Robotdigg heatbed with 130k thermistor for Duet

I got this round silicone heatbed with "3950" thermistor. It reads 130kOhm at room temp.
Is it faulty?
Because 3950 is the standard 100k thermistor in my book...

How would I configure RRF dc42 firmware to work with it?

M305 P0 T130000 B3950 R1000 H?? L??

Is "H" the zero offset and "L" is the gain?
I remember those two terms as two point calibration parameters.
-Olaf

Quote
o_lampe
I got this round silicone heatbed with "3950" thermistor. It reads 130kOhm at room temp.
Is it faulty?
Because 3950 is the standard 100k thermistor in my book...

How would I configure RRF dc42 firmware to work with it?

M305 P0 T130000 B3950 R1000 H?? L??

Is "H" the zero offset and "L" is the gain?
I remember those two terms as two point calibration parameters.
-Olaf

Olaf

I suspect that is the std Chinese Thermistor as used by Keenovo and others (The B Value is a 25-50 degree one so isn't quite right at the point that we use them) and using the Temp table that Keenovo supplied me and DC42's spread sheet I found that a B of 4008 is a closer match (at 100 degrees).

The H and L values I think allow you to tweek the temps to be correct (H I believe adjust the Room temp offset) DC I am sure will be along soon to give you the definitive answer to that

Doug

edit I have attached DC's Spreadsheet and the NTC Temp table for the Thermistor HTH

Edited 2 time(s). Last edit at 11/15/2015 03:59AM by dougal1957.

First, make sure that you have the correct R value. R1000 is correct for older Duet 0.6 and I think also for the Replikeo Duet. R4700 is correct for the Duet 0.8.5. If your temperature reading is around 60C at room temperature, that typically means that you have R1000 in your config.g but you need R4700.

If you have the correct R value set then the temperature reading should be quite close to the actual temperature. However, if you have a Duet with the 1K series resistors (R1000 in config.g) then the reading at room temperature may be a little out. Adjust the H parameter to get the correct reading. It can be positive or negative. Another way is to substitute a 100K 1% resistor for the thermistor and adjust H for a reading of 25C.

In theory, all thermistor channels should need the same H value. Any residual variation is probably caused by the thermistor tolerance.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks both of you for the info.
I played around with the heatbed and added a known thermistor as comparison. With the "T" and "H" value, I was able to set it to show the same room temp. ( R1000 version )
But at 65°C the difference was about 20°C.

Gonna try B4008 and see what happens.
-Olaf
The "L" parameter does nothing obvious...?

Edited 1 time(s). Last edit at 11/15/2015 10:29AM by o_lampe.

The L parameter corrects the ADC high end offset, which affects the readings at high temperatures. It's unlikely you will need to use it, especially with a 1K series resistor in the Duet.

What H value did you need? The maximum theoretical value you might need is +/- 100, but normally it is much lower, e.g. +/- 30 or less.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I had a H value of 40, but actually 20 would be best. I had to make a compromise, because of low ambient temperature in my workshop. ( I get temp-error below 10°C )
With B4008, I got much better readings....until I replaced the second thermistor. It suddenly read 30°C more than before. I must have placed it directly on a heat wire.

BTW: I had a 30cm floatglass bed and only 25cm heatpad. The glass cracked after 5min with constant 65°C.
I should have used a aluminum heatspreader too. The hot wire in the silicone sheet creates to many hotspots/ temp-difference.

Where do I get 30cm round borosilicate glass in EU?
-Olaf

Quote
o_lampe
I had a H value of 40, but actually 20 would be best. I had to make a compromise, because of low ambient temperature in my workshop. ( I get temp-error below 10°C )

That's quite common with Duets that use the 1K thermistor series resistor, and it's why the series resistor was changed to 4.7K more than a year ago in RepRapPro Duets. But I don't think Replikeo picked up the change.

So if you trust yourself to replace two SMD resistors without damaging the board, you could switch to 4.7K.

Quote
o_lampe
BTW: I had a 30cm floatglass bed and only 25cm heatpad. The glass cracked after 5min with constant 65°C.
I should have used a aluminum heatspreader too. The hot wire in the silicone sheet creates to many hotspots/ temp-difference.

Where do I get 30cm round borosilicate glass in EU?

Why not get a heat spreader instead? If you heat 30cm glass directly with a 25cm heater, the edges are going to be cooler than the centre. But if you do want borosilicate, here is a supplier: [www.ebay.co.uk].

Edited 2 time(s). Last edit at 11/16/2015 05:29AM by dc42.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].