Questions on my new PAR meter

Henry Hatch

Guru Class Expert
Aug 31, 2006
My Apogee PAR meter arrived today. I got in on the reefcast group buy. I've been playing with my new toy and have some questions.

1. Most of my lights sit on top of the tank so it's difficult to to simply stick my arm and hand into the tank to take readings. I'm thinking that I would like to construct some sort of device that would make it easier to take readings. I was thinking of just using a piece of clear rigid plastic tubing and attaching the sensor but i'm wondering if that might affect readings with some of the light passing through the tube.

2. I noticed that if I rotate the sensor slightly that the readings go up and down. Should I take readings with the sensor pointing straight up or should I take several readings with the sensor oriented in different postions and establiish a range ?

3. I'm not sure how to interpret the micromol numbers to determimne whether I have a low, medium, or high light tank. I think I read a posting that stated that plants need a least 30 micromols of light to grow. Readings vary a lot based on the depth of the reading. In determining the light level of a tank should I use the substrate reading or some combination of readings ?


Lifetime Charter Member
Lifetime Member
Nov 21, 2007
Los Angeles, CA

1) Yes, setting the sensor upright is tough. Apogee sells a "stand" for the sensor to go on. If I had to do it over, I would buy one and may still do so someday. In the mean time, I simply reach in and keep my arm as far away from the sensor as possible. But, if you have one of those deeper tanks, I can see that being very difficult.

Here is a link to Apogee's leveling stand (even has a bubble level built in):
Apogee Instruments: Accessories (see first item, the AL-100) -- it is $29, not sure how much shipping would be.

Perhaps seeing the Apogee stand will give you a general idea for a DIY stand. I haven't come up with a good solution yet.

2) My understanding is that the sensor should be upright.

3) Here is information I gathered and summarized from a few of Tom's posts re: micromol amounts -- I've learned to remove info from posts and put them in text documents on my computer, otherwise I have trouble finding the info later:

A min level for most plants is about 25-30.
Max levels are about 450-600.
Full sun = 2000.

I can run my tanks at low light, about 100 micmols down to 30-40 with the PC's, or go to the high light ranges with PC+MH and have 80 at the lower ranges/edges, up to 600 in the hot spots.

Low light: 100 micmols near top, 30-40 micmols at substrate
High light: 600 micmols near top, 80 micmols at substrate

If there are errors in my summarizations, someone please correct me.


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
I think all of us will have a problem taking good measurements in the tank. I plan to make a probe holder from the smallest diameter PVC/CPVC pipe I can find, probably with a 90 elbow on the bottom, a stub of pipe and another 90 elbow on that. Then I would just use a rubber band to hold the probe to the last elbow. I hope that will let me hold the probe reasonably vertical, and reach most areas of the tank. If the elbows are just press fit and not glued you could rotate them to enable reaching every spot in the tank.

I think the most important measurements are the reading at the substrate level right under the light, and at front and back glass in the middle, left and right glass in the middle - 5 readings, all at the substrate level. We know the intensity goes up as you get closer to the light, so once you figure out how fast it goes up you don't need to keep measuring those levels later. This will be very difficult for heavily planted tanks.

Some more basic stuff to learn: the ratio of the intensity at substrate level in the tank/water, and the same reading out in the open air, same distance from the bulb. With this for a few geometries of tanks, you can possibly just do measurements in the air and use the ratio to get what it is in the tank. Plus, the effect of age on the bulb. The effect of the reflector vs no reflector. Clean front and back glass vs algae coated glass. ( I believe a lot of light is reflected back to the substrate from the front and back glass, but not after they get algae on them.) More sophisticated: the PAR readings vs the angle of the probe to the vertical, to see if you can pick up the reflected light from the front or back glass. The PAR reading for whatever lights and tanks you have for a data base. Reading for same fixture, but different heights above tank - calibrates the light so you can raise it or lower it to get the intensity you want. PAR of different color temp bulbs. Etc.