With Hanna I thought you cal. with RO water? Yes Each time you use it. Make sure vial is very clean,no dust or anything else on glass. no air bubbles either.
Yes you do! It's supposed to set it to zero, but my results don't seem to be consistent with what I have going on in my tank. So, I need to make sure that it's working correctly!
No, you don't use RO/DI to "0" it. If you are talking about the C1 stage. That should be filled with 10ml of your tank water that you are trying to test. Then taken out and add the reagent to that same vial for the C2 stage.
http://shop.hannainst.com/downloads/dl/file/id/1069/ist736_13_05_10.pdf
I always make sure the do the same steps every single time and have always gotten consistent results. Others have had bad reagents in the past but I haven't had the problem yet but it's always a possibility with any test kit.
I have a pretty good routine down with this checker and do the same with the 713 checker as well. Always handing with a lint free cloth and never touch with fingers though all steps.
1. rinse out the vial with tank water several times and inspect vial for any blemishes.
2. Fill that vial with 10ml of tank water taking care not to have any bubbles on the sides (if filtered can skip step 4.)
3. Get a packet out and flick it a few times to loosen up the powder and move away from the area I'm going to cut in a curved pattern as indicated on the packet (I was doing this before they put that indicator on there as well)
4. Set the vial down with tank water and wait at least minutes doing other stuff ( I used filter out the sample with a coffee filter but this seems just as good and I'm usually doing several things at the same time)
5. Turn the checker on and wait for it to ask for C1
6. Place the vial in with the 10ml mark facing me
7. Cut open the reagent packet and pinch it open so the bottom part is creased to help funnel the powder out into the vial
8. Press the button to read C1
9. As soon as it asks for C2 I pull the vial out (never touching with my fingers at any stage of this using a lint free cloth) and add the reagent
10. Close the vial up and look at the time and gently start mixing the reagent into the water. Taking care not to form bubbles on the insides of the glass. Alternating between slowly inverting and gently swirling
11. As soon as the time changes on the 2 minute mark I press and hold the button to get the 3 minute timer.
12. Continue to gently mix the reagent for 30 more seconds inspecting for any bubbles on the sides or accidental blemishes
13. Place the vial back into the checker and wait for the count down.
14. Rinse immediately after getting the reading.
I find that the reagent for the ULR 736 checker dissolves a lot better then the 713 Low Range checker. Also the end result value is displayed a lot longer on my 736 ULR checker.
So do you recommend that I get the calibration fluid. 
