A word about calibration...

Generally, the thermoplie surface is a ceramic or metal plate with a special coating. It would likely take a good bit of force to put a hole in the surface.

I don't know about you guys... But I'd swear that was a picture of a tweeter module from an audio speaker. One can easily put holes in those. (although personally, I have a tendency of blowing out the woofers first...:shhh: ) LOL

I've only seen thermopiles with flat surfaces myself. That surface looks pretty spherical to me. Do coherent thermopiles use spherical surfaces?

Just askin'

Cheers, CC

@Laser_Ben: LOL, that guy not gently poked it for see what it was ..... he tried to pop it out for open it, thinking it was an inspection cover



@ Curiously_Coherent: no it's a common thermopile with a flat disk ..... it's only the reflex of the light that made it look spherical, but it was flat.

I think the worst thing about the calibration issue is that you would not know if your meter is getting far out of calibration or not. At what point can you not trust your meter?

I wonder if I could use 3 lasers as a reference to see if the LPM readings change in the future.

• Use 3 cheap but stable red lasers just in case one of them loses power sometime in the future so I could know which one changed. If they all change then the meter might be getting out of calibration. (100mW to 150mW to keep costs down)
• Make sure they were under powered to preserve diode life and only put 1 minute on them every year to check calibration.
• Put brand new batteries in every year before the check.

I realize this would not be good enough for professional use but the hope would be that it would be good enough for someone in the hobby creating their own lasers.

I have zero experience with LPMs right now so I don't even know if this is something to worry about for hobby use or not. If I can devise a scheme to test for this though at a low cost it might be worth it. Otherwise I would need to send the LPM off at some unknown(to me) interval to get checked which is something i really want to avoid.

This all assumes laser diodes do not degrade much when off and stored for years which may be an incorrect assumption. It also assumes 100mW to 150mW red lasers are stable enough to use for this test which I also do not know yet.

Any thoughts from those more experienced than I... which is anyone that has touched an LPM .

Using lasers as references like that would be tricky. The diodes might degrade, the windows or lensens could gather residue or degrade, pretty much at the same rate for all those lasers if you are unlucky. Also, the current regulating circuitry can go 'off' with age.

Temperature is a factor too and would have to be strictly controlled: it affects laser diode efficiency, but often also current regulators.

Meter heads with built in meters dont suffer from any such issues. Even if the heater changes resistance over time, power can still be calculated by measuring current and voltage on the spot.

xStatiCa said:

• Use 3 cheap but stable red lasers just in case one of them loses power sometime in the future so I could know which one changed. If they all change then the meter might be getting out of calibration. (100mW to 150mW to keep costs down)
• Make sure they were under powered to preserve diode life and only put 1 minute on them every year to check calibration.
• Put brand new batteries in every year before the check.

Click to expand...

For our Hobby uses.....

This is actually... not a bad idea of using 3 lasers of the same red
wavelength and similar power...IMO
They must me at least over 100mW... the higher the Better...
And yes... under powered with the same type of fresh batteries...

They would be able to see a variation in calibration by all of them
showing the same change on the LPM..
And can check against themselves to see if any one laser had
changed power...even if the LPm has changed.. the readings of
the 3 Lasers would still be relative..

The Lasers can easily be store is separate Zip lock Bags sealed from
dust and grime...

We use a LaserBee I almost daily in the shop... and as you can imagine
we check the calibration more often than an average user.
We have not needed to recalibrate our shop LaserBee Is for the past
year... As I posted else where... we do take care to not damage or burn
the Thermopiles surface and store the LaserBee I in a dust free drawer..


@ C_C... I think I have 3 of those heads in the Shop...and I think
you have one too... At least that head looks an awful lot like them..


Jerry

The problem is still getting the reference lasers, especially if battery powered, they need a darn good current source to provide the same power regardless of battery voltage.

You cannon reasonably expect batteries to be as good as they were a few years ago - rechargables decay over time, and primary lithiums deplete with use.

If you are using a Current driver then a small difference of input voltage would
be negligible if the voltage has enough over head in case of any input voltage
droop of variations....

After all... that is what Laser current drivers (DDL) do... they supply a constant
current to the LD with changes in input voltage... until the minimum regulation
voltage limit (+ Ref.) is reached...

BTW... xStatiCa mentioned using fresh batteries on every calibration period..

There is no reason a Voltage Regulator and wall wart can't be used to drive the
Current Regulated LD drivers in those Calibration Test Lasers...


Jerry

What about building a stable thermal calibrator, and use it for check your thermopile time by time ..... it require a bit of work and can be efficently done only for some heads (for mechanical problems, mainly), but thermal sets are much more stable than lasers (i'm having some problems exactly for this reason, in building reference lasers, but the thermal reference that i've made for my thermopiles heads never failed, in all these years .....

like: stabilized power supply -> precision thermostatic driver -> stable heating element

all in an enclosure that grant you that only radiant heat arrives on the reading disk, with a thermometer for check the stability and an enclosure that take out the assembly from the air flows ..... this is a bit complex to build mechanically, but can give you a decent reference for check your meters, if well built.

The only problem can be finding an adequate insulation between the thermal element and the reading head, for read only the radiant IR part and not the convection flows ..... i've had the luck to find a very thin piece of optical glass of 0,5mm of thickness, but is not easy to find it, at least here, maybe in other countries is more common ..... was thinking to made experiments with underpowered halogen lamps and lenses, but then had no more time and at the moment all my experiments are stopped ..... maybe it can be done more easily using lamps, instead nichrome wire heating elements .....

Like, as example, using a short-filament 50W 12V halogen lamp powered with 3 or 4 V, for have it emitting mainly in the infrared, and a lens for focus it on the reading element, and using the insulation window for take a reflection of a little part of that flow for read it with a photodiode that give you a reading about the output, and at the same time act as feedback for the lamp power supply, or similar ..... must restart this project, if i find the time, one of these days .....