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Old 06-08-2013, 11:42 PM   #166
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I have been asked if these PAR meters will work ok for reef tanks. To find out, I bought a 50-50 36 watt PC bulb, and tested a couple of them with that. One read 32 vs the Quantum meter reading of 33, and the other was only 2 units off at about the same reading. So, now I know they do work with 50-50 white/actinic light.
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Old 06-21-2013, 04:54 PM   #167
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While experimenting with putting 3 photodiodes in one sensor, in order to improve the spectral sensitivity of the PAR meter, I found that I could fit a photodiode into a 1/2" diameter acrylic tube. This led me to redesign the one diode PAR meter to be 2/3 the size of what I had been making - a 1/2" diameter vs. 3/4" diameter sensor.


Assembling this is much easier if you make a simple fixture to hold it while you work on it, while also keeping the photodiode located where it should be. This is just a piece of 3/8" tube, with a longitudinal saw kerf in one side, to make it fit inside a 1/2" acrylic tube easier, with a length sticking above the base equal to the desired distance from the top of the 1/2" tube housing to the top of the photodiode (9/32 inch), cemented into a sheet of acrylic.

Fit the 1/2" tube housing on the assembly fixture, and poke the electric cable through the hole, and up out of the housing far enough to let you strip the insulation about 1/4" back. Don't do this before trying to stick the cable through the hole - it is much easier to do before stripping the wires.


After stripping the wires, gently move the cable back so the stripped wires are inside the housing.


Now, look at the face of the photodiode and locate the mark, either a "+" or just a dot, which identifies the cathode of the diode. This is the lead that connects to the red wire in the cable. Turn the housing over and carefully drop the diode in, so the leads straddle the cable, with the cathode lead on the red wire side. Put the housing back on the fixture, so the diode rests on the top of the stub of tube in the fixture. Use a wood toothpick or something similar to bend up the wires to get them in contact with the diode leads, ready to solder the connections.


Continued on next post:
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Old 06-21-2013, 05:09 PM   #168
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Continued from last post:
Solder the cable wires to the diode leads.


Trim off the wires and diode leads that extend past the end of the acrylic housing. Install a 1/8" long 1/2" tube retainer (the retainers have a saw cut on one side. Use diagonal cutter to trim off more of the acrylic so it will clear the cable). Push the retainer down until it holds the photodiode against the tube stub in the fixture. This locates the photodiode in the right position. Put a drop of acrylic cement on the cable where it comes through the wall of the housing, and one other place on the retainer, to hold the retainer in place. Be sure to pull the assembly partly back off the tube stub in the fixture to make sure you don't cement it to the fixture.


Cut out two 5/16" diameter discs of Rosco #4815 pink filter, and carefully put them on top of the diode, with the assembly turned over and not on the fixture.


Install a 1/8" long retainer on top of the filters, to hold them and the diode in place. This will fit tightly inside the housing, but can be held in place with a drop of acrylic cement, if you wish.


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Old 06-21-2013, 05:23 PM   #169
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Continued from last post:
Cement the bottom of the housing onto the base by putting a puddle of acrylic cement where you want the housing to be attached and pushing the assembly into the puddle, moving it a bit to be sure you have a good contact. Hold the assembly in place for about 3 minutes to let the cement set. The electric leads may slightly interfere with this, so you need to hold it until their push won't separate the joint.


Cut the sensor off the lux meter - disassemble the sensor so you can cut with a long cable lead left ahead of the coiled part of the lux meter cable. Strip the wires on that cable, and the cable to the new sensor. Leave the ground wire on the new sensor extending about 3/4 inch beyond the wires so it can be used as a strain relief for the soldered connections.


Tin all 4 bare wires. Place shrink tube segments on the cables before soldering the two connections. I use shrink tube on both connections, with two layers over the whole cable connection, with the long ground wire wrapped around the lux meter cable so the shrink tubes will grab it to help relieve the strain on the connection.


Cut a 5/16" diameter disc of Rosco #114 diffuser filter and place it on top of the upper retainer in the sensor assembly. This filter is essentially invisible, so it takes great care to be sure it is in place before attaching the top "lens".


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Old 06-21-2013, 05:30 PM   #170
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Continued from last post:
Use black electricians plastic tape to hold the top lens in place, and black out the sides of the sensor, for calibration.


With the sensor shimmed up to match the height of the Quantum meter sensor, measure the PAR from a fixed light with both meters. They should match.


They do match! So, all that's left is to cement the lens in place, put acrylic cement over the cable to housing joint to waterproof it, and cement the socket for a wand to hold the sensor at the other end of the bottom strip.


The last step is to paint the exterior of the sensor housing with black nail polish, two coats, to block all light except that that comes in from the top. You now have a usable PAR meter!
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Old 08-23-2013, 04:26 AM   #171
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Hey Hoppy, how do you calibrate your PAR meter? Thanks.
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Old 08-25-2013, 12:53 AM   #172
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Quote:
Originally Posted by Jalopy View Post
Hey Hoppy, how do you calibrate your PAR meter? Thanks.
I use a Quantum PAR meter, or one of my PAR meters that I have very carefully calibrated several times. I set the two sensors side by side under a 50-50, 36 watt PC AH Supply light that is rigidly mounted above a stand I made for a 10 gallon tank. If my PAR meter reading isn't within 5% of the other meter, I adjust the spacing between the photodiode and diffuser, for the version I am making now, or for the older versions I adjusted the number of white diffuser filters I used, until I get the one being calibrated to be within about 2 digits at about 40 PAR. Since there is no way to adjust the slope of the PAR vs Quantum PAR, I only calibrate at one PAR reading.
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Old 08-27-2013, 07:20 AM   #173
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Hey Hoppy, does the thickness of the 1/2" acrylic disc in this version of the PAR sensor matter? Thanks.
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Old 12-22-2013, 12:35 PM   #174
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How do you convert Lux to par - please
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Old 12-22-2013, 08:43 PM   #175
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Quote:
Originally Posted by Aquaticz View Post
How do you convert Lux to par - please
Lux and PAR are measurements of the same thing - light intensity at a surface. "Lux" is the units used when measuring the light, with the spectrum weighted to match human eye sensitivity. PAR is the units used when measuring light over the complete 400 to 700 nm wave length range, with all parts of the spectrum treated equally. Because lux involves suppressing wave lengths not easily seen by humans, the two sets of units can't be directly converted from one to the other, without using the spectral distribution of the light source to adjust the lux measurement to one that treats all wave lengths equally. So, the conversion factor for going from lux to PAR is different for different types of light sources - fluorescent, the sun, LEDS, CFLs, T8 bulbs vs T5 bulbs of different color temperatures, etc.

Fortunately, for our use, we don't need to know PAR to within 1% or better. If we know it to within 10% that works perfectly well. So, it is possible to use a lux meter with the reading divided by about 60 - 80 to convert the measurement to the equivalent of PAR. Or, you can use various filters on a lux meter to tweak the frequency sensitivity of it, and the overall sensitivity of it, and make it read in PAR units, accurate to +/-10%, and possibly better, for the commonly used lighting types on planted tanks. That is what I was doing in this thread.

I have stopped making these "PAR meters", out of boredom, mostly. But, I think I can just slightly modify a lux meter like the ones I used, so it reads in PAR units almost as accurately as those I made. I'm making good progress towards doing so. That would make it an easy DIY project for anyone, or it would make selling modified units for $25 and making some profit for little work, possible. If this works out, I will start a new DIY thread about how to do it.
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