Silicon Bed Heater Temperature Problems with Marlin 1.1

I bought a new silicon bed heater and aluminum bed does not appear to be getting accurate temperature readings. The product docs clearly states that the thermistor is a "NTC 3950 Thermistor built in the heated bed” with Marlin setting #11 as the setting of choice. But, I am having problems getting correct temperature readings from the bed heater thermistor. The temperature readings all seem to be about 16c degrees HIGH. When the heater is OFF, it would read 41.75c. But when I compare that to my IR Temperature gun, it would read 26.1c.

This is also true when turning on the heater and heating the bed to 55c. The temps goes up as expected to the thermistor reading of 55, but again, when I measure the temp on top of the bed with the IR temp gun, it measures ~39c. After that is steadily maintains what it thinks is 55c.

Just to be sure, I tried the other 3950 Marlin settings (#1, #8, #13 and #75) and all seem to have about the same kind of issue: Thermistor reports about 15-16degrees higher than my IR gun.

To verify that the IR gun is working correctly, I checked it with the hot end temps. The hot end thermistor reads the ambient temp (hot end OFF) as 25.5c. The IR gun, pointed at the hot end tip, reads it a 26.3. Different, but close enough to tell me that the gun is appears to be working correctly.

I would assume that there would be some difference between the place that the bed heater thermistor is and where I did the IR gun readings (center bed, on top). But a nearly 16 degree difference? That seems a little high.

Is there some setting that I can do in Marlin to correct this difference?
Is there another thermistor setting I can try besides the ones above (#1, #8, #11, #13 and #75) ?
Any other suggestions or ideas?

gene
Silicon bed heater (http://a.co/fVSjvr3)
Aluminum bed (https://www.amazon.com/dp/B019BGAB7K)
IR Temperature Gun (http://a.co/4OGZtMX)
Azteeg X3
Octoprint
Marlin 1.1

• PIDTEMPBED is NOT enabled.

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Edited 2 time(s). Last edit at 09/02/2017 08:47PM by gdahilig.

3950 Thermistors are generally 100K @ 25C, Heat water to 25C measure the Resistance.

Could be a bad or maybe it is the defective thermister.

Item:NTC Thermistor
Model:B2-100-3950-1
R @ 25C: K 100K-1%
B25/50:K 3950-1%

"IR Temperature gun" are crap

They cannot measure reflective or metallic surfaces (and most of them the "laser" pointer is nothing like where its looking)

get a thermocouple eg [www.aliexpress.com]

Quote
Dust
"IR Temperature gun" are crap

They cannot measure reflective or metallic surfaces (and most of them the "laser" pointer is nothing like where its looking)

get a thermocouple eg [www.aliexpress.com]

I have a ADM02 with a temperature Probe. What they called a temperature Probe anyway is a thermister hanging an a pair of stiff wires.

Edited 1 time(s). Last edit at 09/03/2017 03:00PM by Roberts_Clif.

I'll have to find/replace my multimeter now

Yeah, it could just be a bad/defective thermistor.

Thanks for the link, Dust! They're pretty cheap, I'll have to pick one up!

I find it interesting that the thermistor difference (at ~ 16deg) is so consistent. When set to 55deg, the measured temp is about 39deg. When set at other temps, the difference is always ~16 off. I concede that the thermistor may be defective, but to be defective consistently? Usually defective products, just don't do what they're intended to do. I would have expected a defective thermistor to just not report any temps or even report random temps, right?

Edited 1 time(s). Last edit at 09/03/2017 05:42PM by gdahilig.