# PAR meter



## faizal (28 Jun 2012)

Okay,...I was just wondering if I was using the Apogee MQ 200 PAR meter properly. It comes with 2 modes of measurement 
1. SUN
2. ELECTRIC

Now it says there on the manual that if we want to measure the PAR from sunlight hitting the leaves we should set it on SUN & if we want to measure the PAR from an electrical source,...we obviously gotta set the mode to ELECTRIC.

Sounds pretty straightforward right? Yeah,...but there's this guy on this forum that I recentely checked out who uses the SUN mode to measure his light intensity in the aquarium because his light tube is rated as 8500 kelvin.

Am I missing something here? The thought of taking wrong readings this past one year is kind of humiliating.


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## ceg4048 (28 Jun 2012)

No mate, you are not missing anything. The Matrix is missing life forms capable of sensing reality. That's why it's such a formidable opponent. 

Have you ever had a dream you felt was so real? What would happen if you were unable to wake from that dream? How would you know the difference between the dream world and the real world?

Cheers,


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## Nutbeam (28 Jun 2012)

take the blue pill


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## gmartins (29 Jun 2012)

I'd think the difference between SUN and ELECTRIC is just one of scale. Full sun will give you far gretaer readings (at least 1 or 2 orders of magnitudes higher) than a normal lamp. Check if the scale at which the readings are outputed changes when you set each of the options.

Otherwise, it makes no sense...

GM


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## ceg4048 (29 Jun 2012)

No, not really mate. The scale is the same for both. When you stick the probe in sunlight on a sunny day it reads up to about 2000 micromoles or so. If you place the probe directly on the surface of an illuminated bulb it will read similar numbers.

The difference in calibration lies in the response characteristics of these particular photoelectric sensors, which have a different response to various wavelengths. At the very edges of the spectral envelop, 400nm & 700nm the sensors are not capable of full fidelity. They fail to exactly record the photon flux of blue (400nm) and they over-respond to yellow (550nm). They are completely blind to far red (650nm and higher) but they respond perfectly to green (500).

The differences in calibration therefore lie in the differences in spectral output of light bulbs versus that of the sun. Different corrections are necessary based on the different outputs of the various types of bulbs. If using a sunlight calibration while measuring fluorescent light the readings will be about 10% high on the Apogee meter and about 8% high if measuring Metal Halide. The accuracy when using the appropriate calibration is within 2%-3% but it's significantly higher when using the wrong calibration.

It's completely delusional to think that just because your bulb is rated at some 8000K, which is a phony marketing value anyway, that it replicates the spectral characteristic of the Sun.This is probably as powerful a delusion as believing your PO4 test kit.

Cheers,


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## spill50 (29 Jun 2012)

So you're basically saying he should be using the "Electric" setting when measuring light levels in a tank as "Sun" will be about 8-10% out?


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## ceg4048 (29 Jun 2012)

Yep, absolutely, unless of course that amount of offset doesn't bother you. It may not matter in some cases I guess.

Cheers,


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## spill50 (29 Jun 2012)

Hmmm it's interesting, I don't have a dedicated par meter but I do have a seneye reef which measures kelvin, lux and par. Not used it yet but may do some measurements tonight see what I get.

It would be interesting to know how accurate these devices are.


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## faizal (30 Jun 2012)

ceg4048 said:
			
		

> The Matrix is missing life forms capable of sensing reality.



 



			
				ceg4048 said:
			
		

> Have you ever had a dream you felt was so real? What would happen if you were unable to wake from that dream? How would you know the difference between the dream world and the real world?





			
				Nutbeam said:
			
		

> take the blue pill



   

I"m sorry. I was away for the weekend with the wife & kids. I have ALWAYS listened to Morpheus,  .....so this settles it for me. I will never get "plugged" into that dull dream world ever again.It had cost me too much in the past. The real world is so much cooler  8) .


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## gmartins (5 Jul 2012)

ceg4048 said:
			
		

> No, not really mate. The scale is the same for both. When you stick the probe in sunlight on a sunny day it reads up to about 2000 micromoles or so. If you place the probe directly on the surface of an illuminated bulb it will read similar numbers.
> 
> The difference in calibration lies in the response characteristics of these particular photoelectric sensors, which have a different response to various wavelengths. At the very edges of the spectral envelop, 400nm & 700nm the sensors are not capable of full fidelity. They fail to exactly record the photon flux of blue (400nm) and they over-respond to yellow (550nm). They are completely blind to far red (650nm and higher) but they respond perfectly to green (500).
> 
> ...



+2000 micromols  on full sun is very common. I've measured it myself. But on a lamp? Never thought it would get this high. 

I thought it was a problem of scale cause some lab gear can switch the scale to suit the concentration (conductivity in fresh versus salt water). I wouldn't be willing to measure the length of road with a 1m ruler  But apparently there is a justification for that which Clive very well explained.

cheers,

GM


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