Lighting - Testing with a PAR meter

[PaulB]

Hi Tom, i have been loaned an Apogee Quantum Meter with separate sensor (model QMSS-E), and i wish to test the lighting systems of some of our plant group members tanks and present this to our group early next year.

Can you advise

1/ How to use the meter - is the sensor waterproof?

2/ The best method to test the lighting on tanks.

The tanks i intend testing have MH, T5, T8 or CF lighting systems.

Thanks

 

[Tom Barr]

1. Yes, it's water proof
2. Generally at a pre set distance from the light source and at the middle, and ends of the tubes etc.

Other locations:
Tops of plants, shaded sections, the low foreground plant areas.
Tops of plants change with time.

So you can take readings at 45 cm depth, 40, 35cm, 30cm, 25 cm, 20,15,10 and 5 and 0.

This will give both lost due to distance and water absorption.
Then you can see how light changes as the plant changes, and also predict how much more CO2, nutrients and how growth rates change as more light is available for the plant.

Many assume that more light = redder color.
Well, we get really intense reds with low light also.
But with faster growth and the same nutrients, the NO3 available for that rate is less and the growth rate is higher because the tips of the plants are closer to the surface relative to the rest of the plant which is often still green.

You can also measure the bulbs over time and see how they decline.

Measure various points along the bulb also, it changes, single bulb vs 2,3 etc also.



Regards,
Tom Barr

 

[VaughnH]

Paul, I hope you keep records that include:
PAR at two depths in the water
Tank dimensions
Light type
K rating of bulbs
Height of bulb above water

I would love to see such a tabulation!

 

[defdac]

I myself was surprised that the lux-reading raised with the water levels. Without water I got like 5000 lux in the middle of the tank. When I raised the waterlevel it raised linearily with the water height to 10000 lux.

Then I understood that the attenuation effect of the water will be rather hard to measure 8)

 

[Tom Barr]

Even lux should decline there.
PAR does, but not much, about 5% at 10 cm depth.
Surface films etc can play a role there as where relative on a length of a FL tube you measure, nearer the edges/ends, the light will be lowest.
I do not see a huge decline due to water, we are at shallow depths.
I do see a large decline due to distance away from the light source...........

Be sure not to assume that the deeper tanks are having more light removed due to water, rather than simple distance related inverse square law.

Regards,
Tom Barr

 

[defdac]

Even lux should decline there.

It doesn't. It raises with the water level. Easy to measure with a clean tank. I guess the refraction index of the sides of the aquarium changes when water pours in making them perfect reflectors, which in turn changes the whole tank to a "light pipe" or one gigantic opto cable tranferring the light down to the bottom.

 

[PaulB]

Hi Tom, what are the recommended / optimal figures to compare the readings i am getting to ?

 

[detlef]

...the refraction index of the sides of the aquarium changes when water pours in making them perfect reflectors.

Daniel,

can you change the sensor's aperture or use a different calotte? I suppose you've done measurements with the widest possible settings. Would be interesting to see which readings you arrive at with a narrow angle calotte reducing refraction from the panes as much as possible.

Anyhow, what plants receive is direct plus reflected light thus the lux meter should sense light from "every where" for testing smaller tanks at least.


Regards,
Detlef

 

[Tom Barr]

It doesn't. It raises with the water level. Easy to measure with a clean tank. I guess the refraction index of the sides of the aquarium changes when water pours in making them perfect reflectors, which in turn changes the whole tank to a "light pipe" or one gigantic opto cable tranferring the light down to the bottom.

Well, could be, I just tested it but they where close, a tad less in the tank.
I know we do not lose much light due to water until we get to 1.2 meters of deeper.
I've done and seen those curves in our light columns at the lab.

They are linear drops until you get fairly deep(PAR units).

But.....they are not glass sides, general reflectors and point source light(MH's) in round tanks........

You may use the Licor Globe PAr sensor for this, at 900$
We use these for our stuff at the lab.

But for the flat surface sensors, like those for the Apogee, they do not account for this, so they do not measure from all sides, just the top.

Note:

You folks can remove the reflectors, or add black paper behind them etc to see what the reflector does...............

Simple test like this can tell you a lot
hint hint...........

I'll borrow the globe LiCOR and see how it compares to the Apogee.

Paul:

I generall find things like HC, really have a min light of about 25 micro mols, most plants we keep are in that range, nice ranges: 100-150, low/med light, med light, about 200-300, high light, 400 and up, few have more than 500.
I ran 850 under 1000w MH at 12" distance from the lamp box, about 18" from the bulb source.

At 8" from the bulb , about 2500!
More than sunlight.

Regards,
Tom Barr
At the surface