Best placement for heatbed thermistor?

Hello guys!
I'm currently building a prusa i3 with a kit I've got from Greg Colbourn at Emakershop.com.
However, I got a heatbed PCB and a piece of glass/mirror and now I'm wondering of where to mount the thermistor to the the heatbed?
I know there is a hole in the hetbed PCB and that some put the thermistor into that hole so it makes contact with the glass.
BUT I've also heard that you quite often remove the glass to clean it. And if I often move the glass won't the thermistor be all messed up if you understand what I mean?

So how should I mount the thermistor? should I still put it into the whole so it touches the glass and also apply some cooling paste to make better contact or should I simply tape the thermistor on the bottomside of the heatbed PCB?

And another question, It is okay if the thermistor cables touches the hetbed PCB on the bottomside right? it wont melt?

Thanks guys!

On the Mendel90, Nophead epoxies it to the bottom of the PCB: [reprap.org] and then insulates the leads with teflon tubing and high temp tape.

The thermistor won't melt at bed temperatures, but the wiring and insulation you choose could behave differently.

You want the thermistor thermally connected to the bed so you get a consistent, representative temperature. Loose in the hole may give different readings based on the looseness of the gap, which is something you can prevent with a solid placement.

Thanks for your answer!

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DaveX
On the Mendel90, Nophead epoxies it to the bottom of the PCB: [reprap.org] and then insulates the leads with teflon tubing and high temp tape.

Alright, according to the pictures the thermistor is going through the pcb hole. I will use Kaptontape to insulate the wires.

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DaveX
The thermistor won't melt at bed temperatures, but the wiring and insulation you choose could behave differently.

But if i insulate each wire with kaptontape and then tape them with kapton tape the wires should be fine right?

Quote
DaveX
You want the thermistor thermally connected to the bed so you get a consistent, representative temperature. Loose in the hole may give different readings based on the looseness of the gap, which is something you can prevent with a solid placement.

Alright so placing it in the hole to touch the glass and at the same time remove the glass after every print would not be a very good solution.
So you would recommend kaptontape it to the bottomside of the PCB, and not stick it through the hole? Sure I will get the temperature of the PCB bottomside but the glass top side will probably be about the same temperature right? (:

thanks again!

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Archelon
Thanks for your answer!

Quote
DaveX
On the Mendel90, Nophead epoxies it to the bottom of the PCB: [reprap.org] and then insulates the leads with teflon tubing and high temp tape.

Alright, according to the pictures the thermistor is going through the pcb hole. I will use Kaptontape to insulate the wires.

Quote
DaveX
The thermistor won't melt at bed temperatures, but the wiring and insulation you choose could behave differently.

But if i insulate each wire with kaptontape and then tape them with kapton tape the wires should be fine right?

Quote
DaveX
You want the thermistor thermally connected to the bed so you get a consistent, representative temperature. Loose in the hole may give different readings based on the looseness of the gap, which is something you can prevent with a solid placement.

Alright so placing it in the hole to touch the glass and at the same time remove the glass after every print would not be a very good solution.
So you would recommend kaptontape it to the bottomside of the PCB, and not stick it through the hole? Sure I will get the temperature of the PCB bottomside but the glass top side will probably be about the same temperature right? (:

thanks again!

In Nophead's picture, it appears to be JB-welded to the end of the 'NOT' in 'DO NOT TOUCH', not through the hole. His maintained guide at [github.com] has some excellent pictures and dscriptions of what he suggests. Perhaps he positioned it off-center to get a place more representative of the entire working surface, not the potentially most central and thermally protected place. Nophead also insulates the bottom with lots of cardboard, so there shouldn't be much of a temperature gradient from the heater to the sensor.

I'm using Kapton on mine without problems, but I haven't the hours of operating time of others.

The temperatures on the top and bottom of the glass, and the top and bottom of the board should be close, but the reliablility is key. Even if they differ by 10C from the top surface to the bottom, if your particular sensor measures the same place the same way every time, it will be fine. When you adjust your temperatures to work well with your plastic, the unknown constant temperature offset from the top, working surface to a reliably mounted sensor would be accounted for in the tuning. If the sensor can move, it might get damaged in the loading and unloading, or measure slightly different things as it moves.

We have built dozens of printers and always just tape the thermistor to the bottom of the HBP with kapton tape.

Thanks guys! I've decided to kaptontape the thermistor to the bottomside of the HBP and also put a layer of styrofoam under the HBP keep the heat from going downwards.

Had to throw this out there in case someone is copying this thread. Styrofoam might melt due to heat, no? Would hate for your prints to start sinking downward mid print. =D

The bottom of my heatbed is insulated with layers of this [www.lakeland.co.uk] and my thermistor is tape to the side of the glass with aluminium tape, the glass is also fixed to the pcb and the mdf sub plate with aluminium tape.

I see no need to remove the glass to clean it I spray it with hairspray.

Ben