How to measure temperature of a nearby object. Cheap?

As a kid I had a book of amateur science projects in which one of the chapt ers was entitled, "Build Your Own Sidewinder Missile," or something like th at. It involved sawing the top off of a germanium transistor and mounting it in a flashlight reflector. I don't remember if the idea was to measure C-E junction leakage or if it relied on temperature dependence of Vbe, but apparently it worked well enough to detect a hand in front of it, and was even more sensitive to flame. Nutation, triggering, sun rejection, guidanc e, and explosives were left as exercises for the student, natch.

So that makes me wonder if a germanium photodiode or phototransistor would be a candidate for (very) close sensing. Calibration would certainly be a pain.

-- john, KE5FX

The product is called an optical pyrometer.

Oh. A plug-in dingus. Or solder-on. Yes. D'oh.

So, Klaus has said that he can paint the metal thing black -- would it work better to paint the RTD sensor black on one side and stick a shiny sticker on the back side, to maximize radiative coupling? My intuition says "yes".

The shiny sticker on the back, probably. The emissivity of Kapton in the thermal IR is just about 1.0 (except for the Fresnel reflection at the surface), so you wouldn't need the paint on the flex.

Cheers

Phil Hobbs

Yeah, a black-chrome plated cavity, spot-welded to the metal, would solve some problems. Maybe a patina of some sort (black in the IR around your temperature range).

For non-contact, you might also consider the Curie law for magnetization 'near' the Curie point. Maybe you could balance an inductance bridge with the unknown metal, against a heated standard? Geometry will have to be controlled (and any strains in the metal will kill your calibration).

Huh, I love kapton tape. We've been using it to stick down heater wire (twisted pair, phosphor bronze) onto various things and (knock on wood) nary a complaint. I like it for sticking little thermal couples on things too. I never thought about emmisivity in the ir?

Can I use it as a NIR absorber? ND ~0.5... (780-795 nm.) (I'll try Monday.) I need a cheap ~low power ND filter.. not a reflector.

I'm meeting with a photographer (down stairs in our building.) to do some various exposures on B&W film, .. I have no idea what exposure level to "stick in the middle" OK a perfect question for sci.optics...

George H.

Hi Klaus,

Could you use the thermal expansion of the metal part to change the gap on a cap sensor?

BTW, still loving the 5444B.

Hi Klaus,

Could you use the thermal expansion of the metal part to change the gap on a cap sensor?

BTW, still loving the 5444B.

Best Regards,

ChesterW

How about a grazing angle laser beam reflection into a position sensor to track the thermal expansion instead? That way the further you stay away from the surface the better :-).

----- Regards, Carl Ijames

That is a really nice idea, which only needs coating of the NTC pcb to reduce effects of condensation :-)

Cheers

Klaus

Response time can be slow, in order of up to a minute

Cheers

Klaus

We even thought about those stickers that change color with temperature and then we "just" need to detect color. But those stickers probably have very low lifetime and cycle robustness

Cheers

Klaus

Yes, we thought of various mechanical solutions like that one, but that has problems with thermal cycling.

Another one is refraction of light reflected from a surface, which has an angle variation to temperature, but price is way to high to even think twice about that one

Cheers

Klaus

huh? A thermistor or IC temerature probe on thin, waterproofed insulated wires won't work because.....

You could talk to these guys:

Another low-tech method would be a Z5U cap. They have high tempco, see graph on page 2:

You could have that and a coil on the target board and then a circuit that determines resonance on the other side of your 5mm gap. Needs to have auto-cal for ambient though because the tolerance of Z5U caps is large.

Thermal expansion is how bimetallic strip thermometers work, and those are relatively robust. Use a Moire pattern background, with a rotating Moire disk instead of a pointer needle, and look at reflectance?

Unless I'm mistaken folk sure are making life hard. What's wrong with a bead thermistor glued on and just keep the voltages below anything that will conduct through water.

NT

There has been many interesting ideas but for a non contact thermometer chip I have used Melexis parts

There are narrow and wide field types. Melexis uses peculiar sort of I2C bus in their sensors.

Digikey and Mouser has them and many other similar types too.