Measuring grid temp

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David Kunkle
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Measuring grid temp

Post by David Kunkle »

Hi all,

Trying to figure out how to measure the temperature of the inner grid. Best I found in a search is someone used a Harbor Freight IR laser pointer temp gauge for the outside of their fusor. I could aim it through a viewport at the grid, but I imagine I would mostly be measuring the temperature of the inner fusor wall behind the grid. Any ideas? Thanks.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
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Rich Feldman
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Re: Measuring grid temp

Post by Rich Feldman »

Someone here (Andrew Seltzman?) recently posted about using an IR imaging camera to measure the temp of his water-cooled grid.

David, if you expect your grid to become incandescent then there's a much lower tech solution.
It's basically an optical pyrometer (note 1).

Make a dummy grid from the same kind of wire as your fusor grid. It can be a plain flat loop. You will heat it with some low voltage amperes, perhaps using a filament transformer and a variac, until its incandescent brightness matches the grid in your active fusor. The temperature of the dummy grid can be inferred from its resistance change.

It helps if you (or your camera) can see both "filaments" at the same time, at similar distances and with small angular separation between them, through the same or similar windows and (optional) red filters.
The dummy grid does not need to be inside your high vacuum fusor chamber.
For many materials and temperatures, the dummy grid could be in ambient air, or in an inert gas at ordinary pressure. Your bag of tricks might include lenses and fixed or moving mirrors.

Note 1. Here are some references on classical optical pyrometers.
http://www.temperatures.com/opyro.html
http://www.temperatures.com/Howopticals ... cument.htm
The second link has 5 slides, of which number 3 looks like this:
Capture.JPG
For more detail, scroll halfway down this document: http://www.pyrometer.com/Pyro_Optical.html
All models are wrong; some models are useful. -- George Box
Jerry Biehler
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Re: Measuring grid temp

Post by Jerry Biehler »

For a IR thermal gun to work you need a window that will transmit in the thermal IR range, this can be anywhere from 5um all the way up to 13um. There are several window materials that will work, ZnSe, Ge, Si. Typical glasses won't work, even sapphire tops out at 4-5um.
RealBorg
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Re: Measuring grid temp

Post by RealBorg »

I think polyethylene is an excellent window material for infrared.

tom
Andrew Seltzman
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Re: Measuring grid temp

Post by Andrew Seltzman »

I use Zinc Selenide (ZnSe) for my IR windows. If you use a 2" lens from a co2 laser cutter, it will fit in a machined out 2.75 CF rotatable flange ring if you machine the ID to 2".

This is the post with grid temperature measurements:
viewtopic.php?f=6&t=8734
Andrew Seltzman
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JohnCuthbert
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Re: Measuring grid temp

Post by JohnCuthbert »

Those IR thermometers measure the average temperature in their field of view. Only a tiny part of that would be the grid so you would, as David says in the OP, be measuring the temperature of the back wall of the fusor (unless the window is opaque, in which case you will measure its temperature.)
The laser is just to tell you roughly where it's pointing- it doesn't measure the temperature of the tiny illuminated spot.
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Rich Feldman
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Re: Measuring grid temp

Post by Rich Feldman »

David,
I repeat that you need to know your grid temperature range of interest.
If it's incandescent then 1) your regular viewport glass is OK
and 2) that's generally too hot for those IR thermometers.

The "Harbor Freight IR Thermometer" (with laser pointer)
only goes up to +520 degrees C (+968 degrees F).
http://www.harborfreight.com/non-contac ... -8905.html

Suppose you are OK with measuring shades of warm, using an exotic window material that transmits longwave IR.
And your solution to the overly wide sensing angle is to get a thermal imaging camera, with enough resolution to have pixels that see nothing but grid wire.
Several makers of industrial and military FLIRs, including a company called FLIR, are taking aim at the consumer market.

One product is "FLIR One", in the shape of an Iphone 5 case and "now available in Apple stores".
In this video we see one demonstrated, compared with the competition, and reverse engineered: https://www.youtube.com/watch?v=IHQr6F7757c
Here is a funny commercial for it: https://www.youtube.com/watch?v=zrHWwjRruEk
The scene temperature range specification is 0 to 100 degrees C.

Here's a demonstration with a similar thermal IR camera, showing (among other things) it looking through a black plastic bag. https://www.youtube.com/watch?feature=p ... LzSo#t=268
Attachments
ir_bag.JPG
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David Kunkle
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Re: Measuring grid temp

Post by David Kunkle »

Hi all,

Got little busy lately, but thanks for all the replies.

To sum up all the info from this thread:
1) I don't expect or want the grid to go incandescent.
2) 520 degrees C would be fine, but I figured the grid is too small for an IR thermometer to read.
3) The FLIR One might be OK if it can give me at least rough actual temperature.
4) Anyone have a rough figure for what an IR gun and ZnSe window would cost?

It's not an absolute necessity to measure the grid temp at this point, but I thought if I could do it relatively cheap, it would be nice.
I don't mind blowing a lot of $$ if it's a necessity, but it may become that down the road.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
Andrew Seltzman
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Re: Measuring grid temp

Post by Andrew Seltzman »

The window will usually be in the $100-$200 range if you use a 2" CO2 laser lens as the window. Definitely use the 2" diameter though since it is so much easier to put on a 2.75" CF flange.

http://www.ebay.com/itm/New-ZnSe-Lens-f ... 58ad26a881

or do an ebay search for:
ii-vi znse
Andrew Seltzman
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David Kunkle
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Re: Measuring grid temp

Post by David Kunkle »

Andrew Seltzman wrote:The window will usually be in the $100-$200 range if you use a 2" CO2 laser lens as the window. Definitely use the 2" diameter though since it is so much easier to put on a 2.75" CF flange.

http://www.ebay.com/itm/New-ZnSe-Lens-f ... 58ad26a881

or do an ebay search for:
ii-vi znse
Thanks for the info- that helps a lot. I've decided I'll hang onto all the info for now, then put it to use if it turns out I really need to monitor the grid temp. I am, however, going to put in an extra 2.75" conflat nipple onto the chamber. Cheap insurance against having to tear everything apart and clean it up again to weld in one lousy fitting down the road.

Thanks again.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
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