pH test kits require judging the color of a test tube vs a color chart. I find it extremely difficult to get closer than .2 in pH. But the ppm of CO2 in a water sample is proportional to 10 raised to the pH power, so a .2 error in pH is a 58% error in ppm ( what you think is 30 ppm is really 30 +/- 17 ppm)

I think pH tests are very accurate, providing you can judge the color correctly. I don't think the reagent has a short time limit before it "goes bad". Judging the color is a major problem though. I find that the easiest color to correctly judge is green. As the color goes towards blue it gets increasingly hard for me to tell one color from another.

Wow, that much off !
I have the same difficulty with PH colors. It's extremely difficult for me; fortunately my wife seems to discern better and helps me out constantly.

I have no confidence in my reading of PH colors; therefore, I am trying to decide on a digital PH meter that's economical but fairly accurate. Do you know of any?

Do you consider the PH test kit unnecessary when using A DC ?

When you use a drop checker you are using the pH test kit. The color of the water in the drop checker indicates the pH of that water. Since green is the easiest color to pick out accurately and it coincides with 6.6 pH, 6.6 pH with 4 dKH water = 30 ppm of CO2. But, if you are .2 off on the pH when you see green water in the drop checker, that is still a 58% error in ppm of CO2. As bad as that sounds, using tank water pH and KH very often causes 200% or more error.

I just recently purchased this same item and tried to follow the same bad instructions (of course, before reading this thread). I contacted the seller for some clarification, and he claimed that the distilled water needed to have some kh added. I had assumed, based on the directions, that the included solution would have been premixed with a 4dkh indicator solution, but this is not the case. He also could not give me details on the contents of the solution that is shipped.

After mixing my own 4dkh water and adding it to the drop checker, adding even a single drop of the included solution (whatever it is) resulted in the drop checker water being an extremely dark yellow-green. When I rinsed the drop checker, I noticed that the water was blue when it was diluted. So, instead of adding drops of the solution directly to the drop checker, I added several drops to my 4dkh water prior to adding the water to the drop checker. This resulted in a drop checker mix that was the correct blue color.

Now the problem, of course, is that I have no idea what I am using as a ph indicator solution...but I'll give this a try for now under the assumption that it is a bromothymol blue solution.

That kind of sounds like my high range pH indicator, when it went bad. It turned a sickly brown yellow color. It wasn't pure bromythmol blue, well I think it had that in it, but had other ingredients too. Bottom line was it was outdated. Actually it was almost like the other ingredients were working, but the bromythmol blue wasn't, because when I would mix it with pure bromythmol blue it would actually give me a color on the chart that was plausible. Anyway I'm wondering how old the solution is in these cases.

You must have gotten your DC from the same source as mine, on eBay, named "joeandfamily". The amber/yellow liquid is useless.
Notice that the supplier says to "put some KH in your water" revealing his total ignorance. The 4dKH water is arrived at with Reverse Osmosis water and baking soda until it reaches a "KH reading (hardness) of 4dKH".

Her is a formula for making your own:
Creating 4dkh Solution - Tropical Fish Forums
You can also make a Google search for 4dKH standard solution.

I purchased my solution on line but can't locate it right now just to be sure I have the accurate KH.

Thanks to your excellent write up, I decided to make some of these for my three tanks.
Picture 01
I brew beer, these airlock/bubblers are available at brew supply stores or online for $1
Picture 02
I cut off the lip at the top of the bubbler, cut the bubbler down to size, cut off most of the stem (I first forced a piece of tubing over the stem to match the diameter of the tubing holders you will see in a later picture, doing this first is much easier than later because the stem is tapered), cut out the lid.
I also cut some of the tyvek from a free envelope from the usps, (excellent tip!)
Picture 03
I straigtened out my cuts on the lip and body for the drop checker with 220 wet and dry sandpaper.
Picture 04
I used Krazy glue advanced formula (hopefully this is inert when cured and will not affect my fish/plants etc)
This bottle has a nice tip that is easy to work with, I wore nitrile gloves (latex type) If you don't wear gloves you will get stuck. I ran the glue a couple of times around the mating face of the body then held the lip in place for 30 seconds or so.
You have to make sure you are relatively centered.
The glue takes a long time to cure if there is no other surface mated, so drips will stick to gloves, fingers, eyeballs etc for some time.
After holding the two parts together, I flip the assembly over and run around a few times to be sure that any voids between the parts are filled.
Picture 05
Here you have them!
I forced the lid over the sheet of tyvek then trimmed with a razor blade.
The top one is the one I pictured making and I chose to leave a part of the tyvek untrimmed to see if it will help see the color of the solution.
The top one was also just put in the tank so it is blue, the second one looks about right to me and the last one just came out of another tank that seems to be showing too high of a co2 saturation, what do you think?
Edit; the drop checkers I've made seem water tight but what if there is a a leak? Is PH test solution dangerous to the tank?

I doubt that the 2-3 drops of pH reagent in the drop checker would be harmful when diluted with 10's of gallons of tank water. The membrane type drop checkers I tried certainly leaked some of the reagent, but I didn't see any effects.

Of the three you made, is the top one a membrane type, and the bottom two air gap types? That's how it looks. You picked a good device to start with in making these. I have never seen an "airlock/bubbler", so I'm stlll not sure just what they look like.

the bubbler is shown in my first picture, you can also see one here;
MoreBeer | Airlock - 3 Piece
I guess it's commonly called an airlock, there is a more useful style (imo for brewing) that is more commonly called a bubbler so I may have led us astray there a bit.
All three are membrane type, I just filled to different levels. Although they still have an open stem so maybe you might consider them a hybrid of the two types.
I made different sizes shooting for as small as possible as I read further into your post and noted your comments on what is essentially lag time. I made one that barely shows any liquid below the cap, but it is pretty much unreadable for me so I settled for the last one I made, pictured on top of the three.
I don't think I will modify the bigger ones, since they will go in a 40g, 10g and 3g tank I'll say they are scaled for the tanks sizes

An interesting thing has occurred that is related to my question about the safety of PH reagent.
I have had the light off in my 3 gallon red cherry shrimp/oto tank because I am trying to eradicate spyrogyra, so I haven't really been able to see much going on in that tank. I just turned the light on and noticed a bunch of juvi cherries in the new drop checker!
I have no idea how long they have been in there but they seemed fine.
I had one drop of reagent in about 5ml of distilled water at 4 dKH kh
Here is a picture of the little bugs, I'm still trying to get decent macro shots but this is the best I can do right now.
(I have a canon sx100 is)