A word about calibration...

[Laser_Ben]

I want to say that I am not trying to bash any products out there, I think it's great that there are low cost meters on the market made by forum members, these are a great asset to the community.

I have seen a lot of people on the forum asking how to calibrate a home made power meter or saying they have tested something with a calibrated meter made by a forum member. The truth is, it's not calibrated.

When a meter is calibrated, it is setup in a temperature and humidity controlled environment and the detector is tested at several wavelengths each at several powers using a know good power/energy meter which is certified by NIST. Every laser is checked before and after testing on the detector in question. Every detector is different and every one gets a different calibration curve, and ratios.

Sometimes you may find a meter on ebay that was calibrated several years ago and has not been used for a while, you may thing it will still be calibrated and accurate, this is not the case. There are many things that can lead to a detector becoming inaccurate. The thermal detectors have specific coatings which allow them to absorb a set amount of light, silicon detectors have a limited lifetime, parts degrade including insulation, thermal compounds, and other materials. The detectors may appear to be fine and may work fine, but they are not calibrated unless they are still within the calibration period.

Every major company understands the reasons to have test equipment regularly calibrated, it's so they can insure consistency and repeatability in their products and be able to give customers exactly what they buy. In some cases, it can cause a lot of problems if you end up selling a product that does not meat the same specifications as you state when selling it.

Some people say that just because the calibration has expired does not mean that accuracy is off. It's the same as saying, just because the safety on my gun is broken doesn't mean it's going to shoot someone accidentally, it just makes it way more likely.

I would encourage people on the forum who own meters from companies who are still around to have their meters calibrated. It usually does not cost much money, I received my meter and head back from Coherent from calibration, it cost about $250 plus shipping. It is worth having done as you can be sure of whatever you measure.

Again, I am not trying to bash anything out there, I am just saying you cannot call it calibrated unless it is NIST traceable.

 

[Cyparagon]

The calibration of rulers is is also especially important.

 

[GooeyGus]

Mine is NIST traceable. I should know, I did it myself :evil:

You don't necessarily need lasers to calibrate a thermal head, but like ben said you do need a temperature controlled chamber (in which the TC and temperature reading device is also calibrated) and having a calibrated power meter makes things a lot easier (because it can be used as a standard)


Just in case anyone was wondering, I am an ASQ/ISO 17025 certified calibration tech. I'm in charge of alllllll calibration for a large aerospace company.

Finally a thread where I can REALLY say I know what I'm talking about :crackup:

 

[Xplorer877]

For me having a LaserBee was good enough. If was fairly accurate and regardless of it's calibration relative to other meters it still gave me a means of comparing my different lasers.

400th post

-Tony

 

[ElektroFreak]

Only a few people even would have a need for a meter that's properly calibrated. It's not like the day the "calibration period" ends the readings go off completely. The might only be off by a tiny fraction most of the time. Professionals probably should be concerned with having their meters calibrated to this degree of accuracy, but the hobbyist really only needs a close ballpark.

 

[Deleted member 8382]

let's say my meter was calibrated long ago and now is reading 5mW under the real power. is it really worth 250$ to fix it?

 

[LarryDFW]

LaserBen;

My Scientech thermal power sensor has built-in power input terminals for calibration.

When I received it, I carefully calibrated it with a new quality multimeter.

The black velvet thermal coating on the thermopile looked very uniform.

I assumed I would have very accurate, repeatable results.

One very cold night I was testing a bunch of Red laser diodes, and getting strange results.

After pulling my hair out for half an hour, I noticed readings changed when the central heating system cycled on and off.

Ambient temp was cool, and the heating vent air flow was considerably warmer. This played havoc with my power readings.

I had learned the reason for a temp. controlled environment.

Now, I try to test when inside and outside temps are similar.

I also have turned off the central HVAC to maintain more consistent temps.

LarryDFW

 

[lasersbee]

When we first started with Laser and Thermal LPMs we found the
same problems with air currents... uneven heating and even lighting.

When we calibrate the LaserBee products we make sure:-
1) the test area is at a stable temperature.
2) the test equipment is warmed up for at least an hour.
3) all fans and heating systems and non-required lighting are turned off
during calibrations.
4) we do all tests 90 degrees to the beam travel to minimize body heat
pickup by the Thermopile Sensor.

Jerry

 

[Curiously_Coherent]

I imagine for the builders and DIY folks that calibration is important, in order to have some credibility in a market full of exaggerated claims. Thanks to Justin at LaserGlow, (who let us test my meter against his pro Coherent), I'm pretty confident that my good ol' Kenometer is at least in the ballpark. Do I need it to be precise? Not really. For me, I just need a meter to track the degradation of my lasers over time and test out their output using different batteries. When I test them, I do them all at once under the same conditions. I don't much care whether that's NIST-accurate, I just need somewhat precise comparisons between my units over time.

I have no illusions that the readings are world-class. But as a hobbyist, they serve my purpose and alert me when diodes and/or crystals are doing bad, or one brand of batteries is better than another as a power source. That's why I bought it, and I can recommend a good meter to anyone serious about this hobby.

The calibration precautions Jerry lists above are also great advice to keep your readings as consistent as your meter will allow. That's why I love this forum. We hobbyists get solid advice from pros like Gooey, Frothy & Jerry, to make the things we can afford perform better.

Kudos to all the smart guys! :bowdown:

Cheers, :beer: CC

 

[HIMNL9]

What about , when you still have your LPM calibrated, to make a laser unit for test periodically the reading, so you can be sure that the meter is still working good ?

I was working on one of these ideas, just cause i had to send my old Coherent to a lab for calibration (2 years after the expiration date, poor old meter ..... but i don't have enough money for make it calibrated all the years, so i'm studying a possible DIY alternative ), and they damaged it (they said they dropped the unit ..... i don't know if it's true or if they just messed up with it and broken it, it's not a Coherent lab ..... anyway they admitted that it was their fault, and promised to get another one identical for me in exchange for free, but are passed 3 months and i'm still waiting ..... crossing all ten fingers ) ..... anyway, the basic idea was to take 3 lasers, a GGW, a LCC and a green module, made a very well stabilized thermal block, some extremely stable driver circuit with 2 different powers for each one of them, like, 15 and 150 mW, and an external feedback circuit for it, and to use it just for check the meters ..... How do it sound, as idea ?


BTW ..... for the funny part, all that calibration cycle resemble the question of the egg and the chicken ..... you need a calibrated instrument for calibrate the laser, but before you need a calibrated laser for calibrate the meter, but before you need a calibrated meter for calibrate the laser, but before ..... (ad infinitum :crackup

 

[daguin]

Scientech has clear instructions for calibration in their older meter's manuals. Their calorimeters have a heater connection just for this purpose.

As far as cool/warm breezes in the room . . . .

The 36001 size calorimeters fit inside the units. One is solid filled with closed-cell foam to insulate the calorimeter from ambient temperature. The other's extended aperture cowl restricts air movement at the thermopile.

Peace,
dave

 

[ninja_tux]

LaserBen;
Ambient temp was cool, and the heating vent air flow was considerably warmer. This played havoc with my power readings.

I had learned the reason for a temp. controlled environment.

Now, I try to test when inside and outside temps are similar.

When we first started with Laser and Thermal LPMs we found the
same problems with air currents... uneven heating and even lighting.

When we calibrate the LaserBee products we make sure:-
1) the test area is at a stable temperature.
2) the test equipment is warmed up for at least an hour.
3) all fans and heating systems and non-required lighting are turned off
during calibrations.
4) we do all tests 90 degrees to the beam travel to minimize body heat
pickup by the Thermopile Sensor.

Yeah I even noticed this after a few hours of using my 365 + 360001, when the resolution is set to 200mW even sticking my finger about a half a meter in front of the aperture raises the reading by about a mW! That may not seem like much but I think it speaks a lot to the sensitivity and potential error of thermal meters like these.

BTW ..... for the funny part, all that calibration cycle resemble the question of the egg and the chicken ..... you need a calibrated instrument for calibrate the laser, but before you need a calibrated laser for calibrate the meter, but before you need a calibrated meter for calibrate the laser, but before ..... (ad infinitum :crackup

Yeah this issue comes up in pretty much all measured aspects of science, precision is everything, but where is the reference? I imagine with lasers that they would use extremely stable (probably gas?) lasers under controlled conditions to properly calibrate meters. Someone correct me if I'm wrong though haha

As far as cool/warm breezes in the room . . . .

The 36001 size calorimeters fit inside the units. One is solid filled with closed-cell foam to insulate the calorimeter from ambient temperature. The other's extended aperture cowl restricts air movement at the thermopile.

Hey dave...I've seen these pictures before in another post of yours, and was wondering, are those something you acquired that originated from Scientech? They look very useful in attempting to reduce error from ambient temperature and air flow.

 

[daguin]

Hey dave...I've seen these pictures before in another post of yours, and was wondering, are those something you acquired that originated from Scientech? They look very useful in attempting to reduce error from ambient temperature and air flow.

They are Scientech products. However, as you have discovered, this series of disc calorimeters is no longer in production. These "housings" are only available (occasionally) as used.

36-0206 (long aperture) & 36-0203 (insulated)

Peace,
dave

 

[Hemlock_Mike]

If you handle the thermo head, wait for it to get a stable reading before checking power. My heads can sense a momentary finger contact on the low power scales.

HMike

 

[lasersbee]

@ n_t.....

If you feel your Thermal head needs extra air flow protection...
you cam easily add an extended shroud to your Head made of
anything opaque...

BTW...

I think it speaks a lot to the sensitivity and potential error of thermal meters like these.

I see sensitivity to detect body heat a Positive... not a Negative...
since it can react to minute changes of Laser Beam power...
It is up to you to place the Thermopile Head into an environment
protected from outside interference...
I could easily make a Thermopile Sensor that will not change readings
by the heat from a human body... but it would not be very accurate
requiring many mW to show a change in readings... which would skew
the actual power of the Laser's beam...


Jerry

 

[ninja_tux]

They are Scientech products. However, as you have discovered, this series of disc calorimeters is no longer in production. These "housings" are only available (occasionally) as used.

36-0206 (long aperture) & 36-0203 (insulated)

As I suspected, thanks for the clarification and information.

If you handle the thermo head, wait for it to get a stable reading before checking power. My heads can sense a momentary finger contact on the low power scales.

Yes, I noticed this too, before I take any readings I let the meter "warm-up" and make sure the ambient reading isn't fluctuating before I zero it out and actual measure the laser's power.

@ n_t.....

If you feel your Thermal head needs extra air flow protection...
you cam easily add an extended shroud to your Head made of
anything opaque...

BTW...


I see sensitivity to detect body heat a Positive... not a Negative...
since it can react to minute changes of Laser Beam power...
It is up to you to place the Thermopile Head into an environment
protected from outside interference...
I could easily make a Thermopile Sensor that will not change readings
by the heat from a human body... but it would not be very accurate
requiring many mW to show a change in readings... which would skew
the actual power of the Laser's beam...

Thanks for the tip! I might try to fashion some protective contrivances in the future .
And I must not have been clear, my bad; I didn't mean to suggest that the heads incredible sensitivity was a negative, but rather a testament to their engineering. Like you said, it's only under controlled conditions (which is the responsibility of the operator; I should have specified "user error" instead) that they can truly live up to their potential and give astoundingly accurate measurements.