PT1000 temps are way off

paboman

@dc42
yes, the temperature read by the thermocouples are higher than what DUET see.
As you can see by the graph with R4700 the error is higher with higher temperatures.

I am using 3 different instruments, a precision multimeter (Fluke 289), one cheap thermocouple digital thermometer with two type k thermocouples attached and one is a Fluke 62 IR thermometer (my build surface is matte black)
All 3 instruments are giving a reading within 1% of eachother, even swapping the thermocouples around.

I have the 1K resistor provided with the pt100 board and some 15, 45 and 47k resistors.
I tryied to connect the 1K resistor to the bed input and it reads 2000°C

how should I proceed to check the temperature against the table?

dc42

I have just tested 1K and 2K 1% tolerance resistors on a Duet WiFi 1.04 bed heater thermistor connector with the heater configured for PT1000 (M305 P1 P0 X500). The readings I got were:

1K: -3.5C
2K: 266.6C

According to https://www.sterlingsensors.co.uk/pt1000-resistance-table the reading should be 0C for 1K, and even if the resistor has 1% less resistance the reading would be -2.5C. So either my resistor is slightly out of tolerance, or there is a small inaccuracy at low temperatures.

The reading for 2K should be slightly over 266C according to the table, so it is giving accurate results at this value.

So I suspect that your sensor is either faulty or is not a PT1000.

The resistor we supply with the PT100 daughter board is 100 ohms, not 1K.

paboman

@dc42
shouldn't be M305 P0 X500 instead of P1 ?

I will source some 1% 1K and 2K resistors and make the same test here.
I have 4 of this silicone heater ordered so I will test another one...the problem is I cant heat it without attaching it to the build plate, as you know ungluing a silicone heater is really a pain

Thank you for your help

dc42

I'm sorry, I did indeed use M305 P0 X500.

dc42

If it's a PTC thermistor instead of a true PT1000, then you might be able to configure it as a thermistor with a negative B value.

paboman

@dc42 like this M305 P0 T100000 B-3950 C0 R4700 is giving 2000°C

dc42

@paboman said in PT1000 temps are way off:

@dc42 like this:
M305 P0 T100000 B-3950 C0 R4700 ?

Yes, but your T parameter will need to be close to 1010 and the B value around -400. Adjust T to get accurate readings at low temperatures, and B to get good readings at high temperatures. If you need to adjust linearity then you can try adding a C parameter too.

paboman

@dc42 is it possible to ask the firmware to show what resistance is the board reading at a given temperature ?

dc42

@paboman said in PT1000 temps are way off:

@dc42 is it possible to ask the firmware to show what resistance is the board reading at a given temperature ?

No. but if you configure it as a PT1000 then you can use the table that I linked to earlier.

paboman

@dc42 Following your advice and instructions I was able to get reasonable good results!
Can I tell to the heater manufacturer they made a 100% mistake with the sensor or this is still unknown ?

Thank you again, I really appreciate

jckray

@paboman

Can you tell me where you got your pt1000 and what your successful M305 config was?

We are testing pt1000 as well and have been noticing some temp reading oddities. Thanks