TC08 (USB) "Crosstalk" between channels

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Dave B
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TC08 (USB) "Crosstalk" between channels

Post by Dave B » Wed May 30, 2018 2:20 pm

I have a problem which is a bit odd with my TC-08 Temperature Logger.
I want to measure a voltage in mV on Channel 1 and measure Temperature on Channel 2.
I have connected up a potential divider on channel 1 which consists of a 1K resistor across the two pins and a 100K resistor in series with the voltage source that I am measuring. I then changed "scaling" so that I the voltage in volts is displayed in mV on the graph.
In Channel 2 I have connected a Type K thermocouple to measure temperature.

When the voltage being measured is reduced the temperature reading reduces on Channel 2 as well??????

Am I doing anything wrong?

Many thanks for your help.
Dave B

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Re: TC08 (USB) "Crosstalk" between channels

Post by Hitesh » Thu May 31, 2018 9:30 am

Hi Dave B,

Are you using the PicoLog 6 software and have you considered using the Single Channel Terminal Board for voltage measurements?

It might be useful for a circuit diagram to show how the connections have been made.

Regards,
Hitesh

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Re: TC08 (USB) "Crosstalk" between channels

Post by AndrewA » Thu May 31, 2018 9:58 am

Hi Dave,

Issue is most likely caused by common mode voltage issue. If voltage is greater than +/-7.5V on the input to USB ground, the channel is pulled out of he common mode range of the TC-08.
This can causes all channels to read very incorrect values. The issue can be on one channel, or between channels.
You need find the cause of the issue by a process of elimination for each channel. But sounds like it is mostly likely the channel#1 with voltage input is causing the issue.
You could check for any AC/DC voltages using a multimeter between the USB ground on the PC/laptop case and the inputs to the channel. (7.5Vpeak = 5.3Vac)
If you cannot isolate the voltage input or thermocouple from the metal surface that is the causing the issue, you can for short term logging, use a laptop running on batteries, that is not connected to mains earth.

Regards Andrew Abbott
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Regards Andrew
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Dave B
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Re: TC08 (USB) "Crosstalk" between channels

Post by Dave B » Thu May 31, 2018 4:40 pm

Andrew
I thought something to do with 0V potential could be causing this. I am going to try a Desktop PC to see if that works any better as its 0V is usually connected to chassis (or almost chassis potential)
Many thanks for your help.
KR
Dave B

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Re: TC08 (USB) "Crosstalk" between channels

Post by mtg » Thu Apr 25, 2019 11:28 pm

I have a similar issue, using a Pico Terminal Board for a pressure sensor connected to a TC-08 with 4 thermocouples also connected. The pressure sensor output is errant and unsteady, and intermittently causes cross talk with the thermocouples.

- If the 5V power supply to the pressure sensor is unplugged, everything works normally
- Checking voltage between the 5V pressure sensor power and the laptop ground reads 0.2VDC
- There is a high powered electrical system adjacent that creates a lot of noise, when it turns on and off, the thermocouple readings jump a few degrees

Any recommendations on what to try next?

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Re: TC08 (USB) "Crosstalk" between channels

Post by bennog » Fri Apr 26, 2019 7:02 am

- Checking voltage between the 5V pressure sensor power and the laptop ground reads 0.2VDC
Have you tried AC measurement on this.

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Re: TC08 (USB) "Crosstalk" between channels

Post by mtg » Fri Apr 26, 2019 7:09 pm

I have not measured AC voltage, will try that next.

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Re: TC08 (USB) "Crosstalk" between channels

Post by mtg » Tue Apr 30, 2019 4:16 am

3V AC are measured between the supply negative and the laptop case ground.

Is there a way to get rid of this voltage between the two without relying on battery power only on the laptop?

Thanks

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Re: TC08 (USB) "Crosstalk" between channels

Post by bennog » Tue Apr 30, 2019 11:03 am

connect laptop ground an pressure sensor ground together should be sufficient.

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Re: TC08 (USB) "Crosstalk" between channels

Post by mtg » Tue Apr 30, 2019 10:53 pm

Connecting the grounds together between the terminal board and the laptop fixed the voltage issue.

However, the TC-08 will not produce any believable values from the pressure sensor. I put in the calibration data from the sensor manufacturer (Omega), which the output signal is ±50mV, and selected that option in the channel properties of the TC-08. It doesn't work. I tried every other option, none produce any real data.

Pico: how can this be made to actually work? I bought Pico terminal boards to plug into our Pico TC-08, followed all of the instructions, obviously spent quite a bit of time troubleshooting, and it doesn't work. Please advise.

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