Muddy fork - Ted has given you the correct theory for determinig the amount of aeration necessary to maintain adequate DO on the pond bottom that involves frequency of pond water turnover.
You(MF) are correct in saying "there are a lot of variables and what works in one pond may not be adequate for another pond"; a very IMPORTANT guideline to always keep in mind.
Aeration should be operated to maintain DO levels of 5ppm on the bottom as UofF recommends. Pond turnover rate is primarily affected by volume & depth of pond, aerator efficiency and aerator run time. How often the pond needs to be turned over or renewed is determined by EACH pond's oxygen demand (primarily in water above the sediment layer).
The only sure way to know "how long and when should I run the aerator?" is to measure or monitor the oxygen levels near the bottom of the pond. Bottom oxygen concentrations may have to and probably should be measured in several places in larger bodies of water to get accurate information about completeness of the pond turnover.
As you are discovering electronic meters that measure oxygen are costly. Chemical tests are lower cost but pretty time consuming and for chemical tests, the water has to be collected with minimal exposure to atmospheric air which can bias the test toward higher oxygen concentrations. This has been discussed in other topics on this forum.
I am not sure that it is practical and cost effective for you to measure oxygen levels with an electronic DO meter just occassionally in the bottom water. This could be an expensive project if you have to purchase a quality DO meter. Cost of doing this DO testing electronically will probably outway any electrical savings you get from running the air compressor less time.
Here are some informative discussions from several topics on this forum about measuring dissolved osygen:
If you are going to order first look at www.aquaticeco.com
. They advertise in Pond Boss mag. They have chem test kits mfg by LaMotte & will be better than you would get at a pet shop. They are pretty accurate, inexpensive and will be your best choice if ordering. Reagents are replaceable. Individual test kits are available for pH, alkalinity & hardness; plus kits for broad spectrum testing.
Chemical test kits are the cheapest way to test for DO, however they are time consuming. DO meters are the fastest most expensive and usu a high maintenence item. YSI brand meters seem to be the most dependable.
B.Lusk says - Inside the magazine are several advertisers who sell all kinds of water testing equipment. In order to collect a sample from bottom water, you will need a special device lowered with a cord. Or, you can buy a meter which measures temperature and/or oxygen, and the cable can be lowered into the lake to different depths.
D. Willis says - Hach Chemical makes a multitude of water quality meters, and they also have simple test kits where you use chemicals, count drops, and watch for a color change. They actually are quite reliable tests. For example, I looked up their dissolved oxygen test kits, and you can do 100 tests from a $48 test kit. I tried to paste the web address for that test kit, but the string was lines and lines long. Perhaps go to their web site, address below, and search for "Dissolved Oxygen, Model OX-2P, 100 tests, Drop Count." http://www.hach.com/
Cody has seen used YSI DO meters reasonably priced on ebay. Buyer beware of repairs and replacement parts. Electronic DO meters are sensitive high maintenence items.
Noblemidwest’s question -We are looking into purchasing a disolved oxygen meter and have acquired a catalogue which has many different types and prices. Would anyone have a suggestion as to the type/brand which my fit the needs of a 2 acre pond for not alot of $$? The price range in the catalog seems to be from $250 to thousands.
Fishman says -- As far as brands go, you can't do any better than YSI. I would recommend a YSI 55 as a good general purpose meter that is durable. That's what most professionals use, myself included, and they work well. You don't want to let the probe dry out, though, and the membrane will need replaced occasionally.
You might check out this website here and see what Randy Rushin can get you a new meter for. You might mention that you got his name off of Pondboss. I've been trying to get him to sponsor the site for some time.
Alternatively, I (Fishman) have an extra YSI 55 with a 12' cord that I no longer need. I would be willing to part with it for $225 shipped. You can check the price of a new one as well as its features here at one of this site's sponsors.
Cody's Comments. As a reward your reading this post this far, I (Cody) went back through some of my DO records from varous water bodies (including my thesis lakes in Canada and local ponds) and have come up with a fairly simple method for you to determine how long to run your aerator. The "when" to run it is ONLY
a debatable issue if you can do a pond turnover in less than 12-18 hours.
NOTE THIS METHOD IS GOOD AND APPLICABLE ONLY FOR PONDS NORTH OF THE OHIO RIVER. IT HAS NOT BEEN PROVEN YET WITH EXTENSIVE TESTING. NO TESTS WERE MADE IN SOUTHERN PONDS. NO TESTS WERE MADE IN AERATED PONDS DEEPER THAN 19 FT. Also, I am not responsible for any fish kills from using this unproven method. For ponds in other areas of the country this may be a worthless method until it is verified by MORE testing. Here are the generalities and or details.
Get yourself a thermometer (preferable electronic) that can measure the temp within one foot of the bottom. In a typical stratified pond and normal water clarity of 18" to 4ft, you want the temperature at the bottom to be as close as possible to that at 1-3 ft deep. Near surface temp and near bottom temp should not be greater than a difference of 4F maybe 5F degrees. The closer the bottom temp is to the surface temp the higher your DO level will be at the bottom. Ideally you want your bottom temp to be within 2F to 3F of the surface temp.
Temp differences greater than 3F between top and bottom run higher risks of low DO in aerated ponds.
I do not guarantee that your DO will be 5 ppm (mg/L) or above 5 ppm, at the bottom, if the bottom temp is equal to or less than 5 degrees of the surface temp, but DO "should" be more than 4ppm or 5ppm on the bottom with only a 5 F temp difference between top and bottom. Your pond and "Mother Nature" will not be aware of these instructions, so exceptions will occur. AS I said earlier, the only way to know accurately is to measure the DO in the bottom water and not just rely on temp.
I would like anyone with a DO meter to take some approriate DO -temp tests during thermal stratification periods and help verify "Codys DO-Temp Theory" for aerated ponds. Thanks in advance. If you are taking some DO temp measurements be sure to take them in the vacinity of the aerator. The pond water column at long distances away from the aerator may not be getting mixed adequately in your situation. It all depends. There are lots of variables with this "aeration thing".
Some of the above numbers were adjusted toward conservative side on May 23, 05.