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Old 06-22-2012, 06:51 PM   #46
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Calibration chart above updated with under water reading.
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Old 06-22-2012, 08:08 PM   #47
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Back to work on the sensor-from-scratch: I drilled the hole for the electric cable, a 6 foot long piece of 22 guage, shielded, round, flexible cable. Then cut it to length. The hole is 1/4 inch from the back of the half sphere end.


With the cable threaded into the hole, carefully, so as not to crack the acrylic.


Soldering the cable to the photo diode was easier than I expected, although it is at the limit of my vision capabilities. Once soldered on I carefully pulled and poked the cable back and pushed the diode up to near contact with the flat side of the half sphere. Looking through the other side I could near center the diode.


The next problem was holding the diode in place before potting the whole thing with Weld On #16. I ended up ripping a sandwitch bag in pieces, wadding the pieces into little balls and poking them in to hold the diode in place. This is primarily to keep the acrylic cement from flowing down over the sensing side of the diode. To be sure the connections were still good, I used a millivolt meter to check for an output at the other end of the cable. I had a couple hundred mV or so.

With the cavity half filled with Weld On #16: (the masking tape is to help hold the cable steady.)
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Old 06-22-2012, 10:30 PM   #48
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Quote:
Originally Posted by Hoppy View Post


This is how the calibration of the DIY meter looks, for the first version, with the sensor head inside an acrylic jar. This should be repeatable if anyone else does one this way.

The next test is the one it will probably fail - submerging it in water to see if it leaks.

EDIT: First a 5 minute dunk, then a 15 minute dunk. No leakage that I can see! Next will be to compare the readings in water to the PAR meter, to see if the calibration changes.

EDIT: Tested in aquarium, 2 feet high (2 feet head of water pressure on sensor). The data point falls on the same line as when calibrated in the air. This definitely works. However, the sensor is still too buoyant, floating up if your fingers slip at all. And, it needs a "wand" to be able to use it effectively without your hand getting in the way.

I think the sensor-from-scratch that I'm making will be a big improvement.
Looks good over a wide range of lighting types.

I think the lux conversion Ole and some others came up with was less, around 60 vs this one which is 76.
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Old 06-22-2012, 10:34 PM   #49
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Quote:
Originally Posted by plantbrain View Post
Looks good over a wide range of lighting types.

I think the lux conversion Ole and some others came up with was less, around 60 vs this one which is 76.
I knew this would be higher because of the loss through the acrylic jar lid. I had checked that before I started.

I just checked the new sensor with a mV meter and got about 380 mV in room light. The acrylic cement potting is still curing, so I will have to wait until tomorrow to calibrate the lux/PAR meter with it.



Quote:
Originally Posted by mistergreen View Post
You might need a semi-opaque acrylic material to make it a real cosine diffuser.
The parabolic curve (output) may be too steep with your frosted semi-sphere. Calibrating that won't be fun. I found out that the light box acrylic I used works pretty darn well. It doesn't have to be a curve/lense.
I think you may be right now that I have it together. I can see the filter on the diode too clearly now. I may coat the spherical surface with white acrylic fingernail polish to fix that. How does that sound?
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Old 06-23-2012, 01:17 AM   #50
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Quote:
Originally Posted by Hoppy View Post
I may coat the spherical surface with white acrylic fingernail polish to fix that. How does that sound?
Give it a try but I don't think it works by blocking out photons but rather bounce photons around in a medium until it hits the photodiode.
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Old 06-23-2012, 02:22 AM   #51
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Quote:
Originally Posted by mistergreen View Post
Give it a try but I don't think it works by blocking out photons but rather bounce photons around in a medium until it hits the photodiode.
Basically spreads the light out nice and evenly over the area of the sensor.

Hoppy, you could wire several of these sensors together and make a rail or a large area meter to measure the light also. Making a grid/linear slice if you will.....
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Old 06-23-2012, 03:45 AM   #52
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As I understand it the "cosine" correction is to get the light from a large solid angle, as much as a half sphere angle, and the "frosting" would be to diffuse that light so it would be uniform over the whole sensor surface. Fingernail polish doesn't go on very uniformly without a lot of care in applying it, but a single coat does diffuse the light very well. I think I will calibrate it as is first, then coat it with the white polish and calibrate again. If I can find that photo diode locally these sensors are extremely cheap, and not too hard to make.

Tom, I'm not sure how I would use a grid of sensors. If they were wired in parallel it should give the average PAR over that grid (I think). Or, reading the PAR with a computer that plots the data points would give an interesting plot, too. But, it is hard enough to find spots in a planted tank where the light isn't partly blocked by foliage as it is, a grid would only work in a bare tank.
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Old 06-23-2012, 07:33 PM   #53
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Calibrating the sensor as it was didn't work very well. It was too sensitive to the angle and not as linear as I wanted, plus it was too sensitive - too high readings. So, I coated it with two coats of white fingernail polish. On recalibrating it, it worked fine, but with a different calibration for LED light and 6500K CFL light. That is ok with me, since I will only use it with one of those. I now dub this the "PAR Volksmeter".


You can see that this has sensor a little smaller than the Quantum PAR meter. It is also easier to use than the Quantum meter for low PAR readings because of the 3 position switch on the readout.

The calibration is:



I will let the nail polish dry a few hours, then test it underwater. I see no reason for it to either leak or change calibration underwater.

Success, an easy DIY of a cheap lux meter to get a very cheap "PAR" meter, usable for most of our uses. I trust the calibration with LEDs more than the one with the 6500K CFL, because the latter was much harder to do, due to the small area of highest light under the CFL bulb. The cost: about $15 plus shipping for the lux meter, about $5 plus shipping for the photo diode, about $2 for the acrylic parts, $2 for the electric cable, plus odds and ends.

EDIT:
I recalibrated the PAR Volksmeter today. Yesterday I modified it by attaching a strip of acrylic to the bottom of the sensor, with a post at one end that will fit snugly into a 3/4" diameter acrylic tube, as a handle, so it is much easier to position and hold accurately.
Then, after a lot of thought I realized how much accumulated error is in each calibration point. For example, the PAR meter is only accurate to +/- .5 micromols, which is a large error at low values of PAR. Then just tiny mistakes in distance from the light source makes a big error in PAR readings. Plus minor mistakes in location of the two sensors adds another error. So, I tightened my positioning control, and weighted the data points by how much possible error each data point had. That gave me a big change in calibration constant, so I corrected the chart above. It also made the calibration constant be the same for the LED and 6500K CFL lights. That constant is 80 +/- 5 Lux per PAR unit. The chart above doesn't include a couple of data points that were way outside the area covered by the chart, but those points were included in calculating the constant. (Very high PAR means the sensor is close to the light source, where errors in distance are very significant and very hard to eliminate.)
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Old 06-23-2012, 08:40 PM   #54
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Interesting. I wonder what the divergence in the CFL and LED. light color?

When I tested the DIY PAR meter, I used T5HO, LED, and spiral CFL. They came out pretty much close to PAR.
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Old 06-23-2012, 09:15 PM   #55
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I don't know why the CFL data was so different from the LED. I'm still reluctant to accept that the data is right for CFL. For that one I taped the two sensors together, and moved them around under the light to get the maximum reading on both. For the others the area lit up about equally was much larger, and I just had the two sensors side by side, for one LED light, and placed at the same spot for the other light. I took several readings at each distance and/or LED current, to be sure I didn't have a sensor in a "dead spot".

I expect to play with calibration some more, just to build up my confidence. Right now I am just tickled to see that the idea works well enough to make a usable PAR meter. We aren't scientists, so "hobby grade" testing is all we really need.
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Old 06-23-2012, 11:46 PM   #56
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Now that I no longer need the sensor head from the lux meter I completely disassembled it. This is the active parts, which are held in by one tiny screw, and a snap-in holder for the photo diode, and 4 snap-in tiny knobs for the cosine diffuser. From this it would be easy to make a little larger sensor like the one above, using an acrylic tube about 1.25 inch inside diameter. Tap Plastics has 1 1/2 inch O.D. clear tube with 1/8" wall, which would work well. It costs about $17 for a six foot piece. Or, there are lots of small acrylic boxes and other small containers on Ebay which would work. I probably won't spend any more on this, unless I get a great idea I can't resist.
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Old 06-24-2012, 12:38 AM   #57
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One big difference I noticed looking at the color filter, the LUX sensor is green while the VTB88... is blue-ish. So I guess it's somewhat more suitable for PAR readings
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Old 06-24-2012, 08:57 PM   #58
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Calibration was redone, and comment #53 revised, above.

I checked my light level in my 65 gallon tank. With the Quantum meter I get 25 +/-5, depending on where I put it, and how much shading I get from the leaves, etc. With the Volksmeter, I get 20, and I couldn't get it in the exact same place I was able to get the Quantum meter, because the handles are too different. I'm pleased with this, and will probably use it for my routine measurements now.
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Old 06-26-2012, 01:43 AM   #59
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If anyone is interested in making one of these, but doesn't have easy access to the plastic parts and electric cable, I can put together a kit of those parts:


With that kit plus the lux meter and photodiode, which anyone can buy on the internet:

There are other vendors for those parts, with different and possibly lower shipping costs.

You can make a usable PAR meter for about $31 plus three shipping costs. PM me if you are interested. You would have to make your own handle to hold it at the bottom of the tank so your hand and arm don't block part of the light. And, of course, for best accuracy you need to have access to a good PAR meter to calibrate yours.
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Old 06-26-2012, 03:16 AM   #60
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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.
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