How are you checking SO2?

[berrycrush]

You can get a re-usable test kit plus chemicals for less than $100. You probably wont need one if your wine goes from carboy to bottle. If you use oak barrel, especially those small ones, SO2 test is a must, because free SO2 drops fast in small barrels.

 

[Monty Knapp]

I test each batch just before bottling to make sure there's enough free SO2.
I use a test kit from Accuvin. About $40-$45 on Ebay. 10 tests per kit, except 2 are High Range, which I've never needed. Accuvin says the kit is good for 10 months from date of manufacture. I checked with the Ebay supplier and his kits still had 8 months life left. Expiration date can be seen in photo.
Simple test. Just pour the liquid in a black capped ampule into a green capped ampule containing a powder. Shack it to dissolve the powder. Use pipette devises provided to draw a wine sample and squirt it into the ampule. Shack it and wait 5 minutes. Then compare the color in the ampule to the chart. Doesn't give you an exact reading, but good enough to know if your wine is within the proper range given it's pH.