[Hoppy]

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

 

[mistergreen]

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.

 

[plantbrain]

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).

 

[Imi Statue]

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!

Imi

 

[Hoppy]

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.

 

[Hoppy]

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...e-jewelry-organizers-and-bead-storage/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.

 

[wastedtime]

Subscribed. I am interested in seeing how this works out for you. This is some great information. Thanks Hoppy!!

 

[Hoppy]

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.

 

[plantbrain]

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.

 

[Hoppy]

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???

 

[mistergreen]

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.

 

[Hoppy]

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.

 

[mistergreen]

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.

 

[Hoppy]

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???

 

[d3snoopy]

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.

 

[DarkCobra]

Curing cyanoacrylate emits fumes which can deposit a white substance on nearby objects, especially if they have any oil on them. It can and has been used by law enforcement to reveal fingerprints. But it's not corrosive.

Most hardware-store silicone produces acetic acid fumes as it cures. Which can be corrosive, like any acid; especially if trapped in an electronic enclosure. I know there's specialized silicones that don't release acetic acid, which can even be used in direct contact with electronics to totally encapsulate them; but I haven't used them and can't make a recommendation.

However, since you have an outer housing which screws on before any of these substances are applied, I'd think you can use anything that provides an adequate seal.

And as for the sensor cap, I think it's just a dust cap. I've never heard mention of any need to keep a photodiode in the dark most of the time.

Oh, and Mistergreen came up with an idea in the DIY PAR meter thread that adding a purple filter gel to reject some of the green, while passing a higher proportion of photosynthetic red/blue, might give results closer to that of a real PAR meter. The filter I see on your LUX meter appears green, doing the exact opposite of what you want. I doubt it's a UV filter, as photodiodes have poor response to that. They are however very sensitive to IR, and if it does remove any of the non-visible spectrum; that would be it.

 

[Hoppy]

I have two sealing needs. One is around the joint between the cap and the box, or jar, both of which are acrylic plastic. The other is around the guts of the lux meter to hold it in place, and also to seal the pass thru hole for the electric cable. I was considering silicone for the second , and acrylic cement for the first (not methacrylic super glue).

My present plan is to leave the lid off the box until the silicone cures around the luxmeter guts, which would also reduce the volume of air in the box, but not in contact with any of the electric elements. Also a fillet of silicone outside around the electric cable. After the silicone cures, probably 2 days later, screw on the lid and apply high viscosity acrylic cement around the joint to seal it permanently. Would that adequately protect the diode, etc. from any fumes? And, do you think the limited adhesion of silicone to acrylic would be enough to seal up the electric cable hole - the cable fits pretty tight in the hole, so it is just the tiny surrounding gap the would be sealed.

Before I consider playing around with optical filters or other methods for changing the response of the diode, I want to try this as is, relying on calibrations using a PAR meter, for each type of lighting I will use it for. If that doesn't work well enough, then I can buy another lux meter and play with it further.

 

[DarkCobra]

Sounds like a good plan to me. roud:

Out of curiousity, I fully disassembled the sensor on my Extech LUX meter. There are no identifying marks whatsoever on the actual photodiode. But here's the filter:

Close to what I'd expect from a LUX meter that's intended to mimic the human sensitivity to green. A bit more blue than expected, but that may be to compensate for photodiode response; which is probably biased towards red.

I also tested it for UV/IR pass. Only slight UV reduction. But it's a very good IR filter, I can't see the otherwise bright flashes from a remote control with my camera at all.

May come in handy later should you ever decide to play with alternate filters.

 

[mistergreen]

Curing cyanoacrylate emits fumes which can deposit a white substance on nearby objects, especially if they have any oil on them. It can and has been used by law enforcement to reveal fingerprints. But it's not corrosive.

Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.

Oh, and Mistergreen came up with an idea in the DIY PAR meter thread that adding a purple filter gel to reject some of the green, while passing a higher proportion of photosynthetic red/blue, might give results closer to that of a real PAR meter.

Shouldn't it be a green gel to deflect green out? Like chlorophyll although magenta makes sense.

 

[Hoppy]

Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.



Shouldn't it be a green gel to deflect green out? Like chlorophyll.

The color of a filter is deceptive. The color can be the transmitted color reflected back from outside the filter, or it can be light reflected off the face of the filter. But, it is the color coming through the filter that is the important one. This filter definitely looks green, and I think it is light that passed through the filter twice that gives it that color. Another guess: it is to adjust the peak transmission into the green band, the peak at 550 nm shown on that little chart. It might even come closer to a PAR characteristic with no filter at all.