DIY PAR meter, Yeah you heard me - Page 2 - The Planted Tank Forum

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post #16 of 733 (permalink) Old 11-09-2010, 10:41 PM
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Truly some pioneering DIY action! Great job!


"The fewer our wants, the nearer we resemble the gods." — Socrates

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post #17 of 733 (permalink) Old 11-10-2010, 12:11 AM Thread Starter
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Thanks. Trying to make the hobby more accessible.

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post #18 of 733 (permalink) Old 11-10-2010, 04:03 PM Thread Starter
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I don't think I showed you how I'm reading the values so far.



It's all through USB... It's supplying the ardiuno power and a way for the ardiuno to communicate to my computer...
This is my serial monitor outputting the values.


The values jump/dance around like it's being hit by photons, kinda cool. So I wrote the code to find the average within every second...

NON-PAR code...(NOT FINAL)
Code:
unsigned long average = 0;
unsigned long time;
int counter = 0;

void setup() {
  Serial.begin(9600);
  time = millis();
}

void loop() {
  int sensorValue = analogRead(0);
  average += sensorValue;
  counter++;

  //every second or 1000 millis
  if(millis() > time+1000) {
    average = average/counter;
    Serial.println(average);
    
    //reset timer & counter to get ready for the next second.
    time = millis();
    counter = 0;
    average = 0;
  }
}
Oh, I double checked the readings by going outside... Full sun will give me the maximum value of 1023.. In the shade, around 100-120.

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post #19 of 733 (permalink) Old 11-10-2010, 04:08 PM Thread Starter
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For those wanting to play with a microprocessor, check the Texas instrument one. It's only $4.50!!!
http://processors.wiki.ti.com/index....+launchpadwiki

I haven't played with it but my dad got one for me.

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post #20 of 733 (permalink) Old 11-10-2010, 05:28 PM
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I think PAR in full sun at midday is around 2000. That suggests your readings may be about half of the true PAR values.

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post #21 of 733 (permalink) Old 11-10-2010, 05:39 PM Thread Starter
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Yeah, I'm mapping out the values now to see the relationship...

Hoppy, I have a question about your PAR - distance chart.
Your graph implies a parabolic curve, meaning the closer you get to the bulb, the more it gets to infinity or to the max value (the sun 2000 PAR). This is assuming your bulb is outputting at the level of the sun? It doesn't seem right.

I'm definitely getting a fixed value no where near the maximum with my DIY meter.

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post #22 of 733 (permalink) Old 11-10-2010, 06:51 PM
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post #23 of 733 (permalink) Old 11-10-2010, 07:02 PM Thread Starter
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Here you go..


The proportions doesn't look like a x2 constant calibration... It looks like a gaussian curve (bell curve).... Will work on the equation unless some math wiz out there beat me to it

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post #24 of 733 (permalink) Old 11-10-2010, 07:41 PM Thread Starter
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Ok,
I found the equation... There were some guessing going on because I didn't know what hoppy's chart start value is.

It looks like these bulbs output are bell curves rather than parabolas.

f(d) = A * (11e^(-d^2/(2*35^2)))
A is the value from my DIY sensor, d is distance.

I'll graph this out to see how close they fit.

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post #25 of 733 (permalink) Old 11-10-2010, 07:43 PM
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Great idea. This might be my next dig as well.

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post #26 of 733 (permalink) Old 11-10-2010, 07:55 PM
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The inverse square relationship of PAR and distance only works within a relatively small range of distances. When you get close to the bulb it breaks down. I determine relationships like this by just plotting the data on log-log graph paper. If the points form a straight line, it demonstrates that one variable is a function of a power of the other variable, with the slope of the straight line being the power.

It is very hard to measure distances very accurately with tests like this, and PAR meter readings can fluctuate as you read the meter, and vary from hour to hour. So each data point can be assumed to have a pretty large uncertainty. Considering all of that, if a line with a slope of minus 2 will fit the data points reasonably closely within the distance range that is relevant for aquariums, it is a good approximation to assume the inverse square relationship. Then, when you toss in values for multiple bulb fixtures, using the PAR number divided by number of bulbs, and those data points follow the same line, you can see that it is a good approximation to say that n bulbs give n times the PAR of one bulb.

It makes no difference to us, when we are looking for an aquarium light, whether or not the PAR varies with square of distance at distances of 1 inch to 4 inches, so I never bothered to measure it at those distances, except with single LEDs. Single LEDs are so near a point source that the inverse square relationship will hold true at even 3-4 inches from the single LED. It gets much more complicated when you have an array of LEDs.

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post #27 of 733 (permalink) Old 11-10-2010, 07:56 PM Thread Starter
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Doh! there's no way to tell the sensor what the distance is to the light source... will have to rethink the equation.

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post #28 of 733 (permalink) Old 11-10-2010, 07:58 PM Thread Starter
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Quote:
Originally Posted by Hoppy View Post
The inverse square relationship of PAR and distance only works within a relatively small range of distances. When you get close to the bulb it breaks down. I determine relationships like this by just plotting the data on log-log graph paper. If the points form a straight line, it demonstrates that one variable is a function of a power of the other variable, with the slope of the straight line being the power.

It is very hard to measure distances very accurately with tests like this, and PAR meter readings can fluctuate as you read the meter, and vary from hour to hour. So each data point can be assumed to have a pretty large uncertainty. Considering all of that, if a line with a slope of minus 2 will fit the data points reasonably closely within the distance range that is relevant for aquariums, it is a good approximation to assume the inverse square relationship. Then, when you toss in values for multiple bulb fixtures, using the PAR number divided by number of bulbs, and those data points follow the same line, you can see that it is a good approximation to say that n bulbs give n times the PAR of one bulb.

It makes no difference to us, when we are looking for an aquarium light, whether or not the PAR varies with square of distance at distances of 1 inch to 4 inches, so I never bothered to measure it at those distances, except with single LEDs. Single LEDs are so near a point source that the inverse square relationship will hold true at even 3-4 inches from the single LED. It gets much more complicated when you have an array of LEDs.
hmm. In that case, I use a simpler equation like a regular parabola.

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post #29 of 733 (permalink) Old 11-11-2010, 12:19 AM
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I plotted your data for the 3 bulb unit on log log paper and got:

The line is the slope if the data follows the inverse square rule. You can see how much scatter there is, and that the close to the bulb readings diverge from the other data. This isn't much different from the data I got from other sources. And, it is why the charts I made are only good for approximating the PAR - you have to measure in your tank with your lights to get really accurate measurements.

If I were to add your data to my charts I would read off data points from that line, not your actual data. That reduces the scatter a great deal, and makes it possible to do this.

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post #30 of 733 (permalink) Old 11-11-2010, 12:36 AM Thread Starter
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This is the best I can do, with my sensor
(multiply 8 to the single bulb data)



But as we know I went outside into the sun I'd get 8000 PAR

I'm curious to see if the mamamatsu photo diode would give me a better output. I think the Infra Red is affecting the readings.

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