DIY Thermal LPM for under $50

[lazeerer]

No problem buddy.

-Tony

What about the multimeter will that one work For me?

http://cgi.ebay.ca/DT830D-Digital-M...wItem&pt=2_Way_Radios_FRS&hash=item4a9ca99099

 

[TTerbo]

im using a 10 dollar DMM

 

[LazyBeam]

Harbor freight has a good (acceptable) $21 DMM. That's the one I'll be getting. I would avoid the analog MM and $10 DMM... both mine are pretty useless for things we're doing but fine for "around the house" DIY stuff. YMMV

 

[Meatball]

I'm still not sure what kind of paint to use to coat the TEC though.

-Tony

You can use some flat black high temp paint for outdoor grills, or paints for engine headers. Those are found at any auto parts store.

 

[TTerbo]

mine isnt just a cheapie shit one......dads friend gave me it for 10 buks

 

[LazyBeam]

carbon black is an excellent absorptive medium.
I would suggest a matte/flat high temp paint... very light coat.
While the paint is still wet, throw/sprinkle a pinch of carbon black on there.
I'm probably going to try this if I can;t get ahold of some 2% spectralon.

 

[aXit]

Cool thanks buddy.

Another thing I just thought of is that the TEC will generate a voltage proportional to the difference between the two ceramic plates. So the hot side would be the side that the laser shines on and the cold side would be attached to the heat sink. So a consistent reading will also rely on the same ambient temperature of the room at the time it is calibrated. The Laserbee would also be affected by this. It might mean that the reading may shift quite a bit depending on the temp of the heatsink.

If the room was 60 deg and you got 100mW lets say, then if you test again in 80 degs with the same laser the difference would be -20 degs resulting in a reading less than 100mW. It could be significant if it was a really hot day.

-Tony

Actually, the ambient temperature doesn't matter. The laser in this case isn't heating the front plate to a stable temperature above ambient, it is applying power, implying a constant rise of the temperature. The temperature difference you're measuring is the thermal resistance of the junctions between the two plates. Thermal resistance is measured in ºK/W, and as you can tell, that represents a difference in temperature per watt of power transferred through that junction. If you have less power going through the junction, there will be less of a temperature difference between the two plates. The heatsink on the back of the sensor serves only to pull heat away from the back plate, so that the heat can continue to flow through the junction. Technically, it would work without the back heatsink, but let's not try that.

One inaccuracy of this is that the thermal resistance of silicon can change over different temperatures, but I have no information on this, and I'm just speculating.

 

[Xplorer877]

What about the multimeter will that one work For me?

DT830D Digital Multimeter Meter AC DC sound Pocket Mini on eBay.ca (item 320455938201 end time 28-Jul-10 01:29:46 EDT)

I don't think that'll be a good one. It needs to be able to measure 1-1000mV and that one can only do 200-1000mV. As long as it can measure mV accurately then you should be good to go. Also use the same meter to measure the power that you used to calibrate it. Two different meters can be a bit off from each other respectively.

Actually, the ambient temperature doesn't matter. The laser in this case isn't heating the front plate to a stable temperature above ambient, it is applying power, implying a constant rise of the temperature. The temperature difference you're measuring is the thermal resistance of the junctions between the two plates. Thermal resistance is measured in ºK/W, and as you can tell, that represents a difference in temperature per watt of power transferred through that junction. If you have less power going through the junction, there will be less of a temperature difference between the two plates. The heatsink on the back of the sensor serves only to pull heat away from the back plate, so that the heat can continue to flow through the junction. Technically, it would work without the back heatsink, but let's not try that.

One inaccuracy of this is that the thermal resistance of silicon can change over different temperatures, but I have no information on this, and I'm just speculating.

Cool, that's good to hear. Thanks.

-Tony

 

[Xplorer877]

carbon black is an excellent absorptive medium.
I would suggest a matte/flat high temp paint... very light coat.
While the paint is still wet, throw/sprinkle a pinch of carbon black on there.
I'm probably going to try this if I can;t get ahold of some 2% spectralon.

A few days ago I'd agree with you but black carbon and burn white hot if the laser is powerful enough. If you have a radiometer or a Crooksmeter attempt to burn the black veins and you'll see what I mean.
http://laserpointerforums.com/f42/weird-radiometer-burning-effect-52866.html#post738987

-Tony

 

[lazeerer]

I don't think that'll be a good one. It needs to be able to measure 1-1000mV and that one can only do 200-1000mV. As long as it can measure mV accurately then you should be good to go. Also use the same meter to measure the power that you used to calibrate it. Two different meters can be a bit off from each other respectively.


-Tony

OK thanks ill find one better.

 

[lasersbee]

I don't think that'll be a good one. It needs to be able to measure 1-1000mV and that one can only do 200-1000mV[/COLOR]. As long as it can measure mV accurately then you should be good to go. Also use the same meter to measure the power that you used to calibrate it. Two different meters can be a bit off from each other respectively.
-Tony

That's not correct... That meter will work just fine for a DIY LPM...
It actually measures from 1mV to 2000mV on the 2000mv scale
and as to accuracy... any accuracy problems are not in the meter
(even a <$10.00 one) it is everywhere else... IMO

- precision of the passive components
- precision of the power supply
- precision of the Amplifier
- precision of the TEC
- precision of the Broadband coating
- precision of the LPM you calibrate it to..

Those things will throw off your readings a lot more than
the DMM referred to..


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[lazeerer]

That's not correct... That meter will work just fine for a DIY LPM...
It actually measures from 1mV to 2000mV on the 2000mv scale
and as to accuracy... any accuracy problems are not in the meter
(even a <$10.00 one) it is everywhere else...
- precision of the passive components
- precision of the power supply
- precision of the Amplifier
- precision of the TEC
- precision of the Broadband coating
- precision of the LPM you calibrate it to..

Those things will throw off your readings a lot more than
the DMM referred to..

Jerry

So if i want to measure 1 to 200mv aka 1 -200mw i put it in the 200mv section and if i want to measure higher then i put it to the 200 to 2000mv aka mw section?

 

[lasersbee]

So if i want to measure 1 to 200mv aka 1 -200mw i put it in the 200mv section and if i want to measure higher then i put it to the 200 to 2000mv aka mw section?

Read the post carefully...I just stated....

That meter will work just fine for a DIY LPM...
It actually measures from 1mV to 2000mV on the 2000mV scale.

Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[lazeerer]

Read the post carefully...I just stated....


Jerry

OK sorry i got it.:thanks:

 

[lasersbee]

N/P....


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[Xplorer877]

Oh I see, yeah the specs were kinda misleading.

DC voltage 200mV-1000V ± (0.5% +2 dgt)

You're right there are many factors that will affect the accuracy of the meter but that is why the calibration is needed in the first place. There is no such thing as a material that will absorb 100% of the light. Otherwise you wouldn't even a reflected dot. But once calibrated the tolerances won't matter anymore. The accuracy of the meter doesn't matter as long as the same meter is always used. It'll be accurate relative to the Po because the system was calibrated to that particular meter.

-Tony