Nozzles with a slot for a thermistor or a groove for wrapping nichrome?

Hello,

I am wondering what the opinion is for using a nozzle with a slot milled in it to hold the thermistor. What I am thinking about doing is to take a Large Brass Nozzle (BigHead style) and mill a 1/8" slot in it to hold the thermistor.

Also, I could make some with a groove in them to allow nichrome wire to be wrapped around them.

Due to the space available, there is only enough space for either a slot or a groove.

If there is any interest, I'll make them available.

Thanks,

Brian

Edited 1 time(s). Last edit at 06/17/2010 11:44AM by reifsnyderb.

Actually, something that has been working flawlessly for me is a Nasa technique for attaching thermocouples to rocket motors -

[www.techbriefs.com]

Basically you drill a 0.5 mm hole in the side of your nozzle, not all the way though, insert the 2 thermocouple wires, and peen them in there. It works great, and if you use non-welded thermocouples, you get an automatic fail-safe - if one or more of the wires comes loose, I get an out of bounds temperature reading, and my software knows to shut down the heater. Hasn't happened yet.

This requires a thermocouple and the thermocouple measuring circuit, but it's way more accurate than a thermistor, tougher, and you can cut your thermocouples into many short thermocouples.

Wade

Do the wires need to be electrically insulated when they are inserted into the hole?
I have been trying to figure out how to get the temperature right at the tip of my nozzle.
I am using 36 gauge thermocouple wire, so I can certainly make a small hole right at the tip.

Nope, the thermocouple wire should be bare at the tip. It doesn't matter if the two wires actually touch inside the hole, as long as there is a K to Brass to T connection in there somewhere, you'll get the average voltage. This does require that your nozzle is electrically insulated from the rest of your electronics though - make sure you don't have a short from the nichrome to the nozzle. And if you use furnace cement, be aware that you'll get strange readings from the thermocouple until the cement cures - the wet cement and the different metals make a bit of a wet cell.

Wade

Hello,

I hadn't thought about a thermocouple.

Drilling a 0.50 hole near the tip could also be an option as there is plenty of room for one.

Regards,

Brian

Thanks Wade. I sort of figured the nozzle would have to be electrically isolated, but was not 100% certain. For now I will just attach on the main body of the barrel and insulate with kapton, like I have been doing.

Reif, thermocouples are very easy to use and you can make them smaller than any thermistor. The wires on mine are about 5 thousands of an inch. I built a small capacitive discharge spot welder to weld them together and I wrap them in 1 layer of Kapton tape. I do work under a stereo microscope when I do this since the wires are so small. Working with a larger gauge would not be much different. In fact, as Wade has mentioned, they do not really need to be spotwelded, but it makes for a more secure bond if you do.

... for temp sensing i use mostly PT100-resistors as they're linear and i can measure until 800 centigrades.

The smallest i found were flat SMD-types with 1mmx3mmx0.5mm ceramic bed, but the best reliable are cylindric ones with 2mm diameter and 6mm length.

With the linear characteristics they're easy to calibrate, but i use them mostly for my industrial process temp-controllers from Watlow, where i can select the sensor between different types - JK, thermocouple, PT100/PT1000 or others more ...

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Viktor
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@Viktor...Have you got a part number for either of those resistors? What would you need to use one. Just an ADC pin?

That is very intriguing.

Hi Arvin,

i've got them from Heraeus as samples, but you can find them in every electronics shop - search for PT100 and then select the grade A-types, which are specified until 500 centigrades ... but they are really pricey with 6 to 15 Euros

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Hello,

I just sent out a nozzle with a thermistor slot and a groove, for testing. Hopefully, the results will be positive.

Regards,

Brian