Dirt cheap PAR meter (kind of) - The Planted Tank Forum
 
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post #1 of 14 (permalink) Old 12-20-2010, 06:37 PM Thread Starter
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Dirt cheap PAR meter (kind of)

A true PAR meter is expensive, but a LUX meter is cheap. As I understand, the PAR meter measures only part of the spectrum, while a LUX meter measures the entire visible spectrum.

So it is possible to measure the LUX intensity and knowing the spectrum, to calculate the PAR intensity!

See an example here: http://www.olino.org/us/articles/200...t-articles#PAR
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post #2 of 14 (permalink) Old 12-20-2010, 06:58 PM
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You really have to be careful to what light spectrum the LUX meter is sensitive to. This is dependent on what instrument you have. If it's sensitive to ultraviolet or infrared, it's pretty useless as a PAR meter.

here's a formula to convert lux to PAR

Quote:
Multiply the Lux by the conversion factor to get PPF. For example, full sunlight is 108,000 Lux or 2000 μmol m-2 s-1 (108,000 ∗ 0.0185).
different light sources have different factors.
Quote:
Sunlight 0.0185
Cool White Fluourescent Lamps 0.0135
High Pressure Sodium Lamps 0.0122
High Pressure Metal Halide Lamps 0.0141
It's neat but not very user friendly.


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post #3 of 14 (permalink) Old 12-21-2010, 03:51 AM
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Lux is a measure of light adjusted to the sensitivity of human eyes. PAR is a measure of light with no adjustments. There is no easy conversion factor between the two, if there is any at all.

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post #4 of 14 (permalink) Old 12-21-2010, 09:13 AM Thread Starter
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Quote:
Originally Posted by Hoppy View Post
There is no easy conversion factor between the two, if there is any at all.
I think that the conversion factor can be computed empirically. Just measure the LUX and PAR of a given light source simultaniously and you can find this "conversion factor". Is there anyone on this forum with a LUX and PAR meter?
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post #5 of 14 (permalink) Old 12-21-2010, 01:12 PM
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on Tom Barr forum i have found a thread about buying just a SENSOR from apogee instruments and use multimeter to get the data from it. It is a lot cheaper than the whole thing. The sensor is calibrated, so it is possible.
However, i did not find there any data on how to convert measurements into PAR values.
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post #6 of 14 (permalink) Old 12-21-2010, 02:42 PM
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Quote:
Originally Posted by artemm View Post
on Tom Barr forum i have found a thread about buying just a SENSOR from apogee instruments and use multimeter to get the data from it. It is a lot cheaper than the whole thing. The sensor is calibrated, so it is possible.
However, i did not find there any data on how to convert measurements into PAR values.
I believe it is 2millivolts per PAR unit (from apogee).
The sensor isn't that cheap either, ~$120-$200.


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post #7 of 14 (permalink) Old 12-21-2010, 02:45 PM
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Quote:
Originally Posted by MosMike View Post
I think that the conversion factor can be computed empirically. Just measure the LUX and PAR of a given light source simultaniously and you can find this "conversion factor". Is there anyone on this forum with a LUX and PAR meter?
I wish it was this easy or we'd all invest in LUX meters years ago.


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post #8 of 14 (permalink) Old 12-21-2010, 04:13 PM
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Quote:
Originally Posted by mistergreen View Post
I believe it is 2millivolts per PAR unit (from apogee).
The sensor isn't that cheap either, ~$120-$200.
Oh! I found that doc. That's 5 units per 1mV. So, for 40 we would have 8mV. That pretty low for your usual multimeter. This sensor is 150$
There is another one for 200$ with 5V supply which gives 0.5 units per mV,
so, for 40 it is 80mV, which quite measurable. But 200$.

Though, one can build a 100x differential amp for the first one for a couple bucks and power it from 9V battery.

On a side note: why not put a yellow filter above the sensor and strip off the yellow range?
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post #9 of 14 (permalink) Old 12-21-2010, 04:37 PM Thread Starter
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Originally Posted by mistergreen View Post
I wish it was this easy or we'd all invest in LUX meters years ago.
I think that this is what the guys did here: http://www.olino.org/us/articles/200...t-articles#PAR

They use a LUX meter and a spectrograph (can be done for $20) to calculate PAR output.

Quote:
So, for 40 we would have 8mV. That pretty low for your usual multimeter.
True, spend $90 on a Fluke 18B
or a Mastech
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post #10 of 14 (permalink) Old 12-21-2010, 06:08 PM
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Quote:
Originally Posted by artemm View Post
Oh! I found that doc. That's 5 units per 1mV. So, for 40 we would have 8mV. That pretty low for your usual multimeter. This sensor is 150$
There is another one for 200$ with 5V supply which gives 0.5 units per mV,
so, for 40 it is 80mV, which quite measurable. But 200$.
Oh! It depends on the sensor... Didn't know they have several types.

Have you guys seen my DIY PAR meter thread?
https://www.plantedtank.net/forums/di...u-heard-6.html

I used a photodiode that fits within 400nm - 700nm.


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post #11 of 14 (permalink) Old 12-21-2010, 07:27 PM Thread Starter
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I saw that thread. Actually you made a "selective" LUX meter. Do you plan to compare it face to face to a true PAR meter ?
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post #12 of 14 (permalink) Old 12-21-2010, 08:05 PM
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Quote:
Originally Posted by MosMike View Post
I think that the conversion factor can be computed empirically. Just measure the LUX and PAR of a given light source simultaniously and you can find this "conversion factor". Is there anyone on this forum with a LUX and PAR meter?
You would have to do that for each manufacturer's bulbs, for each color temperature bulb by that manufacturer. Of course this is easy to do, if you have all of those bulbs - but then, since you would obviously be wealthy, why not just buy a good PAR meter?

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post #13 of 14 (permalink) Old 12-21-2010, 10:15 PM
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Quote:
Originally Posted by MosMike View Post
mistergreen
I saw that thread. Actually you made a "selective" LUX meter. Do you plan to compare it face to face to a true PAR meter ?
Actually what I made is a meter sensitive to 400nm - 700nm, not necessarily a LUX meter and irrespective of 'light temperature color' or Kelvin. LUX corresponds to the human eye.

I studied a paper written
Peter Fielder B.C. Ministry of Forests Research Branch 712 Yates Street Victoria, BC

to make my sensor.
http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf


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post #14 of 14 (permalink) Old 12-22-2010, 03:33 AM
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I thought you guys might find this interesting.... A way to understand how a lux meter works is to take a peek inside and see what it's made of....

Quote:
The Lux Meter is usually used to measure illumination. The illumination is how level of luminous flux is falling on a surface area. The luminous flux is visible component that is defined in radiant flux (light power) divided by relative sensitivity of human eyes over the visible spectrum. This means the Lux is well fit to light level from sense of human eyes.
However there is a difference between spectra response of ordinary silicon photo diode and human eyes, it cannot be used for the lux meter. Some photo diodes for illumination sensor have a color compensation filter on the window to correct its spectra response.
The photodiode in a Lux meter is filtered spectrally for the human eye.
http://elm-chan.org/works/lux/report.html

My meter does not.


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