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Old 08-04-2011, 06:29 AM #1
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Default Method of Calibrating LPMs

Hey everyone. So I have been playing around with trying to figure out sneaky ways to calibrate DIY LPMs... and so I thought this would work.

The idea is that I would take a 1Ohm 10W resistor (so it has room to spare) and then shove 1A through it, that way it should be dissipating 1W of heat, yes? Anyway, so I did this, and just laid the resistor on top of my thermal DIY LPM, and the reading I am getting is about 500mW or so (480mW at the time of this writing, but it's still increasing, very slowly). So what's going on here? Anyone have any ideas?

Shouldn't this method work? Thermal LPMs work by measuring the temperature different between the two sides of the TEC module. The temperature difference between them is proportional to the watts that are being transferred to one side by a laser/light/flame, etc. Now, I would think that, because we can EASILY calculate the fact that it would emit 1W of heat, why is my LPM reading so very far off? I realize that it should be a bit off due to calibration, but it's not off by HALF.

Anyway, anyone else have any ideas?


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Old 08-04-2011, 07:16 AM #2
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Default Re: Method of Calibrating LPMs

I don't think that will work.

You are trying to put one watt worth of energy on the thermocouple and use that to calibrate it, but a laser doesn't put 100 percent of it's energy to the head and neither would the resistor so you would still have to calibrate it for the difference in power transfer of the resistor vs. A laser. Since every diy lpm would be slightly different (paint on the thermocouple, resistor used, how resistor is attached for calibration) you would need every on to be calibrated itself, which is no different than before and you are adding another step which reduces accuracy.
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Old 08-04-2011, 07:27 AM #3
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Default Re: Method of Calibrating LPMs

Hmm. Makes sense. And you're right about that - lasers aren't fully absorbed, or else we wouldn't be able to see the dot on the LPM =p
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Old 08-04-2011, 10:39 AM #4
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Default Re: Method of Calibrating LPMs

Hm, intersting idea. But you would have to use a small resistor and somehow manage to get it to transfer ALL of its heat to the TEC. If you use one of the big 10W resistor types, then most likely a lot of the heat will not hit your sensor. This could be an explanation for your low reading.
And of course you would need to find out how much a laser beam gets absorbed by your coating and if there are differences in wavelength.
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Old 08-04-2011, 10:58 AM #5
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Default Re: Method of Calibrating LPMs

Quote:
Originally Posted by benmwv View Post
I don't think that will work.

You are trying to put one watt worth of energy on the thermocouple and use that to calibrate it, but a laser doesn't put 100 percent of it's energy to the head and neither would the resistor so you would still have to calibrate it for the difference in power transfer of the resistor vs. A laser. Since every diy lpm would be slightly different (paint on the thermocouple, resistor used, how resistor is attached for calibration) you would need every on to be calibrated itself, which is no different than before and you are adding another step which reduces accuracy.

I'll need to disagree with that statement..
The power leaving the Laser's aperture gets put onto the
LPM's Sensor 100% ( if the total beam profile is in the active
area of the LPM's sensor).
There is no loss of power from that beam unless the air between
the Laser's aperture and LPM sensor has a massive amount of
suspended particles....or is blocked by some object...(ie ND filter)

The problem with the 10Watt resistor is that the DIY LPM sensor
has a large surface area and only a small part of id is seen by
the sensor... The rest of the resistor's power is radiated into
the surrounding ambient air...

The other thing is Thermal conductivity/resistance and sensor
coating bandwidth/efficiency as "chefla" mentioned...


Jerry
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Last edited by lasersbee; 08-04-2011 at 11:01 AM. Reason: Spelling errors
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Old 08-04-2011, 04:39 PM #6
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Default Re: Method of Calibrating LPMs

Righto. Seemed like an interesting idea. But, like most interesting ideas, it was interesting but had no real world application. Thanks, everyone, for clearing this up.
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Old 08-04-2011, 04:50 PM #7
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Default Re: Method of Calibrating LPMs

Quote:
Originally Posted by lasersbee View Post
I'll need to disagree with that statement..
The power leaving the Laser's aperture gets put onto the
LPM's Sensor 100% ( if the total beam profile is in the active
area of the LPM's sensor).
There is no loss of power from that beam unless the air between
the Laser's aperture and LPM sensor has a massive amount of
suspended particles....or is blocked by some object...(ie ND filter)

The problem with the 10Watt resistor is that the DIY LPM sensor
has a large surface area and only a small part of id is seen by
the sensor... The rest of the resistor's power is radiated into
the surrounding ambient air...

The other thing is Thermal conductivity/resistance and sensor
coating bandwidth/efficiency as "chefla" mentioned...


Jerry
Yes I see what you are saying. The way I worded that made it sound like I meant the energy was somehow being lost in between.

All of the energy "goes" to the sensor, but not all of it stays. What I meant by that statement is that you can never have a coating on the sensor that absorbs 100 percent of light at all the wavelengths we use. Obviously some of the light hitting the sensor is reflected or, as wolfman said you wouldn't even see the dot. Also I'm sure some of the energy radiates off the sensor just because of the temperature difference between it and the air around it.

Hopefully that cleared things up?
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Old 08-05-2011, 06:31 AM #8
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Default Re: Method of Calibrating LPMs

Some scientech power meters have a resistor that can be used to calibrate the meter, but only after it previously has been calibrated. You can heat a tec based sensor by applying an AC voltage. This will heat the inside of the tec and not the surface, so it will differ significantly, but it may be fairly constant when the same tec is used on a good heatsink.
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Old 08-05-2011, 12:22 PM #9
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Wink Re: Method of Calibrating LPMs

The built-in resistors from Scientec heads can be easily used for calibration, also if you don't have them previously calibrated, but you need to know two parameters with precision, the resistance of the heater, more precise possible, AND the absorption factor of the reading surface ..... then is only matter of Ohm law and calculation.

I mean, suppose the resistance of the heater is exactly 100 Ohm ..... then, if you put a 10V PRECISE voltage on it, you're heating the reading plate with exactly 100mW in thermal power ..... then, if your plate have an absorption factor of, say, 80%, that mean 0.8, you can multiply the power for the absorption factor (in this case 20*0.8), and obtain 80, that means you have to set the reading instrument on 80mW, and your LPM is set correctly (the absorption factor is indispensable, cause with the beam, the measured power is the part that is absorbed, where instead with the heater, the measured power is the 100% of the thermal energy transferred from direct contact)
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Old 08-05-2011, 03:53 PM #10
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Default Re: Method of Calibrating LPMs

The internal heater of a scientech sensor won't heat the sensor in the exact same way incident light does, but it's good enough to do a calibration and use a predetermined calibration value for the sensor that relates the resistor power to measured power.
Maybe such a calibration value would be relatively constant for a certain type of tec, so it would mean you could calibrate your own DIY lpm by ensuring the construction is equal to a reference design. Of course this will still have a large error, but far better then pushing a hot resistor against the surface.
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Old 08-05-2011, 04:00 PM #11
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Default Re: Method of Calibrating LPMs

Well, the better way at hobby level, and where there is not available a sample reading head for set-up a calibration station, can be to still use a laser, with an APC circuitry.

This still require a calibrated LPM for set the calibration unit for the first time, ofcourse, but then the APC system will keep the output power stable enough for have a decent reference value.
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Old 08-05-2011, 07:08 PM #12
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Default Re: Method of Calibrating LPMs

What's APC circuitry? o.o
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Old 08-05-2011, 07:32 PM #13
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Default Re: Method of Calibrating LPMs

Is a circuit for automatic power control of the diode.
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Old 08-05-2011, 07:41 PM #14
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Default Re: Method of Calibrating LPMs

Oh. So it's like an LPM attached to a driver or something?
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Old 08-05-2011, 07:47 PM #15
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Default Re: Method of Calibrating LPMs

More or less

The APC electronics controls the power through the reading of a photodiode placed under the same laser diode.

You need a laser diode with a photodiode inside.

Many red and some blue laser diodes have it.

Here you have a simple and inexpensive kit APC to control up to 150mA output current.

This one is compatible only with diodes that have the common anode pin of laser diode, and the photodiode is the cathode.

Meredith Instruments : APC Laser Diode Driver Kit [LDD-15] - $15.00 - The source for laser surplus

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Old 08-06-2011, 08:36 AM #16
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Default Re: Method of Calibrating LPMs

You can also do it yourself style, if you don't find LDs with built-in photodiodes ..... my old calibrator work in this way, i just placed a beamsplitter glass in the path of the beams, and redirected the resulting beam on external photodiodes (in a dark chamber, so they are not influenced from external light), and it's still working good from years.

Now i'm building a better one, and have an evily plan for the old one, when the new one is finished
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Last edited by HIMNL9; 08-06-2011 at 08:36 AM.
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