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PT-104 Negative Resistance Readings

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PT-104 Negative Resistance Readings

Postby Tone5 » Thu Oct 19, 2017 10:39 am

Hi all

I'm trying to configure a PT-104 data logger to record 2 wire resistance measurements. I have 3 of these loggers and am recording 10 channels. 9 of the channels are giving negative resistance readings and one is giving a positive reading. The magnitude of the readings is correct, but I'd like them to all read positive (negative resistance doesn't make sense).

Can anyone suggest what I may have done wrong in the setup which is causing the negative readings please?

I've referred to page 9 of this guide: https://www.picotech.com/download/manua ... 4.en-2.pdf
It says to connect pin 1 to pin 3 and pin 2 to pin 4, then to connect the wires across which I want to measure resistance to pins 2 and 3.
I've tried swopping the wires connected to pins 2 and 3 around but this doesn't change the polarity of the reading I get (I still get a negative reading).
Any suggestions?

Thanks
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Re: PT-104 Negative Resistance Readings

Postby Martyn » Fri Oct 20, 2017 6:39 am

What value of resistance are you measuring, and how have you got the channel set up in the software ?
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Re: PT-104 Negative Resistance Readings

Postby Tone5 » Fri Oct 20, 2017 9:09 am

Hi Martyn
I'm measuring 10 channels, using 4 PT-104 loggers. 2 of the channels are about 60 ohm and the remainder are in the range 8-10 k ohm.
The channels are setup as follows:
Realtime continuous, repeat immediately, restart delay 1s
1s sampling interval, stop after 500 samples, single reading per sample
Converter type PT-104(USB)
Data type: Resistance 0..375 ohm (for the 60ohm channels), Resistance 0..10k (for the 10k ohm channels)
Circuit: 2 wire
Filter not enabled
I have not set any scaling, alarms or calculated parameters.
Thanks
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Re: PT-104 Negative Resistance Readings

Postby Martyn » Sat Oct 28, 2017 7:29 am

I did a quick test using a terminal adapter moving it across channels, and could see occasional negative readings when actually moving the adapter, but they settled to correct positive values. I have been away from the office for a week so will verify with suitable multiple values in channels.

Can you test with just one logger, just to rule out any interaction between devices, and verify the connections are secure. If using a laptop also remove the power supply so that it runs on batteries.
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Re: PT-104 Negative Resistance Readings

Postby Tone5 » Tue Oct 31, 2017 3:18 pm

Hi Martyn
I've found the problem. Unless I'm mistaken, it seems that the pins on the PT-104 screw terminal adapters don't match the diagram on the datasheet (page 2 of https://www.picotech.com/download/manua ... -guide.pdf).
I identified the terminals which correlate to the pins 1-4 and changed the positions of the wires in the terminals and now all channels give positive resistances.
Cheers
Kevin
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Re: PT-104 Negative Resistance Readings

Postby Martyn » Wed Nov 01, 2017 7:04 am

What did you find the pin connections were ?
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Re: PT-104 Negative Resistance Readings

Postby Tone5 » Mon Nov 06, 2017 12:23 pm

The loggers are being setup at a site which is not my normal place of work. I'll check the pin numbers and their corresponding terminals next time I go there, which could be in a few weeks time.
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