Problems setting up bed thermister

I am trying to setup the heated bed for my printer. I am having a few issues. Once wired up I set it to heat to 40C as a test. I monitored the temperature with repetier-host and also an infra-red thermometer. The infra-red thermometer showed the temperature on the top of the bed hit near 50C while the beds thermistor eventually got to 40C.

The infra-red probe showed the temperature slowly drop to 45C and stabilised there, all the while the electronics insisted that the temperature was 40C. When I set the temperature to 45C the infra-red probe showed a jump to 55C(ish) and a drop to 50C then stabilised. It seems that the temperature is reading off by 5C.

I brought this (eBay) thermistor and am using Marlin's thermistor table 1 (on Megatronics 2.0).

Am I missing something?

I think it is something to do with the thermistor. I've been running an experiment on my Huxley. I plugged my new thermistor into the hotend sensor input and tapped it to the top of the Huxley's bed. Then I set the bed temp to 60C and waited.

The Huxley's bed sensor reports it at 60C but the new sensor is reporting 55C (see attached). Would it be something to do with the thermistor table?

Well tried making my own table, using infrared thermometer to give temp and multimeter to give resistance of thermistor. That didn't work, ended up with it displaying temps far far lower then actually was when re-connected with the heated bed.

I am at a total loss and have run out of ideas. is there anyone who can help me, please?

Hi, others will insist that the best thing to do is to use the beta value for the thermistor and the available python script to generate a table. They're probably right as they're more experienced people that me.

I however struggled the same as you and decided to build my own table experimentally. I was able to tee fore Aldo customise the table with closer values near the key temperature points, meaning that I could get more stable temperatures more quickly. I wrote about it on my blog http://blogger.kritzinger.net/2012/04/recalculating-thermistor-tables-more.html

Good luck,
Craig

Found out part of my issue, my multimeter is so far out of calibration it is a bin job!

I used a higher quality multimeter to read the resistances and I am nearly spot on. Thanks for the blog post, kept me going thinking that it can be done

Now I just need to sort out the nichrome wire burning through the kapton tape. I was thinking fire cementing it into place?

smartroad Wrote:
-------------------------------------------------------
> Found out part of my issue, my multimeter is so
> far out of calibration it is a bin job!
>
> I used a higher quality multimeter to read the
> resistances and I am nearly spot on. Thanks for
> the blog post, kept me going thinking that it can
> be done

No problem.
>
> Now I just need to sort out the nichrome wire
> burning through the kapton tape. I was thinking
> fire cementing it into place?
Sounds like you have some of the "fake", or lower temp kaput on tape, as I know others use in chrome directly under laptop.

However, fire cement (exhaust putty) should do the trick if you can get it to stick to the bed surface.(I'm assuming you're still talking about the bed).