Converting a Lux Meter to a PAR Meter

VaughnH

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About a year ago I discovered that we can buy a lux meter, made in China, for about $13-$25 on Ebay and Amazon. This aroused my interest in trying to find a way to convert one into a PAR meter, and at that price experimenting isn't very costly. So, I bought a few of them and started a long, long project! The model number of the lux meter I used is LX1010B, which seems to be available from more than one Chinese manufacturer

Initially I took the easy way out, and simply put the sensor, which is separate from the readout meter, in a waterproof housing. This gave me a lux meter that would work under water, but holding it down against the buoyancy proved to be more than I wanted to work with. Next, I cut up the sensor, and put just the minimum part of it in a water proof housing. It was still too buoyant, but this also let me experiment with filtering the light to try to approximate the PAR spectral range. Unfortunately, the photodiode sensor in these has a built in, one piece filter that eliminates the IR and UV portion of the spectra, and adjusts the visible spectral sensitivity so it reads little more than the green light.

I then found that we can get a free sample book of Roscolux gel filters from any retail dealer for those filters, and there was a local dealer near me. I picked up a book of samples, each of which is shown in their website, http://www.rosco.com/filters/roscolux.cfm with a graph of the spectral response for that filter, plus a tabulation of the response for every 20 nm portion of the spectrum. This enabled me to spend many hours with a spreadsheet trying to find a filter or combination of filters that would magically transform the lux sensor to a PAR sensor. (It can't be done!)

Fortunately, a poster on The Planted Tank was working on building a PAR meter from scratch, and he found a readily available photodiode that has a much better spectral response. So, I chopped off the sensor from my lux meter and started designing a sensor from scratch, using Roscolux filters. This is the last version, which has worked very well:

View attachment 4506

To be continued:
 
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VaughnH

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The calculated spectral response for this design is:
[attachment=1540:name]

The "secret" to assembling this is to make each part with an accurate length, because the lengths of the stubs of acrylic tubing are what sets the dimensions of the optical path, which has a big effect on the calibration. The assembly steps are:
1. Make all of the parts ahead of assembling.
2. Attach the photodiode to a small piece of circuit board by soldering the leads to the copper strips. One lead per strip, of course.
3. Pull one end of a 3 foot length of Belden 8451, 2 wire, flexible, 24 gauge, shielded with ground wire cable through the small hole in the side of the 3/4" dia acrylic tube, and out the end of the tube.
4. Strip about 1/4" of insulation, then about 1/8" of insulation on each of the two wires. Cut off the ground wire and shield.
5. Identify the photodiode lead that is positive, marked with a "+" on the case, and solder the red wire to it, and the other wire to the other lead.
6. Work the cable back through the hole so the mounted photodiode is inside the 3/4" tube, at about the right position.
7. Insert the slotted piece of 5/8" dia acrylic tube, with the slot clearing the cable.
8. Align the photodiode in about the right position, and, using Weld-On #16 acrylic cement, glue the 1" x 5" strip to the bottom of the 3/4" and the 5/8" tube. Use a scrap piece of 5/8" tube to hold the inner acrylic tube to the bottom along with the outer tube.
9. Install the 1/4" long 5/8" tube on top of the mounted photodiode, so the piece of circuit board is trapped between the two pieces of 5/8" tube. To hold it in place, first remove it, add a drop of #16 cement to the circuit board/inner surface of the 3/4" tube, and shove the 5/8" tube in place to be stuck in place by that drop of cement.
10. Install the Roscolux [HASHTAG]#4815[/HASHTAG] (pink) filter on top of the 5/8" tube. Place the Roscolux [HASHTAG]#102[/HASHTAG] diffuser filter on top of the pink filter, and place one [HASHTAG]#114[/HASHTAG] diffuser filter on top of that.
11. Install the 1/16" long 5/8 tube on top of the filters to hold them in place temporarily.
12. Use a piece of black plastic electricians (or other) tape to hold the 3/4" dia acrylic disk on top of the 3/4" tube, and block any light from entering the side of the assembly.
13. Solder the electric cable to the lux meter cable, keeping the red wires together. I use shrink tubing to strengthen the joint and insulate it, and I extend the ground wire so it is attached to the outside of the lux meter cable to act as a strain relief.
14. Hold the sensor in the light from a typical aquarium light, along with a known good PAR meter, preferably a Quantum meter.
15. Compare the meter readings - the lux meter will read on the 50,000 scale. It should read about 10% high.
16. If it does read about 10% high, add another Roscolux [HASHTAG]#114[/HASHTAG] diffuser filter on top of the existing one, and repeat the calibration.
17. When you are satisfied, carefully glue the acrylic disc on top with #16 cement. Use the same cement to seal the electric cable into the hole in the sensor outer tube. Paint the exterior of the sensor with black nail polish.
18. You now have a PAR meter!

PARVolksmeter Spectral Response.jpg
 
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Gerryd

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Vaughn,

Great job and documentation. Thanks so much for this. I don't understand 90% of it but others will :)

I made it a sticky as well. Do you have any pics of the build?

Thanks!
 

VaughnH

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View attachment 4565I have tried to take photos as I build one, but it is so small, and is transparent, so my camera doesn't work well enough for the photos to be legible. I have a close-up option on the camera, but it doesn't seem to work right. All I can do is post drawings, and the photo of the completed assembly. I hope someone else will step up and start making these to sell. It gets boring to make them after the first 30 or so.

ImprovedPARSensor_zpsf553d120.jpg
 
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Feidlimidsmith

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Jun 18, 2013
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its not working well here i am annoyed and frustated with this problem how could it be helped i need of few recommendations for getting of this situations here,
 

cube

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Mar 18, 2013
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Where can i get the diode Excelitas VTB9441BH ?

Hello!

I was reading you post and really want to make a meter after reading.
Problem is, i found everything needed except the diode Excelitas VTB9441BH.
In another forum post, you mentioned of Newark Electronics. But found nothing as Excelitas VTB9441BH.Rather i found this one VTB8441BH.

So i would be very glad if you kindly tell me where can i get those diode online.
And thanks for the whole post.
 

VaughnH

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Sorry to be so late replying. The Excelitas part number on my article has a typo. The one to use is the one you found - VTB8441BH and Newark Electronics is where I buy them.
 

VaughnH

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[attachment=1605:name]
This is the latest configuration I am using. It solves some of the adjustment problems I have had, but is still very sensitive to the dimensions being corrrect, so the distance between the bottom of the diffuser to the diode, and the length of the diffuser rod are both very close to what the sketch shows. When it is, it isn't difficult to adjust the sensitivity so the readout is accurate.

PAR Meter Sensor 7.jpg
 

VaughnH

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Jan 24, 2005
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View attachment 4766
This is the latest configuration I am using. It solves some of the adjustment problems I have had, but is still very sensitive to the dimensions being corrrect, so the distance between the bottom of the diffuser to the diode, and the length of the diffuser rod are both very close to what the sketch shows. When it is, it isn't difficult to adjust the sensitivity so the readout is accurate.