The Planted Tank Forum - View Single Post - Convert Lux meter to PAR meter?
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post #158 of (permalink) Old 12-13-2012, 07:10 PM Thread Starter
Planted Tank Guru
Join Date: Dec 2005
Location: Sacramento, CA
Posts: 21,015
I have been working steadily, day after day, trying to make this DIY PAR meter work better, and be easy to duplicate. I think I now have it about as good as it is going to get. After a lot of wheel spinning, and wasting parts, I finally sat down and figured out what it should take to get where I want to be. First, the cosine diffuser, I figured out, is just a way to put a light source for the sensor's photodiode at a fixed distance from the diode, and with a brightness that is proportional to the brightness of the light being measured. The amount of light this provides to the diode for measuring depends on how far it is from the diode, with the brightness dropping with the square of that distance, making the distance a critical dimension.

Next, I figured out that filtering the light to reduce its intensity at the diode, and modify the spectral sensitivity of the diode, isn't a good idea. This is because the diode does have some small sensitivity to infrared and UV, even though there is a glass filter mounted on it to greatly reduce its sensitivity to the non-visible light. But, the Rosco gel filters are not able to block the infrared light at all, or at least they only block a small portion of the infrared. That means as I filter out visible light to flatten the spectral response, and reduce the intensity, I am also making the IR light sensitivity go up as a percentage of the total sensitivity - not good at all. So, I have to use a minimum of filtering, preferably only one relatively weak filter.

But, you can "filter" the light by reducing the aperture the light shines through. The amount of light going through is roughly proportional to the area of the aperture - the square of the aperture diameter. So, the aperture is another critical dimension.

The best way to be able to reproduce both the aperture and the distance between the photodiode and the cosine diffuser is to make each of those parameters be set by an individual part, with one easy to control dimension. Along with that, the design has to position the photodiode in a fixed location.

With all of that in mind I came up with this: (Using an Excelitas photodiode|plid| instead of the lux meter diode, to get around the filter on the lux meter diode.)

This is made by first making the 6 acrylic pieces. I used plastics from Tap Plastics, Also, I had a scrap piece of 1/16" thick acrylic I used for one piece.

Starting at the top: I drilled a centered .25" hole through one of the 3/4" diameter acrylic discs. Then I used black nail polish to paint the inside face of that disc black. I poked and tapped in a short piece of .25" dia. frosted acrylic rod in the hole with a bit of Weldon #16 cement on it. When it cured I hack sawed off the excess rod and used fine sandpaper to trim it flush on the top side of the disc, with the rod already flush with the bottom side.

For the aperture, I drilled a .125" diameter hole through a 3/8" square of 1/16" acrylic. and painted the inside face of it with black nail polish. When that cured I cemented it on the bottom of the disc with the frosted rod in it, so the hole was centered over the frosted rod end. This is the cosine diffuser plus aperture.

Next, I cut a piece of 3/4" OD, 1/16" wall acrylic tube about 3/8" long with square ends. (I used a small cheap miter box and hacksaw.) Before I cut it, I drilled a hole centered on one side, with the hole diameter the same as the electric cable diameter.

To mount the diode, I used another of the 3/4" diameter acrylic discs, with a carefully centered .312 dia. hole through it. (Drilling centered holes is easy if you first make a jig from a piece of plywood, drilling a shallow 3/4" dia recess to fit the disc, then using the center of that hole to locate a .25" or .312" hole the rest of the way through the plywood. Push the disc into the hole, clamp the plywood to a workbench with the disc captured between the plywood and the bench, then drill the hole through it, using the hole in the plywood as a centering guide.) The Excelitas diode is .312 in diameter, with two parallel cutoffs making it a rounded rectangle, so it fits snugly into the .312 hole in the disc. With the face of the diode at the top of the disc, I used Weldon #16 to glue the diode into the disc.

Poke the electric cable through the hole in the 3/4" tube piece far enough so you can strip the wire so you have two stripped wires about 1/2" long. Use the plywood drill guide to hold the disc that holds the diode, so you can locate the electric cable in between the two electric leads to the diode, and wrap one wire around one lead and the other around the other lead. The positive lead is identified with a "+" marked on top of the diode. The red wire goes around that lead. Solder the two connections and cut off the excess wire.

Carefully push and pull the wire back to get the diode inside the piece of 3/4 tube, with the disc sitting on top of the tube. Use Weldon #16 to cement it on, being careful not to get it on the diode sensing surface. Apply a filet of Weldon #16 around the cable inside the 3/4 tube, and outside as well. This seals the water out of the inside, and acts as a strain relief to avoid pulling the wire off the diode.

Cut a .25 inch long piece of the 3/4" acrylic tube to be the spacer that positions the diode .25" from the cosine diffuser. Keep the faces parallel and square, and use fine sandpaper to get the length accurate. Use Weldon #16 to glue that on top of the diode mounting disc.

Cut a rough circle of Lusco #3313 Tough Minus Green filter and put that on top of the diode - it can be loose since it can't go anywhere.

Use Weldon #16 to glue the cosine diffuser/aperture to the top of the assembly, with the aperture to the inside. After that cures, glue the assembly to a strip of acrylic about 1.25" x 5" to seal off the bottom and act as a handle for the sensor. Add another coat of Weldon #16 to the outside of all cemented joints to be sure they are waterproof.

Use black nail polish to paint the exterior surfaces, being very careful not to overlap the frosted rod diffuser disc.

Connect the end of the electric cable to the luxmeter readout. I cut the cable to the readout off as close to the luxmeter sensor as I can, to have a straight section of wire to make the soldered connections. Connect red to red. Use shrink tubing to insulate and reinforce the connection. Since the luxmeter cable wires are very thin, and weak, I use the ground wire on the electric cable, not otherwise used for anything, to overlap the insulation of the luxmeter cable by an inch or more, and use black electric tape to tightly hold it to the insulated cable to relieve the load on the soldered wires.

The performance of this PAR meter is:

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