Convert Lux meter to PAR meter?
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Old 06-13-2012, 05:03 PM   #1
Hoppy
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Convert Lux meter to PAR meter?


There are a lot of cheap Lux meters available now that look like they could become a hobby grade PAR meter with some modifications. Just for the fun of it I bought one. It is a Mastech LX1010BS, from Cybertech, in California, for only $14.99, $21.53 after shipping and tax. I ordered it last Saturday and it arrived yesterday, Tuesday. You can't beat that!

Here it is:


It takes a 9 volt battery, which I don't have right now, so I haven't tested it at all. I did partly disassemble the sensor though - 2 screws.


I have a LED light and a CFL-in-a-dome reflector light to calibrate it against a PAR meter. I'm guessing there will be a different conversion factor for the different types of light, but that's fine for what I want.

My requirement is to be able to use this under water, so that is the challenge. Ebay has a nice 4 inch diameter acrylic box that I could just seal the sensor in, but that has problems: temperature and atmospheric pressure changes would flex the box and probably break the seal, It would float so would be hard to push down to where I want it, it is too big for convenience. Another option is to discard the sensor housing and use a small 2 inch diameter screw top acrylic box as the housing. For the moment that is my plan.

What am I overlooking? And, is there a better way?

EDIT: To help those who don't want to read through 10 pages of frustrating failures, the final successful project starts at post #158
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Last edited by Hoppy; 12-13-2012 at 08:20 PM.. Reason: Edit: add direction to the final successful effort
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Old 06-13-2012, 05:08 PM   #2
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Can you find the name of that photodiode? Maybe we can look it up to find the spectrals for it.
I notice they use a ultraviolet filter on top of the photodiode.
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Old 06-13-2012, 05:20 PM   #3
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I think the lux is divided by 60.6 for umol.

This assumes the PAr is 400-700nm though, lux meters measure the bell shape at 555 nm.

If the source of light is standard, this should work okay.
Better than nothing.

The other thing is to tie the probe next to a PAr meter and go a measuring various light sources and distances and see how close things are to the 60.6 conversion factor(this is what I'd do).
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Old 06-13-2012, 06:02 PM   #4
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I'm very interested in this thread! I hate the idea of paying the "Brightest" price for PAR performance.

You rock for checking this out Hoppy!

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Old 06-13-2012, 06:32 PM   #5
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Quote:
Originally Posted by mistergreen View Post
Can you find the name of that photodiode? Maybe we can look it up to find the spectrals for it.
I notice they use a ultraviolet filter on top of the photodiode.
I can't see the actual diode for the filter and a case below it. Here is another photo:


It looks like it would be pretty easy to remove the whole sensor assembly to put it into a different, and waterproof case. I think the white plastic dome, the cosine filter? - has to be the only optical thing between the light and the sensor, so I couldn't have that part inside the watertight case. Is that correct? The only effect I can figure out from having a layer of acrylic covering the dome is reflection off the flat layer, but there is reflection off the dome too, so I can't understand why that does much harm.

The 60.6 conversion factor could change if there is acrylic over the dome, I think, but that's not a problem either. And, if the conversion factor is accurate to +/- 5% that would be outstanding.
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Old 06-13-2012, 08:25 PM   #6
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I found an acrylic round, screw-top box, 2.75 inches in diameter, which will work great as a housing for the sensor. It is available online, http://www.consumercrafts.com/store/...torage/1125-53 Unfortunately, only one lid comes for the 4 boxes, but it also only cost me $2.25 at Michaels. Here is what it looks like:


I will have to cut the sensor housing down in length but it is empty in that area anyway. Now, how to make the sensor watertight? I'm thinking I could essentially pot it with silicone, so the hole for the electric cable will also be sealed. The lid will have to be cemented on, either with silicone or with acrylic cement.

The acrylic lid attenuates the meter reading from 99 to 89 lux with it sitting on my desk and the overhead CFL light lighting the room. That would be a 10% loss due to the lid, which would just change the conversion factor from 60.6 to 67. Not a problem, that I can see.

There is a tiny "instruction manual" with the luxmeter, which has this chart on it:


Clearly this is not consistent with a true PAR meter, but calibration against a PAR meter for specific light sources should take care of this easily enough.
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Old 06-13-2012, 09:04 PM   #7
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Subscribed. I am interested in seeing how this works out for you. This is some great information. Thanks Hoppy!!
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Old 06-13-2012, 09:43 PM   #8
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I always believe that when you decide how to do something, it is a good idea to go ahead and do it, so:


I sawed off each half of the housing separately, after first cutting the electric cable to it. Then, I found that it didn't quite fit in the little box, so I had to carefully file off the corners. Now it fits tightly.


I think I will do some more filing to make it fit less tightly, then take it back apart - one screw now - clean it with my handy air blower (my mouth), and go back to thinking about the sealing problem.

Does anyone have an idea about how critical it is to keep the sensor dark when not in use? These always come with a cap for the sensor, which I won't be able to use, but this one also came with a nice black cloth case which should do the same thing.
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Old 06-13-2012, 10:00 PM   #9
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Quote:
Originally Posted by Hoppy View Post
The acrylic lid attenuates the meter reading from 99 to 89 lux with it sitting on my desk and the overhead CFL light lighting the room. That would be a 10% loss due to the lid, which would just change the conversion factor from 60.6 to 67. Not a problem, that I can see.

There is a tiny "instruction manual" with the luxmeter, which has this chart on it:


Clearly this is not consistent with a true PAR meter, but calibration against a PAR meter for specific light sources should take care of this easily enough.

The scale is no good.

What you need to do is compare the PAr meter to this meter once it's set up and done.

Then see how the readings compare and then use that difference for the factor.

I suspect most of the typical white FL bulbs will be somewhat close as far as a PAR to Lux comparison.

Put another way: some standard measure of light is better than NONE.
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Old 06-13-2012, 11:48 PM   #10
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I bought some 22-2 flexible shielded cable, 4 feet for $1.50, to reconnect the sensor to the meter. This, because the coiled cable that it comes with is too short for dunking the sensor 2 feet down in the tank. The cable didn't need to be shielded, but that was the closest I could come to the 24-2 cable on the unit. I soldered and taped the connections, with the new cable going through a tight fitting hole in the acrylic housing. To act as a strain relief I just knotted the cable right inside the housing. Without that the cable could be jerked loose from the diode. And, good news, it still works!


Now to decide on how to waterproof this. Suggestions???
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Old 06-13-2012, 11:55 PM   #11
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I use a heyco grip cord to waterproof where the cable meets the housing but I'm going from beneath because it needs a flat surface, not a curved one from the side.

I use silicon to seal the cap.
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Old 06-14-2012, 12:21 AM   #12
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Quote:
Originally Posted by mistergreen View Post
I use a heyco grip cord to waterproof where the cable meets the housing but I'm going from beneath because it needs a flat surface, not a curved one from the side.

I use silicon to seal the cap.
So, the curing silicone fumes don't do any harm? And, I know it doesn't adhere strongly to acrylic, but there shouldn't be any stress on it to keep it from sealing. I can't see any way to ever get back into the sensor housing once I seal it, whether with silicone or anything else, so is there a good reason not to use high viscosity acrylic cement to seal the lid on permanently? The solvent fumes? Remember it is threaded on too.
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Old 06-14-2012, 03:08 AM   #13
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Fumes from super glue will corrode the photodiode. Silicone seems to be fine. I'm assuming the acrylic cement has the same stuff as super glue.
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Old 06-14-2012, 03:15 AM   #14
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From the MSDS for "acrylic cement":


Do those look like what would corrode the diode?

It may not matter if I screw the lid on, then apply the high viscosity cement in a line where the lid meets the main body. Little, if any, fumes would then get into the box. I think???
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Old 06-14-2012, 03:23 AM   #15
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Quote:
Originally Posted by mistergreen View Post
Fumes from super glue will corrode the photodiode. Silicone seems to be fine. I'm assuming the acrylic cement has the same stuff as super glue.
I don't think so. Super glue is cyano acryliate. It like something like weld-on is dicholromethane. They work on two different principles - super glue adds a plastic-ish compound between the surfaces that grips both surfaces. Acrylic cement melts the acrylic, causing the two surfaces to re-crystallize as a single structure.
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