection.

I don't understand? You have an LJTick-CurrentShunt connected to FIO6/FIO7, correct? If you wanted to test without a signal, you could remove connetions from the LJTCS and instead jumper INA+ and INA- both to GND. This would result in something close to 0 volts, but is not a very useful test.

A better test is look at Figure 6 from the LJTCS datasheet, but replace the sensor with a 1k resistor. That means you should have a current of 18mA in the loop, which means the output of the LJTCS should produce 2.12 volts.

#18 embayweather

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Posted 31 January 2013 - 09:56 AM

I have about 8 or nine different sensors going through the U3, one is going through a LJTick amplifier. The input from the temperature transmitter is going in through FI06, which does not have the amplifier in. Essentially I did as you advised and removed all the connections other than the temperature transmitter. I was still getting the variable voltage which bore no relation to the output from the temperature transmitter. I was monitoring that through a DMM. I did find that as I was disconnecting the connection of the transmitter to FI06, when I had removed the GRND connection, not the input to FI06, I was still getting a variable voltage reading on the FI06 terminal measured with my computer. Thus although there apparently was not a complete circuit from the temperature transmitter in place I was still getting a voltage reading at that terminal. I disconnected the FI06 wire at the temperature transmitter end also and the voltage did not changed. that meant I had just a single wire hanging out of the U3 and I wa apparently getting a reading out of the UŁ via the computer. I replaced that wire with one about 3 inches long and again got a variable voltage, much lower but still varying, out of FI06 with one wire, unconnected at one end, and only connected at the U3 end. It appeared to me that is seemed to be acting like an antenna. My question therefore is can this happen, especially whilst in close proximity to my iMac on which I have the Labjack and the DAQFactory software running? It seems that that is where my problem lies in an induced voltage of some form in the U3. Thank you for your advice. Mike

#19 LabJack Support

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Posted 31 January 2013 - 02:01 PM

A voltage reading is the difference in potential between 2 points. If you only have 1 point, you are looking at something that is floating. For example, imagine trying to use a DMM to measure a voltage with only using the red probe and not using the black probe at all.

The following all result in FIO6 floating:

- Nothing connected to FIO6.
- A wire hanging out of FIO6.
- A signal connected to FIO6 that is not referenced to GND in any way.

When FIO6 is floating, the voltage is undefined, so a reading is meaningless. See the following:

http://labjack.com/s...s-guide/2.6.3.8


Back to your testing. If you have a 2-wire current loop sensor, and are using the LJTCS, you need to use Figure 6 from the LJTCS datasheet. If you are not using the LJTCS, then you need to add a load resistor and use Figure 2.6-4 from Section 2.6.3.7 of the U3 User's Guide (just use 0 ohms from the bottom of the resistor to SGND, which means a jumper wire).

Either way, a good general troubleshooting technique is to replace the sensor with a 1k resistor to get a simple fixed current.

http://labjack.com/s...shunt/datasheet

http://labjack.com/s...s-guide/2.6.3.7

#20 embayweather

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Posted 18 February 2013 - 07:41 AM

Managed to fix the problem. I installed a galvanic isolator between the psu and the temperatiure transmitter and all is well. Nice smooth curve working as it should. Thank you for bearing withe me on this problem. Best wishes Mike


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