Hans-Wehr open source CO2 laser power meter

Discussions on optics for laser cutter/engravers

Hans-Wehr open source CO2 laser power meter

Postby lasersafe1 » Sun Oct 21, 2012 4:23 pm

How many of you have wished you had a CO2 power meter, but find them to be too expensive for your hobby laser?
What if I told you that you can measure your laser power using common household items?

Materials required:
1. Styrofoam Cup
2. Wesson Cooking oil
3. Thermometer
4. Timer

Tim Wehr and I (John Hansknecht) had an idea to figure out a method of measuring power that only uses common materials available to any of us. We are calling this power meter the Hans-Wehr Meter. Remember, for all eternity this should be known at the Hans-Wehr CO2 laser power meter.

Oil, being high in hydrogen, will readily absorb the 10.6um light better than almost any other material. Water would have also been a choice, but it is too close to it's transition temperature and would have local boiling. I took two styrofoam cups and placed one inside the other for good insulation. I put 40ml of Wesson cooking oil in the cup. We don't want to focus on the surface of the oil, which might ignite it, so I lowered my z axis so the oil would be a good 3 inches away from the focus. I could have just as well removed the focusing lens. I measured starting temperature 26C. I applied what my 40W machine calls 70% power for 60 seconds. It was completely uneventful in the oil, no smoke or fire(good thing). I then put in my temperature probe and stirred the oil and measured 48C. Clearly, with a known thermal capacity and time, Watts can be derived.

Tim did the following calculation:

1tbl Vegetable oil = 14 g
1 tbl = 14.787 ml
40 ml / 14.787 ml/tbl = 2.705 tbl
2.705 tbl * 14 g/tbl = 37.87 grams in target

Test
37.87 grams * 22 degrees = 833.14 watts
833.14 watts * 1.67 thermal capacity = 1391.34 watts / 60 second
1391.34 / 60 seconds = 23.2 watt/s

So since all calculations are linear, if we choose the right amount of fluid and the right time period, we can come up with a poor man's power meter where each degree C is 1 watt. We are already close with my 60 seconds and 40 ml of oil because it indicated 22 degrees increase, which may (or may not) be 23 watts. We could also measure for a shorter period of time or different volume and calculate accordingly.

What are the unknowns at this point? Is the thermal capacity of Wesson cooking oil exactly 1.67? Is at least 95% of the laser absorbed?

Some might say you need a calibrated laser power meter to prove the result. Actually, perhaps you don't. If you agree that >95% of the light is absorbed by the oil, then you can verify the calibration using a power supply and a power resistor. If you put 1 amp through a 25 ohm power resistor, you have 25 Watts. If you drop this resistor in your oil bath for 60 seconds, the temperature difference will give you the data you need to calculate the actual thermal capacity of the oil. Notice that we are using a Styrofoam cup. This is important, because we don't want the thermal capacity of the vessel holding the oil to be a factor in the measurement.

The end result "laser" power measurement would be at most 5% off if you have 5% reflected away. ....... BUT..., If we, as a community, can agree on a certain simple volume of Wesson cooking oil in a styrofoam cup, then we can all perform power measurements that are easily comparable to each other within a couple percentage points.

This method is also useful for performing a sanity check of your system before engraving or cutting an expensive piece of material to make sure the power is what you expect.

Now you must also realize the following: You need to begin your test with the oil stabilized at room temperature. If your oil was already warm and was naturally cooling off at 2 degrees/minute then you would have a 2 degree error in your measurement. The $500 Synrad laser power meter has a "patented" system method that compensates for this. Before you take a measurement, you must hold it out of the beam and push the "measure" button to zero the meter. When you push the measure button, it reads the aluminum block temperature for about 2 seconds and checks if there is a downward trend. It then takes this into account when it does the "before" and "after" measurements of aluminum block temperature to calculate watts absorbed over time.

Tim and I have been busy with our own projects. Perhaps others on this forum will help work on this Hans-Wehr meter and do the calibration runs. The end goal should be to come up with a recipe that anyone can follow where we get 1 watt per degree of temperature change over 60 seconds. Who sells good cheap digital thermometers? How precise should we be in oil volume? Should we use a volume measurement or a postal scale and go by weight? Is oil brand/type critical or are they all about the same? Can someone with a 100W laser try it without bursting the cup into flames? Do we go with smaller oil volume and reduced time to make the measurement faster? Once we are all in agreement we will make a "Sticky" that gives the standard method for building and using the Hans-Wehr Laser power meter.

Looking forward to your comments... I think...., John
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Re: Hans-Wehr open source CO2 laser power meter

Postby iGull » Tue Oct 23, 2012 7:53 pm

Hi

Thought I'd continue here as it's connected with the previous thread of mine :D
Yes, the idea is quite sound - if not a wee bit presumptious in the naming ! It's just a calorimeter after all - I'm sure Mr Lavoisier or Mr LaPlace would have something to say about the naming convention LOL :D

To make it worldly wide, it would be useful to find a common oil that would suit - I'm unsure what 'Wesson' oil is - canola/rapeseed/groundnut ? I'm supposing that all oils of the same genre don't have the same physical properties - I'm no chemist, but would assume not? (does it even matter?)

I'm all for ballpark measurement - or some kind of sanity measurement - let's stick with SI units for worldwide sanity 'though - we can at least sing from the same hymnbook with them - I had to wonder what a 'tbl' was until I was told it was a 'tablespoon' measurement commonly used in cookery :D

It really would be good to find a worldwide 'standard' oil (don't suppose a McDonald's hamburger would work :-) ) - I'm wondering if '3-in-1' lubricating oil is sufficiently common - even if it would be suitable?
The styro cups are more than likely ok - weighing is probably best, but volume is probably cheapest. You could of course make a styro container to a specific size, but no point in getting anal over what is essentially an uncalibrated measurement (maybe a McDonalds cup - but they're probably bigger in the US :-))

Distance from the lens is an issue as we have varying focal lengths - perhaps 2X focal length?

Digital thermometers are a non-issue, you can pick them up for peanuts. Depending on the range, even medical ones can be had for the equivalent of a US dollar!

So, all you need to find is that common oil and the correct weight/volume for a 1 deg/W change and you're sorted :D (apart from all the other variables like temp, barometric pressure ... - but it's only a sanity check, not a golden ruler :D )

It would be really cheap and simple to lash up a small picaxe microcontroller controller with a temp sensor and small lcd display that could do all the calcs for you.

Certainly worth carrying on with and a good 'poor man's' method!

Cheers

Neil
EMOs are a sign of weakness ...
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Re: Hans-Wehr open source CO2 laser power meter

Postby lasersafe1 » Wed Oct 24, 2012 2:21 am

Presumptuous with the name, perhaps, but yet in the three years I have been on this forum all I have heard was complaints that power meters are too expensive and no suggestions. There have been people who "think" their laser is dead, but need to confirm burn depth through some type of material at some speed to get comparisons with others on this forum.

Since this universally common method/formula for the CO2 power meter will need a name, why not name it myself?

I'm willing to bet that oil type or brand makes very very little difference (<<1%). The key point about the oil is that it is high in hydrogen molecules and it is far away from a boiling point, even for the temperature of the light. As you are probably aware, once you reach this transition temperature for a given pressure, the fluid can absorb vast amounts of heat without changing a single degree. Also with oil, let's just say "vegetable oil" for now, your local barometric pressure will not be a factor, even on Mt. Everest.

I agree that weight would be most accurate and we used tablespoons because it was handy at the time. Does every country have some version of a post office where you can go and weigh a package to calculate postage? That is one option. I have my own postal scale and cheap digital scales are available at cooking supply stores or at Harbor Freight here in the states. I would say we could probably say XX grams if you can weigh it, but XX ml if you don't have a scale.

Yes, with a microcontroller you can not only do the initial and final readings, but it can give it's power measurement in real time as the test is in progress since it will "know" seconds. The only problem with that is the fact that there is substantial stratification in the oil and I could not get an accurate reading until I stirred it. This is why some companies make these systems with an anodized aluminum block. The aluminum reflects a little, which is bad, but it is an excellent thermal conductor for the temperature measurement.

By the way Neil, I got your little zing about the McDonalds cups being larger here, yes, we are fatter here. You don't have 2 liter cups of Pepsi with your happy meals?
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Re: Hans-Wehr open source CO2 laser power meter

Postby lasersafe1 » Wed Oct 24, 2012 1:34 pm

I will get a power resistor and power supply and test a few different types of oil this weekend. If several types give the same temperature rise over the same time period with the resistor (which they should), then I will test them under my laser to see if the absorption is the same by again looking at the thermal response.
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