Calorimeter based Laser Power Meter (LPM)

Hey All,

After doing a bit of searching here I was puzzled to not be able to find anybody trying to make an LPM using a simple calorimeter.

One can possibly make such calorimeter using known amount of water, heat insulating container, and something that will be suspended in the middle of the container to absorb laser light and convert it to heat (small dark colored material for example).

Something like this:



For an even heat dissipation throughout the water in the container a small stirrer can be used (or some other way of agitating the liquid inside).
Shoot your laser at the suspended object and at the same time start the timer. Record initial and final temperatures. Using specific heat formula get the energy. Knowing time calculate the Power.

I realize that it is not the fastest and by far not the most efficient way of calculating power, but if will cost you almost nothing to make.

This is not a guide, or tutorial, just a thought.

let me know what you think. If this has been discussed before, I was unable to find the info and I apologize in advance.

Thank you,
gillza

Nice project ... to agitate the liquid could be used a magnetic stirrer to prevent heat exchange with the outside.

Yep that is pretty much what I meant. But this can potentially add some uncertainties too due to heat produced via friction by the stirbar and the bottom of the container. But i'm sure it is a VERY insignificant change.

gillza said:

Yep that is pretty much what I meant. But this can potentially add some uncertainties too due to heat produced via friction by the stirbar and the bottom of the container. But i'm sure it is a VERY insignificant change.

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Nope, the new magnets have a particular shape to minimize this phenomenon

Arayan said:

Nope, the new magnets have a particular shape to minimize this phenomenon

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Even better

I was just wondering if anybody here tried this approach.

Only problem i see is that most lasers here are built with a duty cycle and probably don't want to run for the time required to perform this test.

Mohrenberg said:

Only problem i see is that most lasers here are built with a duty cycle and probably don't want to run for the time required to perform this test.

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This is not necessarily true. Generally the higher powered lasers will require smaller duty cycles, and vice versa. For this testing, the higher the output the less time one will require to measure the heat supplied, thus a short duty cycle.

It is not necessary to change the temperature of the liquid drastically for this to work.

I think it can be done in theory, but the practical side of things make it difficult. Water has a huge heat capacity, and you would need a fair amount of it to suspend the thermometer and stirring bar. Even if you could use only 10 ml of water, it would take 42 watt-seconds to heat it only one degree.

If you tried to measure the output power of a 42 mW laser, it would take over a quarter of an hour to heat the water only 1 degree. The isolation would have to be very good in order for that to be detectable with any precision. One problem is that you need to be able to shine the laser into the water, so you would need at least a window that is exposed.

If i had to build it, i'd take a 20 ml glass vial, and put it in a large piece of styrofoam with a hole drilled in the side to allow you to shine the laser in. Using ink instead of water, the laser energy would be absorbed directly and no target is needed. There are plenty of small stirring magnets for this type of vial, so that wouldn't be a problem either.

Benm said:

I think it can be done in theory, but the practical side of things make it difficult. Water has a huge heat capacity, and you would need a fair amount of it to suspend the thermometer and stirring bar. Even if you could use only 10 ml of water, it would take 42 watt-seconds to heat it only one degree.

If you tried to measure the output power of a 42 mW laser, it would take over a quarter of an hour to heat the water only 1 degree. The isolation would have to be very good in order for that to be detectable with any precision. One problem is that you need to be able to shine the laser into the water, so you would need at least a window that is exposed.

If i had to build it, i'd take a 20 ml glass vial, and put it in a large piece of styrofoam with a hole drilled in the side to allow you to shine the laser in. Using ink instead of water, the laser energy would be absorbed directly and no target is needed. There are plenty of small stirring magnets for this type of vial, so that wouldn't be a problem either.

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Yes you are right. After doing actual calculations the time for low powered lasers needed to heat up just 20mL will be too long. 33.3 minutes for 42mW laser to only raise it one degree, heh well that is too long

Thank you for your post, actually pushed me to do some calculations.

Yup, its a no go for low power levels... but it could be a simple method to measure the higher powereld lasers like the casio 445s. It would still take several minutes to take a measurement - i guess it makes a nice physics class demonstration, but not a very practical meter.