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Approximating laser power without LPM - A scientific way

tsk1979

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For starters let me clear two things.
You cannot measure power of a laser, or brightness output of a flashlight, or similar things, by just "looking" at a beam, or seeing how fast something burns etc., etc.,
Accurate measurements require specialized equipment.
What if you don't have specialized equipment?

Here is a way to approximate laser power without an LPM to a degree of accuracy of 10% with ease.

All you need to do is follow the footsteps of Mr. Joule.

So what we would need is joules apparatus and a copper vessel(black painted inside) with about 100g of water, and a thermometer.

We use the forula
W*s = T*4.18*100

W = watts
S = time of exposure
T = Change in temperature of water
100 is the mass in grams.

So if you have a laser which you shine for 50 seconds, and this causes the temperature of 100g of water to rise by 1 degrees, you would get approx power as

W = 1*4.18*100/50 = 9W

If you have a 100-200mw Laser its best to use around 10g of water in a vial or tube with base painted black.

If tiny amount of ink is added to water to make it black, I guess it will absorb the energy instead of copper doing the job and fudging up readings.

couple of 100 years back, Joule used this to measure mechanical power equivalent of heat. Today you can do it to measure Optical power equivalent of heat using his apparatus.
 



Photonz

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sounds like a good idea:beer: but can we get someone with an LPM to compare results (i.e. measure a laser with an LPM and then use the above method and see if they are the same or close)
 

ARG

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Interesting! I would have a go at it but I don't have a Joule's apparatus.
 
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Leodahsan

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The air around the vessel wouldn't affect the calculations? Convection heat dissipation?
 

tsk1979

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The air around the vessel wouldn't affect the calculations? Convection heat dissipation?
Yes, thats why your reading with this method will be around 10-15% lower than actual reading.However, if water is at room temp, this will start affecting more if you are trying to raise water temp by 10 degree or so. For 2-3 degree Delta not much of a difference.
 

tonyt

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I will go home, get my lpm and try this in an hour.

Also, in theory this method is sound.
 
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tsk1979

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I will go home, get my lpm and try this in an hour.

Also, in theory this method is sound.
It better be sound. After all the entire system of Thermo-Mechanical engineering has its basis in what Mr. Joule did!
Mechanical Work <=> Heat
Heat <=> Work

The basis of all engines, heat pumps, exchangers and what not. Heck the Cg of water of a value of 4182 was calculated using this method!

:)

I am excited about the results you get. I think you need to experiment with various vessels, and doing stuff like putting a black ball bearing inside the water container to absorb most of the laser energy.

Start with only 50g of water first of a 1W laser, and see how much time it takes to raise temperature by 1 degree!
 

Cyparagon

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In theory, BRILLIANT! In practice, I'm afraid it's useless. What you're describing is called a calorimeter. Calorimeters don't work at tiny powers like this mostly because of thermal uncertainties (insulation, water impurities, absorption spectra, ambient temperature fluctuations, and others), but also because thermometers are not terribly accurate when it comes to <1 degree ranges.
 

tsk1979

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Cypaagon I just realized that heat capacity of copper is much lower. So temperature of copper can be increased with application of much lower energy. Therefore even 1W laser can be measured to a reasonable accuracy.

In reality my experiment may be totally crap outside the lab. However, why not experiment.

All LPM owners, I am sure you can create a copper 10g pellet and drill a small hole inside, and enclose in insulator. then we will look for 5 degree Rise in temperature.
So if your LPM says 1.2W, and this experiment ways 1W, and we are consistently able to replicate this experiment, we know how to get reading within 20% accuracy without LPM

Of course if you get only 600mW, then this crap, we may need other materials and learn by trial and error

So W*s = Cg*T*M
M = Mass Lets use a 10g heat sink with a hollow cavity to shine laser into
T = Temperature change lets look for 5 degrees
Cg = 0.385

So if your laser is 1W

5*0.385*10 = 20

So 20 seconds of shining laser inside a small heatsink pellet should increase its temperature by 5 degree. You may need to enclose copper in Styrofoam because otherwise it will radiate heat quickly.
 
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lasersbee

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You seen to have a good grasp of what exactly you
want to do.

Why not invest in a calibrated Laser Power Meter and
do the experiments yourself. Then you could report back
with your findings.

Like Cyp stated and I gotta agree...
In theory, BRILLIANT! In practice, I'm afraid it's useless.

and impractical...


Jerry
 
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tonyt

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Like Cyp stated and I gotta agree...
In theory, BRILLIANT! In practice, I'm afraid it's useless.

and impractical...


Jerry
I agree, as I said, the theory is sound.. But as you said.. Highly impractical.
 

Noahwhite2014

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I will go home, get my lpm and try this in an hour.

Also, in theory this method is sound.
First time on the forums and I am learning a lot! But I have a request for you or any other member.

So I have also tried this method with a "200mW" green laser I bought. I was expecting somewhere around at least 100mW seeing as I purchased it for about $55 (I know cheap for a green laser of this strength which is why I wasn't expecting much). So I decided to test this method with multiple lasers and they were all pretty close to what was advertised using this method. Except the one I just purchased. I also tried another method using an IR thermometer and black foil multiplying the change in temperature by 3.13. I used these since I don't have a LPM. I'd like to return the laser I just bought but they don't believe my measurement of 40-45mW through both methods above though it seems quite accurate. I just want to get my money back or a replacement of the proper strength. In order to do so I'll need a picture of a LPM reading at 40-45mW as they think I've measured the power using an LPM. I noticed you have a LPM and a green laser of variable strength. So if you or any other member Who would be so kind as to send me a picture of this measurement? I'd be very grateful, and may be able to offer a small bit of compensation via PayPal for your help! I'm learning a lot and lasers really interest me! :) if anyone can help please let me know through Private message!!

PS: I replied here since I can't find where the new thread/post button is. I'm on an iPhone and am having a hard time :p
 

lasersbee

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First time on the forums and I am learning a lot! But I have a request for you or any other member.

So I have also tried this method with a "200mW" green laser I bought. I was expecting somewhere around at least 100mW seeing as I purchased it for about $55 (I know cheap for a green laser of this strength which is why I wasn't expecting much). So I decided to test this method with multiple lasers and they were all pretty close to what was advertised using this method. Except the one I just purchased. I also tried another method using an IR thermometer and black foil multiplying the change in temperature by 3.13. I used these since I don't have a LPM. I'd like to return the laser I just bought but they don't believe my measurement of 40-45mW through both methods above though it seems quite accurate. I just want to get my money back or a replacement of the proper strength. In order to do so I'll need a picture of a LPM reading at 40-45mW as they think I've measured the power using an LPM. I noticed you have a LPM and a green laser of variable strength. So if you or any other member Who would be so kind as to send me a picture of this measurement? I'd be very grateful, and may be able to offer a small bit of compensation via PayPal for your help! I'm learning a lot and lasers really interest me! :) if anyone can help please let me know through Private message!!

PS: I replied here since I can't find where the new thread/post button is. I'm on an iPhone and am having a hard time :p
Yeah.... right...:whistle:

We are supposed to believe that you have done those experiments
and they were accurate and you want an LPF Member to falsely send
you a pic of an actual LPM screen showing your non verified results
above by shining a Laser that you don't own with the 40-45mw output
so that you claim your Chinese Laser puts out so that you can lie to
the seller....:thinking:

I DON'T THINK SO.....:tsk:


Jerry
 

Livinloud

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I actually agree with Jerry on this one. Tsk tsk on your other thread and asking a community of TRUTHFUL and TRUSTED members to help you with, well for lack of a better word, a SCAM! Yeahhhhh okay, whatever you say
 




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