Strange RTD behavior

D

DenGuleGiraf

New member
Hi,
I have an old Mega-RTD 8-channel RTD DAQ v4.1 board that I just started playing with.
I have some strange readings that I can't really understand.
Without anything connected the channels 1, 2 and 4 returns -259.77276611328125
Channel 3 returns 2.0954757928848267e-07
All channel 5-8 all returns -1324.67529296875

The readings does not change when I connect a 2-wire PT-100 (at app. 110 ohm).
Any ideas?
 
D

DenGuleGiraf

New member
Have now tried with a 1k-ohm resistor and still seing the same readings.
Then switched back to the PT100 and then after a few seconds at -1255 it jumped down to -188 where it then stays..
It seems there is some kind of life, but not something that makes sense..
Could it be due to the board being v4.1 while software is made for newer versions?
I cannot find any info on the old version.. Is it possible to switch between PT100 and PT1000 somehow? or is it only meant for one of them?
 
alexburcea

alexburcea

Moderator
Staff member
Hi,

The RTD card is designed for 3 wires sensors and for and the ADC reference is powered trough sensor so the readings with no sensor are unpredictable because there is no reference for ADC.
If you wish to use a resistor or 2 wires sensor you need to make a sort circuit between 2 pins exactly how is printed on the card to simulate the 3 wires sensor.
The card is designed for PT100 RTD probe so you can not use PT1000.
The software is compatible with all versions.
 
D

DenGuleGiraf

New member
Thanks a lot for you advice, I had already guessed that a short circuit might be needed, so I tried that - unfortunately I still got strange values, so I skipped that again.
Now, I have done these tests again, and it seems there might be some sense in it.
Channel 1, 2 and four shows -259,7-ish with small variations.
Channel 3 shows 0.0000
The other channels seems to be alive, but with results around -188 at room temperature but increasing to -185 when hated by hand..
Could it be a malfunction in channel 3 that ruins channel 1, 2 and 4. and then a range issue on channel 5-8?
I have checked the resistance of the RTD. It is around 110 ohm at room temperature.
 
alexburcea

alexburcea

Moderator
Staff member
The calibration must be done in 2 points . Make a sort circuit between all 3 terminal and run "rtd 0 cal 5 0" (if the card was not repowered since last calibration command).
I did not realize how old this card it is.. but still needs to work. If we find something wrong with this card, we will replace it with the last version because we do not have this type of card .
 
D

DenGuleGiraf

New member
OK, I tried to do a two point calibration by first shorting all three terminals and 0 calibrating, then removing the short, and calibrating by measured value (110 ohm). Result as below..
1706021171084.png
 
D

DenGuleGiraf

New member
Hmm. After repowering it is showing -1100'ish (quite jumpy though - seems to jump between -1050 and -1145). Calibration does not change anything.
Tried moving it to channel 6, this shows same pattern.
Also tried on channel 4, but this just lies flat on -259.772766 as it always seems to do.
 
alexburcea

alexburcea

Moderator
Staff member
Please try to reset the calibration "rtd 0 calrst <channel>"
and run "rtd 0 readres <channel>" for diferent channels while your sensor or short(on 3 pins) is connected
 
D

DenGuleGiraf

New member
Readres returns 0.0000 for all channels no matter if PT100, short or nothing is mounted.
Have tried the Calrst command and it ruturns an OK, but does not change anything..
I have ordered two pieces of your new input hat on kickstarter, so if you decide to replace, you could maybe ship it together.
 
D

DenGuleGiraf

New member
Ok, just did another test This time, I tried a clean install on another ras-pi.
Now I get clear signals from channel 5-8!
channel 1-4 is still not reacting, with channel 1,2 and 4 returning -259.772766, and channel 3 returning 0.00000.
Are they grouped in two groups? In that case it could be channel 3 ruining the others somehow..
Channel 5-8 must be a software or hardware issue on my other Raspberry. will try with a clean install here as well.
I can survive with 4 inputs, so you won't have to send a replacement, but thanks a lot for the offer.
I am very impressed with the level of service, thanks a lot for that!
 
alexburcea

alexburcea

Moderator
Staff member
Yes, for that version of card we have 2 x ADC, one for channels 1-4 and second foe channels 5-8. We did not use that topology anymore because Texas Instruments did not produce that ADC or if it does you have to pay a lot of money for it. Current card uses 8 small 24bits ADC's.
It is kind of obvious that one of the ADC's is bad so if you need a replacement (new version only available) just let me know.
 
Top