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PicoLog (TC-08) inaccurate reading at higher temperature

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PicoLog (TC-08) inaccurate reading at higher temperature

Postby awtchiang » Mon Jan 17, 2011 2:39 pm

I have noticed that ever since I changed my laptop PicoLog has been reporting rather inaccurate readings at high temperatures. It seems fine at around room temperature and ice water temperature. But when I stick the thermocouple into the furnace (set to 310C, which should give around 300C given the furnace), PicoLog keeps reporting a temperature of around 148C. I have checked the thermocouple and there isn't any apparent damage and I have also tried logging on another computer, and the reported temperature was fine (~300C in the furnace). Is it my computer's problem?

My computer is a MacBook Air with Windows 7 installed on it via BootCamp.

Please let me know there is a way to fix this, I can't keep on borrowing someone else's computer now can I... =(
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby ziko » Wed Jan 19, 2011 5:58 pm

Usually when there is a problem with the temperature readings on a TC-08 it is due to one of the following reasons:

1) You are using it on a Laptop with the power supply connected, thus causing a ground loop issue. If this is the case then try disconnecting it form the mains and leave it floating.

2) The thermocouple has a voltage injected into it which exceeds the common mode rejection ratio of the TC-08, ensure that the thermocouple is getting a voltage to it.

Kind regards
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby awtchiang » Fri Jan 21, 2011 12:41 am

That did solve the problem. Thanks!
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby secretphysicist » Fri Nov 01, 2013 4:26 pm

Hi - the laptop issue sounds like it may help with half of the problem I have been seeing - but how do I know which is the correct readout?

The situation is this:
I'm using the TC-08 to log temperature on an oven. The power supply in my building is never very stable - and we often see issues with the pico readings after leaving the unit on over night or weekends. Sometimes these troughs coincide with power cuts which knock out other analystical kit in the labs (e.g. time stamps on data files from other kit match the pico)

Now, from what you say, perhaps when the power goes off, and the pico is running just from the laptop battery it should be more acurate, rather than less accurate? But how can I test this? - especially as the temp of the oven obviously falls due to the power outage.

Sometimes there are so many fluctuations in a 24 hour period that it becomes hard to know which is the correct temperature. Also, the power may not actually go off, but the pico plots on a given channel may jump bewtween a lower and a higher read, which it holds happily for an hour or so at a time. Both states look stable, but are obviously artifacts of the power surges. Can I test this?

I have a single channel terminal board, but haven't worked out whether this would let me meaure something relevant - if so, what is the simplest thing to check?

Thanks for any suggestions
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby Martyn » Thu Nov 07, 2013 7:26 am

What type of thermocouple are you using ?
What are the thermocouples touching in the oven ?
What are the temperatures involved ?
Are the oven and laptop/TC08 on the same earthed mains circuit ?
Does the lap top power supply have a metal earth pin (3 core cable) or plastic one (2 core) ?
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby secretphysicist » Mon Nov 11, 2013 10:16 am

All thermocouples are K- type, not touching the oven, and temperatures should be between 50 and 150C, depending on time within the schedule.

Yes, the oven and the laptop are plugged into the same circuit, so when the power goes off, it affects both. Yes, it's an earthed power lead. But, it happens even when working in other locations - e.g. where the oven in on a different circuit, as per the example below.

The quirky thing is that on some fluctuations the channels don't all jump in the same direction. I've attached a really strong example in the pic - look at the segment between 3 hours and 3.45 - here the power was being switched on and off at the fuse box to fix the supply on a different circuit in the building. Ch 4, 5 and 6 do different things if you look closely.
Attachments
picolog-31Oct.png
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby Martyn » Tue Nov 12, 2013 3:04 pm

Where do the thermocouple cables go, through screened trunking or just free air ? They may be picking up noise which is taking the TC08 outside it's common mode range.

Do you get strange readings if you are running the laptop on it's battery ?
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby secretphysicist » Thu Nov 14, 2013 5:51 pm

Through free air I guess - what's screened trunking and would it help?

The laptop is old enough that running it on its battery isn't really an option - it lasts about 5 mins...
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby Martyn » Tue Nov 19, 2013 10:44 am

Can you send a copy of the plw file to support@picotech.com referencing this forum topic. We need to look at this in a bit more detail.
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby wintm1 » Thu Dec 12, 2013 11:50 pm

secretphysicist wrote:All thermocouples are K- type, not touching the oven, and temperatures should be between 50 and 150C, depending on time within the schedule.

Yes, the oven and the laptop are plugged into the same circuit, so when the power goes off, it affects both. Yes, it's an earthed power lead. But, it happens even when working in other locations - e.g. where the oven in on a different circuit, as per the example below.

The quirky thing is that on some fluctuations the channels don't all jump in the same direction. I've attached a really strong example in the pic - look at the segment between 3 hours and 3.45 - here the power was being switched on and off at the fuse box to fix the supply on a different circuit in the building. Ch 4, 5 and 6 do different things if you look closely.


We have eactly the same problem Random noise on the thermocouple input. We use a desktop which has a second common earth between the furnace and the PC. We havent yet tied one side of the thermocouple to ground. Our errors start at about 530°C and vary by 100 to 200°C then by 700 they come right again.
We use N type thermocouples that do touch the metal in the oven....... HELP
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby Martyn » Fri Dec 13, 2013 7:28 am

That sounds as if the thermocouples are touching the walls of the furnace, which is possibly ceramic.
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby vampire790410 » Thu Mar 06, 2014 7:54 am

Hi everyone, i have a problem with fluctuations on reading from tc-08 at temperature of 1100 °C. I have a desktop system grounded as the electrical oven we use for test. The oven have an equalization block that is think is aluminum oxide where the thermocouples are placed for isolation. I use two tc-08 for read different channels and we run a test with 6 K type (connect on the 1st device) that fluctuate on less temperature but we solve grounding them. The problem is on the reference temperature a ceramic S type (that is connected to the 2nd device) that also if it is grounded negative wire with capacitor we try on separate and same ground of the other thermocouples, this reduce the fluctuations a +-3 °C but we need a steady read from that. There is any other way for obtain a stable reading? Am i missing something? There is any suggestion?
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Re: PicoLog (TC-08) inaccurate reading at higher temperature

Postby Martyn » Fri Mar 07, 2014 12:04 pm

This does sound as if the change in property of the ceramic at high temperature will be the underlying cause. I am not an expert in this area so can you please contact support@picotech.com
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