Author Topic: Thermal measurement fixture  (Read 418 times)

ggallant571

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Thermal measurement fixture
« on: May 13, 2018, 12:49:00 PM »
As a result of a discussion in another thread I have started work on a fixture to try an measure the energy disapated by a diode laser. The basic idea is to build a semi enclosed chamber that contains a temperature sensor. Fill the chamber with a known amount of water and shine the laser beam into the chamber. Hopefully it will result in a change in temperature with no damage to the chamber.

First prototype is on the printer. It consists of two parts, base and a lid. The inside diameter is 50mm and the height is 30mm. The lid has two holes. One for the laser beam and a second for a DS18B20 stainless steel temperature probe. The lid is finished and looks good. The base has 3 hours to go!!
 
Plan is to use an Arduino Nano and a small OLED or TFT display for posting results.
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ianchia

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Re: Thermal measurement fixture
« Reply #1 on: May 13, 2018, 05:46:08 PM »
Thank you 🙏 Iíd be delighted to build and beta test your build on the other side of the world. I only have access to an ABS 3D printer though (An Up Mini v1). I hope that will be suitable?

- Ian

ggallant571

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Re: Thermal measurement fixture
« Reply #2 on: May 13, 2018, 10:35:57 PM »
First pass of the fixture shows promise. Put approx 31 grams of water and ran 1.6W laser at full power for about a minute. Water temp rose about 0.7C. Not a good good test because I did not let things stabilize or accurately record the time.

Used a generic Arduino and an old SparkFun color display.

Issues:

   1. The stainless steel probes have slightly different diameters.
       My drill set is in 0.5mm increments. The 6.0 drill is too small
       and the 6.5 is too big. Might need a
   2. 3 hours to print a simple chamber hold water is too long. Be
       nice to find a usable glass vessel. Also could watch the water react.
   3. My scale only does imperial units with a resolution in 0.1oz.
   4. The lid needs a tighter fit to the water chamber.
   5. The DS18B20 cable needs a tie-down to keep it away from the laser beam.
   6. Be nice to add a graph to the display.
   7. Will need a metal plate at bottom as the 1.6W laser beam is heating
       the plastic enough to form small dimples. Also makes little cracking sounds.
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ianchia

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Re: Thermal measurement fixture
« Reply #3 on: May 13, 2018, 10:54:32 PM »
Iíve been thinking about this.

Would a glass beaker suffice? We could measure milliliters pretty accurately with a pippette.

Something like this?

5mL Graduated Borosilicate Glass Beaker Volumetric Glassware For Laboratory

https://banggood.app.link/cOwEqJg9TM

As for some black metallic material - how about a small square of aluminum foil with a light spray of black matt paint. The only problem is that we cannot guarantee the exact thickness of the foil or the layer of black paint, but perhaps the tolerance wouldnít matter all that much? Maybe could paint a 10 x 10mm square of foil and then rest it onto the bottom of the beaker?

Alternatively, cut a 10x10mm square from standard 3mm (at least in Australia) aluminium flat bar, like this:
https://www.bunnings.com.au/metal-mate-10-x-3mm-1m-aluminium-flat-bar_p1063146

A small piece of aluminium would be easily machined to be fairly accurate just using hand tools and some cheap calipers.

We could also conceivably 3D print a lid for the beaker that would hold the probe parallel to the beam as well.

Thoughts?

- Ian

 

ggallant571

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Re: Thermal measurement fixture
« Reply #4 on: May 14, 2018, 05:25:10 AM »
A concern/need is getting the proper amount of probe submerged. The ones I have are designed to be completely immersed. Think that a 5ml beaker would make a good base. Then make a fixture that contains:
    1. Metal (or other) plate to absorb laser energy
    2. Holder for temperature probe
    3. Lid

BTW - there are space constraints if we are to use it with the laser attached to the etching machine.
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wild.bill

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Re: Thermal measurement fixture
« Reply #5 on: May 14, 2018, 05:47:08 AM »
Following with interest:
First thoughts - with water there is always a chance that the laser could make steam and that would allow energy to escape and not be measured. I sometimes put a cup of water under the end of the Co2 laser while measuring something  and have seen steam coming off and felt the water temp didn't rise.

With aluminum you need to be careful because it's an excellent reflector and would reflect energy away, that is why the commercial meters use anodized aluminum because it is not as reflective. I do have some bar like you showed that is anodized I can play with.

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ggallant571

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Re: Thermal measurement fixture
« Reply #6 on: May 14, 2018, 06:00:05 AM »
Nice to have a third voice. I expect you will post a finished working model before I finish my morning coffee.

Couple of thoughts:

   1. Defocus the beam while measuring. We want total energy delivered.
   2. Use a piece of black slate.
 
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wild.bill

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Re: Thermal measurement fixture
« Reply #7 on: May 14, 2018, 10:55:52 AM »
I won't be that quick. I had hand surgery 10 days ago and have not been able to do much. I just go back from a Dr. visit and for the first time in more than a week I can actually use my right hand to use a mouse and type.

I always use a defocused beam while measuring power. With the Co2 at its lowest power and defocused is where I was getting steam off the cup of water.

I like the slate idea, black will help absorb the power without reflecting. My first thought was its way better than water or aluminum. I have a few unused K type thermocouples sitting around and can epoxy one to some slate and do some tests.

We back to doing my once an hour PT.
Laser: was an A3 2.0w TTL L6 Z axis now 2'x3'
          OpenBuilds ACRO 510 w/ 2.0W TTL laser Cohesion Mini Z axis Homing switches
          K40 Cohesion Mini 60W LightObject LPS
          80W Red-Black 500x700

cool is simply a subjective state of mind

ggallant571

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Re: Thermal measurement fixture
« Reply #8 on: May 14, 2018, 05:12:08 PM »
Second attempt at thermal measurement. I ran the 1.6W laser at full power for 12 minutes. The chamber had 40g of water and the temperature rise was 2.7C.

The possibility of recalling anything from my last thermodynamics class is zero. Hope to find an online calculator.
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ggallant571

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Re: Thermal measurement fixture
« Reply #9 on: May 15, 2018, 07:15:44 AM »
From what I found via Google:

         Cal = G * dT
          Ws = Cal * 4.18
         W = Ws / Time;

         G = 40
         dT = 2.7
         Time = 12 * 60

         Cal = 40 * 2.7 = 108
         Ws = 108 * 4.18 = 451
         W = 451 / 7200 = 0.63

Conclusion:
         1. The 1.6W laser is really delivering 0.6W.
         2.  My math is faulty.
         3.  The fixture (procedure) is flawed.

Attached are couple of pictures of current fixture.
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ggallant571

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Re: Thermal measurement fixture
« Reply #10 on: May 15, 2018, 07:18:07 AM »
Attached are the stl files.
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ianchia

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Re: Thermal measurement fixture
« Reply #11 on: May 15, 2018, 06:07:22 PM »
Thank you for the calculations and the files. It sounds like the basic principle for the calculation is something like this?

https://sciencing.com/calculate-time-heat-object-8223103.html

Iíll have a go over the coming week with a glass beaker and a digital thermometer. Iíd like to try and properly replicate your methodology but itíll take 3-4weeks as I need to order in the probe. My only electronic store in town doesnít stock the waterproof version, just the PCB version. Iíll need to get the probe via eBay or banggood and print up the STLs as well.

Thanks for going the extra mile @ggallant571 - Iíll keep this thread updated with my progress in due course.

All the best,

- Ian

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Re: Thermal measurement fixture
« Reply #12 on: May 16, 2018, 05:42:03 AM »
I am thinking that the stainless steel DS18B20 temperature probe may not be the best choice in this application. The ratio of the mass of the probe relative to the mass of water is quite large. At a minimum, we need to account for the energy expended in heating the probe. An additional but untested feature is whether the probe itself is generating heat.

The same but to a lesser extent is the chamber itself. It also must be absorbing energy. Perhaps moving to a thinner wall or even a glass vial would help.
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ianchia

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Re: Thermal measurement fixture
« Reply #13 on: May 16, 2018, 05:52:08 AM »
I was giving this some thought today and did a little bit of research on how lasersí output is measured with water temperature. For high powered industrial lasers, there was a paper discussing an insulated copper chamber with water entering and exiting at two different points and the temperature delta was measured between these two pipes, all of which sounded lovely but too complicated to produce.

It did lead me to wonder about using a better insulated container for the water - perhaps we could put a probe into a thermos where heat loss would only likely be through exiting a custom lid? If we placed a crushed ball of aluminium foil in the bottom, perhaps the beam would be reflected somewhat randomly throughout the thermos container. If we also managed to construct a lid out of crushed foil with a hole poked through with a skewer for the beam, that would provide additional insulation for the lid, forming a somewhat primitive pressure cooker. The probeís wires could trail down the sides of the ball ...

Thatís about the extent of my hacky, kludgy experimental setup, as an idea anyway.

Opinions and editorial thoughts most welcome

- Ian

ggallant571

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Re: Thermal measurement fixture
« Reply #14 on: May 17, 2018, 10:22:38 PM »
I should be prohibited from doing math.
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