Thermistor Reading Too Low

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Thermistor Reading Too Low

Postby snagglepup » 2018-Jan-Wed-18-Jan

This post might not belong in Troubleshooting, but I will start with it here.
I am back to the forum, after having my printer down for ages. I just got the old PB+ v1 back up and running. It has a few mods, now, but is mostly original.

While doing some preliminary tests, I measured the bed temp with a (cheap) IR laser thermometer. I'm not sure if the glass on the bed could have affected it, but I was reading 120 degrees C. The reading from the thermistor was 85 C. It is not affecting my prints, but I would like an accurate thermistor.

What do you all recommend to sort out this discrepancy?
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Thermistor Reading Too Low

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Re: Thermistor Reading Too Low

Postby RetireeJay » 2018-Jan-Wed-20-Jan

IR thermometers can easily be fooled by the nature of the surface they are looking at. The ideal surface is a matte black. But a reasonable substitute would be blue painter's tape. You could put that on the bed just for the purpose of verifying the temperature, and then remove it for printing.

I have a PB+ also, and I seem to recall from my startup days that it was difficult for it to reach 110C for ABS printing, so an actual temperature of 120 sounds very unlikely. That's well above the boiling temperature of water. In fact, that's hot enough to melt most plastics that we use; that's extruder temperature. What happens if you lay a piece of PLA or nylon or T-Glase on the bed? Does it melt?
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
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Re: Thermistor Reading Too Low

Postby snagglepup » 2018-Jan-Thu-08-Jan

I do not have any PLA, nylon, or T-glase. But I have always wanted to see if I could boil a small bit of water on the bed. Obviously this is a lot of thermal mass, even with the small amount I used, so I did not see the same temperatures from before. I saw small bubbles, but it did not really boil.
At the same time, I tried blue painters' tape and saw reasonable temperatures in the low 90s C when the water was almost boiling - that is, from the IR thermometer. The thermistor was reading high 70s.
This tells me that both are somewhat inaccurate, which is what I thought. What can I do to adjust or better place the thermistor?
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Re: Thermistor Reading Too Low

Postby RetireeJay » 2018-Jan-Thu-10-Jan

First off, does your thermistor read reasonably close to room temperature when the 'bot has been sitting with no heat at all for several hours? Given the tolerances of the components and the nature of the circuit, a reading within 3 degrees C of actual room temperature would be acceptable.

Then, look at how the thermistor is coupled thermally to the bed. It should be touching the underside of the heat pad, and for best results you should have some form of insulation covering the thermistor to prevent ambient air from affecting the temperature that it's sensing. You could even go so far as to add some thermally conductive grease improving the thermal contact between the thermistor and the heat pad.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
User avatar
RetireeJay
My next printer is...
 
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Joined: 2013-Jan-Wed-13-Jan
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Re: Thermistor Reading Too Low

Postby snagglepup » 2018-Jan-Thu-22-Jan

Yes, the thermistor reads room temperature normally. It is currently secured by Kapton tape below the center of the bed. Below that is a quarter inch of cork, and below that is the wood of the Y-axis.

I might try that thermal compound, if I can find some around.
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Re: Thermistor Reading Too Low

Postby RetireeJay » 2018-Jan-Fri-09-Jan

So your apparent error at operating temperature is about 12 or 13 degrees? If your thermistor reading is at the low end of its expected error band and your IR reading is at the high end of its expected error band, these readings may actually be pretty consistent.

In the long run, it wouldn't matter if your thermistor was calibrated in "Degrees Snaggle"; all that really matters to you is that you get good adhesion to your bed, the prints don't warp, and that at the end of the day you can remove the prints. You find the setpoint temperature that gives you good results, plug that into your standard routine, and go on from there with a smile on your face. :D

Two caveats:
(1) If your IR thermometer says the bed heats up much faster than your thermistor says it does, then there's a reason to go ahead with the thermal grease. But if they both register essentially the same rate of rise, then there's no need to worry about the grease; you have sufficiently good thermal contact.
(2) If your temperature control is very poor (temperature overshoots, then keeps wandering back and forth across the setpoint by more than three or four degrees) then again you might not have good thermal contact. Or, just possibly, you might have fatigue in your wires and they are failing. But if the wires to the thermistor actually break, the printer will shut down on an error saying the temperature is too low.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
User avatar
RetireeJay
My next printer is...
 
Posts: 4900
Joined: 2013-Jan-Wed-13-Jan
Location: Greenville, SC
Reputation: 490


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