Convert Lux meter to PAR meter?

[Hoppy]

Originally Posted by Chlorophile

I would have taken the acrylic round box you had and sprayed it black.

Then I would have just taken the cosine filter and cut a hole in the top of this.
Cut the hole equal to roughly the diameter of the cosine filter and siliconed it in place so that the dome of the filter protrudes through the hole.
Once its dry just seal it up with the diode fixed beneath the cosine filter.

That would work, but gain very little. The big problem with that one was the size and buoyancy of the sensor assembly. It was very difficult to use it because of that. If you want to do that, just remember that the layer of acrylic over the dome reduced the lux reading by 10%.

Using just those parts, I would use a smaller acrylic container, and just the sensor and cosine filter in it. That helps a lot with both the buoyancy, if most of the remaining volume is filled with silicone, and the size of the sensor assembly. Incidentally, when I tore that original assembly down, I noted that there was very poor adhesion between the acrylic and the silicone. It came apart extremely easy.

[Hoppy]

I stopped by Tap Plastics today and found a tiny acrylic box, about 2" x 1" x .75", which should be perfect for mounting the guts of the original Lux Meter in a waterproof box.


I will have to do some minor trimming of the photo diode package and the cosine diffuser before they will fit. But, at 90 cents, the little box is a perfect size and price.


The only trimming I had to do, to this point, is file a small, shallow notch on the top edge of the box so the round diode filter holder will fit flush with the top. It is glued in place with Weld On #16 now.

It needs some "ballast" to help it lose its buoyancy. So, I added some pool filter sand and some more WeldOn #16.

[plantbrain]

Hoppy, you might also look into R/B ratios for lighting types, sometimes the makers give this data, this will skew the PAR/Lux data some but is interesting none the less.

Ivo Busko used to discuss this, he's a pretty smart guy and did a lot with lighting on the APD a decade or and 1/2 ago.

[plantbrain]

Hoppy, just make a rod like we have for the PAR meter, no need for weighted ballast.

[Hoppy]

Originally Posted by plantbrain

Hoppy, just make a rod like we have for the PAR meter, no need for weighted ballast.

I have to eliminate most of the air trapped in the little box, or I found that it gets difficult to hold the sensor steady due to its constant attempts to float up. Rather than just pour more acrylic cement in, which eventually softens and distorts the whole box, I decided to use mostly sand as the filler. So it helps in two ways.

[Hoppy]

All of the ballast sand in place. After it sets (dries) some more the top gets glued on. The sand grain on top of the filter, I blew off.



This is before I glued on the cosine diffuser, which is acrylic too.



Complete, waiting for the cement to set/dry. I like the size of this, and the lack of buoyancy, but I suggest using something more coarse than filter sand as the ballast/filler. This is certainly better than my initial try.



The two sensors I now have. I still like the last one, but this one is a nice size too.

[audioaficionado]

I got one of the first six Good Enough PAR meters. The only issue is that the sensor wants to float like a cork. I don't want to open it and break the water tight integrity of it. I really like how you got the one on a flat base. You can get it right down at the substrate level while mine will be about an inch above. I'm happy I got that PAR meter even if the sensor isn't as compact as a commercial unit. $60 vs $400 makes it worth dealing with the float issues.

[Hoppy]

Originally Posted by audioaficionado

I got one of the first six Good Enough PAR meters. The only issue is that the sensor wants to float like a cork. I don't want to open it and break the water tight integrity of it. I really like how you got the one on a flat base. You can get it right down at the substrate level while mine will be about an inch above. I'm happy I got that PAR meter even if the sensor isn't as compact as a commercial unit. $60 vs $400 makes it worth dealing with the float issues.

I agree, and we now have the third option of just buying a cheap lux meter and modifying it. The Good Enough PAR meter has the advantage of reading out PAR directly, instead of having to use a conversion factor to convert a reading to a PAR number. I suspect it may also measure a spectral range a little closer to the PAR spectral range. But, anyone who doesn't want to pay $300 for a real PAR meter can pay $60 or so for the Good Enough PAR meter, unless even that is too much for their budget. Then they can make the PAR Volksmeter for around $30, using that last sensor design. I think anyone who wants to measure PAR should now be able to do so.

[Hoppy]

The calibration constant for this one is 67. The lux reading divided by 67 equals the PAR. I took great care keeping the two sensors the same distance from the light. Did two readins at 26 and 45 micromols of PAR, and got 1750 and 3030 lux. I haven't decided which sensor to use for my PAR meter - most likely this one. When I measured the PAR with this at the substrate level in my tank, I got 22 micromols of PAR, in between the 20-25 readings I got with the PAR meter and my other sensor.

Now if anyone wants to make their own PAR meter, this version is as easy as possible to make, and costs:
$15 for the lux meter
$8.50 for the Weld On #16 cement
$2 for the electric cable
$1 for the little acrylic box
$26.50 plus tax and any shipping costs

And, it took me about 2 hours to make the sensor, at most.

[Hoppy]

Is anyone interested in buying one of these?

It would be complete - the lux meter, the sensor, with a 4 or 5 piece holder and rod attached, to make it easy to hold it in position in the tank - for $35 shipped. Until Amazon runs out of the lux meters I can make these in batches of 5. And, they only have 19 left as of this morning.

PM me right away if you are interested. I will only do this if I can do 5 at a time. And, this will not be profitable for me - I just want to see more PAR meters in use.

[Bad-Daddio]

PM'd.

Hoppy - Just one question. How stable can I expect readings to be over time? Would any re-calibration be necessary?

[Hoppy]

Originally Posted by Bad-Daddio

PM'd.

Hoppy - Just one question. How stable can I expect readings to be over time? Would any re-calibration be necessary?

Of course I don't know from experience, but the lux meter maker says the photo diode slowly loses sensitivity, with the loss being proportional to the intensity of the light and the time it is exposed to that light. I doubt that using as we do, with a couple of readings maybe every 6 months, will cause any degradation at all. Plus, the lux meter is made to read up to 100,000 lux, which, for this modification, would be 1500 micromols of PAR, almost 100 times the intensity we will use it with.

[mistergreen]

you should cover the sensor when not in use.

[Bad-Daddio]

Got my Volksmeter in the mail today.

Wanna know how I know that it's really cool? As I explained what it is, my wife's eyes slowly rolled back in her skull and my daughter called me a nerd and walked away. My step son got all excited - He gets it!

I wasn't planning on setting my tank back up until after summer - Probably not going to be able to wait that long now....

Thanks again Hoppy!

[Hoppy]

I made 5 of these and sold them at cost, primarily to share this, but also in hopes of getting some more PAR data on various lights. One glitch has shown up - the tiny amount of solvent left inside from the acrylic cement causes some frosting of the acrylic box. This slightly reduces the sensitivity of the meter. I corrected for that when I calibrated them, so I don't think it will ever be a problem. After another week or so I will again recalibrate mine to see if any more frosting occurs.