Cheapest PAR Meter DIY - Page 2 - The Planted Tank Forum
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post #16 of 65 (permalink) Old 12-28-2013, 04:00 PM
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So the style that I have pictured below has the alternative photodiode correct??

I just wonder because if its the photodiode that comes with the lux meter ill have to guesstimate my par a little more.. add the blue and red in and give my best judgement...

But if I have the new diode it should be pretty bang on correct??

Basically what im asking is what photodiode and filters does this particular meter have??

Ive been using it for 6 months ish now and have been setting my photoperiod by this par meter.. its seems to b working for me..
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post #17 of 65 (permalink) Old 12-28-2013, 06:42 PM Thread Starter
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Originally Posted by jeffdenney View Post
So the style that I have pictured below has the alternative photodiode correct??

I just wonder because if its the photodiode that comes with the lux meter ill have to guesstimate my par a little more.. add the blue and red in and give my best judgement...

But if I have the new diode it should be pretty bang on correct??

Basically what im asking is what photodiode and filters does this particular meter have??

Ive been using it for 6 months ish now and have been setting my photoperiod by this par meter.. its seems to b working for me..
That uses the new photodiode, Excelitas VTB8441BH. It should continue to be accurate for a long time, possibly needing a new battery at times. This is the only photodiode I have used in any of the PAR meters I made.

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post #18 of 65 (permalink) Old 12-29-2013, 07:29 PM Thread Starter
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Starbrite liquid electrical tape is not a good sealant to use. It is a noxious mix of unsafe solvents, in a tube where it can't be mixed as it is supposed to be, and so runny it gets on your hands and drips as you apply it. Brushing it would work better, but the noxious fumes are enough to avoid it.

I'm going to try my reliable WeldOn #16 again. With a good technique it should work fine.

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post #19 of 65 (permalink) Old 12-29-2013, 10:44 PM
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..in a tube??
I've never seen liquid elec tape in a tube, usually it's in a can like PVC glue..and pretty thick

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post #20 of 65 (permalink) Old 12-30-2013, 04:01 PM
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That uses the new photodiode, Excelitas VTB8441BH. It should continue to be accurate for a long time, possibly needing a new battery at times. This is the only photodiode I have used in any of the PAR meters I made.
Awesome. Ive been using it for a while and it seems to be working great.. havent even changed a battery yet..

Thanks
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post #21 of 65 (permalink) Old 12-30-2013, 06:46 PM Thread Starter
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..in a tube??
I've never seen liquid elec tape in a tube, usually it's in a can like PVC glue..and pretty thick
This is what I tried:


I also looked up the msds for this, and found the warnings were the most severe and varied of anything I have seen a msds for. It is just not good to use unless nothing else will work, and then only in a very well ventilated room, with hand and eye protection.

Today I finally was able to seal a sensor with WeldOn #16 acrylic cement, after several tries. The secret was to apply the cement inside, when assembling the sensor, then again over the outside of each joint. Without that last step the sensors just fill with water in a few minutes.

Next step is to put all of this together in one usable sensor. I'm inclined to concentrate on the least expensive build - using the original photodiode. Then I can try it with various kinds of light and see how much the calibration changes with the different kinds.
Parahnoyd and Parahnoyd like this.

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post #22 of 65 (permalink) Old 12-31-2013, 07:35 PM Thread Starter
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I finally have a cheapest PAR meter assembled, and it calibrates against the Apogee Quantum meter very well, within 5% of the reading. I had to abandon my attempt to make it read on the middle range scale because I couldn't get it to read close enough. So, I started all over, trying to get a broader spectral response and make it read on the lowest scale. Lots of spreadsheet calculations later I have a 5 colored filter, plus one diffuser filter combination that works. And, it has a broad spectral response, so it has a chance of working with a variety of light types. I'm waterproofing it now, to see if it continues to work the same afterwards. Then I will try it against a few more lighting types. And, finally, buy a few luxmeters and see it the results are the same with all of them.

The work required to modify one lux meter is pretty easy, goes fast, and doesn't require a lot of mechanical aptitude. But, arriving at that configuration was a royal pain.

Yesterday I tore down a readout module from a couple of my failed luxmeters, and to my surprise, the printed circuit boards in them are not the same, or even close to the same, for the same LX1010B meters. One circuit has 4 adjustable resistors, obviously used to adjust the readout to make it read correctly. (Unfortunately that readout isn't working well enough to test out how well it can be adjusted.) The other has no adjustable resistors that I could identify - either they were able to just use precision resistors, with no adjustment needed, or the circuit is considerably different. But the different Chinese companies making these appear to work out the design details independently.

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post #23 of 65 (permalink) Old 01-01-2014, 05:21 PM Thread Starter
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There is always something to learn, and often it is learned with egg on your face!* My attempt to flatten out the spectral response of the lux meter photodiode by using colored filters didn't work out at all like I expected. The "PAR" meter response matched the Apogee meter response very well with a 50-50 bulb (half actinic), but was off considerably with a 10,000K bulb. Since the spectral response wasn't uniform, there are apparently big enough differences in peak responses for the two bulbs that one has peaks where the photodiode has peaks and the other doesn't. My attempt at forcing the lux meter diode to act as a PAR meter diode was a failure.

So, I changed the filters to neutral grey filters, leaving the spectral response a peak in the green band, but making the meter read the same numbers as the Apogee meter reads. Then I checked it with the two bulbs, and it works the same for both of them. That means it is better to just measure the light in a small part of the spectrum than it is to measure it at a combination of different parts of the spectrum. That isn't what I expected.

Next, I need to try this with a couple more different "white" bulbs, and sunlight.

*Egg on my face because I sealed the sensor before discovering this! I had to use a hammer to get it back apart. No problem, since I have about 50 left over sensors to replace it with.

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post #24 of 65 (permalink) Old 01-02-2014, 10:24 PM Thread Starter
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After almost 2 years of working on this, trying everything I could think of, I finally shifted my brain into "park", and guess what? This lux meter has a closely fitting cap over the sensor, to block out the light to make the photodiode last longer when it isn't in use. What do you suppose would happen if you left that cap in place, but drilled a tiny hole in the center of the part over the diffuser dome? Duh! It acts as a very effective filter, reducing the lux meter reading as much as you might want to. So, I drilled a 1/16" diameter hole in it, and it reads about 25% low compared to the "PAR" that it should read. No filters needed, just a small increase in the hole size, probably to a #49 drill size (about .073 inches diameter). I will buy a drill bit that size and see if it works. If it does that means no parts, no filters would be required to make the lux meter read "PAR", and you could leave the drilled cover on to read PAR and take it off to read lux. At worst, you would need to add a tiny piece of diffuser filter from the Rosco sample book, over the back of the hole to adjust the reading down a bit.

I started thinking about this when I noticed that even the Rosco neutral gray filters badly distort the spectral response, and will probably make it mis-indicate some types of lighting, where a simple green light measurement would be more likely to be accurate. I should put my brain in "park" more often!

Here is what the modification looks like as a sketch:


Today I found that the best diameter for the tiny hole in the protective cover is .067 inch, which is a #51 drill size. I did this by trying 4 different drill sizes, plotting the resulting meter readings and using that to interpolate to the best diameter, giving the same reading on the lux meter as on a PAR meter. I still need to buy a #51 drill and try it to be absolutely sure of the size needed.

This is an absurdly simple way to make a usable PAR meter. Drill a tiny hole in the protective cover. Use silicone sealant to make the sensor body waterproof. (Silicone worked best for this, by testing various sealants with partially assembled sensors.) The lux meter is then unharmed and, by removing the protective cover, reads lux as it was designed to. This enables you to read from zero to about 50,000 PAR! Take it on a trip to the sun and measure the PAR there!

Hoppy

Last edited by Hoppy; 01-04-2014 at 02:50 AM. Reason: add graphic and more details
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post #25 of 65 (permalink) Old 01-04-2014, 03:25 AM
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Well done! I don't know how you could make it any easier. When I have all my other projects done, I'll try one.

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post #26 of 65 (permalink) Old 01-04-2014, 04:58 PM Thread Starter
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I was able to make the lux meter read almost exactly what a PAR meter reads, but not just by adjusting the hole diameter. A very small change in the hole diameter makes a big change in the lux reading. I wasn't able to find the #51 drill (.067") I wanted, so I got a #50 (.070) instead. That made the lux meter read 37% too high! But, I added a Rosco #111 and #101 diffuser filter under the hole, and the reading was then right on the button. This may be an advantage for other reasons. By sealing around that pair of filters inside the protective cover it prevents water from filling the cover, making it less "sloppy" to use in the tank. Still, if I could find a #51 drill, it should also be very accurate.

Now, I need to combine sealing the sensor to make it water tight, with the drilled protective cover, and calibrate this with various lighting types. Since this sensor measures only green light I expect that there will have to be a correction factor for some types of lighting.

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post #27 of 65 (permalink) Old 01-18-2014, 07:37 PM Thread Starter
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I finally got my hands on our club Apogee Quantum PAR meter, and calibrated this meter with two colors of PC bulbs, an incandescent bulb, a 6500K CFL bulb, and open shade outdoors. The results are:

The meter reads a bit low, from 3 - 14% low, so if you wanted better accuracy you could add 8% of the reading to the reading and be much more accurate. One thing I discovered is that the meter works well with a light typical of an aquarium light overhead, but out in the sun or in shade outdoors it doesn't do well. A really good PAR meter will have a cosine diffuser on it, so it is measuring light from a large conical area above the meter. With the tiny hole as the primary filter on this meter it doesn't have a very effective cosine filter, so it misses a lot of the light out doors. For our typical planted tank use it works fine though.

Two more tasks to be done:
Demonstrate that I can waterproof the sensor without interfering with the geometry of the tiny hole to the diffuser to the photodiode.

Demonstrate that this works the same when I make more of these. (Sometimes something will work ok once, but due to small geometry variations, not work well when repeated.) So, I guess I need to buy a few more of the lux meters.

Hoppy
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post #28 of 65 (permalink) Old 01-19-2014, 04:00 AM Thread Starter
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Obviously the easiest and cheapest way to use one of these lux meters as a PAR meter is to just use it as is, measure the light intensity in lux instead of PAR units, then divide the reading by 78 to get the PAR reading. That coversion factor I got by reading the intensity with the small hole filtering the light, then removing the cover withe hole in it, and reading the same light again, in lux units. Dividing the lux reading by the PAR reading, then multiplying it by .92 to adjust for the approximately 8% low readings from the modified lux meter, gave me 78 as the conversion factor. Perhaps if I were to do this again another time with another lux meter I would get a slightly different factor, but 78 will always be true for this specific lux meter.

I ordered a couple more lux meters, which I will test the same way to see if the 78 factor stays the same.

Hoppy
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post #29 of 65 (permalink) Old 01-19-2014, 04:11 AM
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Hoppy, so these results basically are from two #111 and #101 filters, and the 0.067" hole?
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post #30 of 65 (permalink) Old 01-19-2014, 03:07 PM Thread Starter
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Hoppy, so these results basically are from two #111 and #101 filters, and the 0.067" hole?
Yes, that's the configuration I used. The question is whether this will work for a random lux meter. This particular meter is one I replaced the photodiode with a Excelitas diode, then reinstalled the original diode. Doing that may have slightly altered the optical path, which would mean this wouldn't be the right combination for a new lux meter. The two new ones I ordered should be adequate for answering that question. I should have them by the end of this coming week.

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