I wanted to apologize for creating such thread. Probably there is much to find out using the search and forum archive but I'm a bit busy right now and I don't have enough time for a serious research.

A friend of mine wants to purchase a dissolved oxygen meter. He's is a simple pond owner and he wants to do measurements some times a year in 2 ponds only. His main goals are:

1) being sure about sufficient amount of oxygen in winter.
2) to find out if one of his ponds can be good enough for trouts (after purchasing fish, oxygen monitoring could be useful during hot summer period).

As you see, not much measuring at all and it seems like that some kind of simplest meter could be used.

I'd be grateful if somebody could explain in a few sentences what do I have to buy and how much could such thing cost (maybe some example too, please). What additional parts of chemicals do I have to buy additionally and how often?
The max. depth is about 4-5 m, just in case.

I saw similar thread by Michael P but he seems to have a serious company and that's why I think that advices given to him might not be the best for my case.

You answered part of your question with the 5 meter depth. See who has a cable offering that long. Have to add "out of the water length" tho. That should eliminate a few.

Next,
What are you looking for in accuracy?

Thanks for response. At least now I got something (as I said, I had no idea what do manufacturers offer).

About accuracy - no freakin' idea Let's say - 0,5 mg/l - is that a great accuracy of not? Nobody's here a scientist and high accuracy is not necessary. If dissolved oxygen is ~4 mg/l then that +/- 0,5 doesn't matter a lot - no trout will live there.

If you can get accuracy within 1 mg/L it will be fine for what you want to do. Accuracy also depends on how you take the reading. The probe needs to be constantly moving(slightly up and down or figure eight motion) or the reading will not be correct.

That's not a problem. In winter that's easy but in summer boat is an option.

Can you please list a few models that have good recommendations?

Do you want models that people have used or are currently using and are satisfied with?

Yes.
I usually trust not advertisements from media but people's experience.

YSI550/YSI550a. Repeatable measurements, easy to calibrate. You should have extra membranes on hand, when one goes bad the meter in unuseable until it's replaced and re-calibrated.

Downside? For me, it's the time lag between turning it on and having it stabilize ready to take readings. Other than that, it's pretty easy to use.

You must move the probe thru the water to keep constantly new fresh water hitting the probe to get an accurate reading. If not, it'll read low.

Thanks!
I'll certainly gather information about those devices when I'll have a bit more free time.