On Tom Barr's website, http://www.barrreport.com/forums/
, we have been discussing how to get a more accurate measure of ppm of CO2 in our tanks. I think Tom came up with a winning idea:
The problem with the 1.0 pH drop technique is that it depends for accuracy on the "degassed" water having 3 to 4 ppm of CO2 in it, but testing has shown that not to be a good assumption. In fact, it looks like it takes 48 hours or more to get the pH to stop rising in the "degassing" sample. So, here is a way to work around this.
1. Measure the pH of the water in the tank. Record it.
2. Take a sample of water from the tank and let it sit in the open for 48 hours. The sample needs to be big compared to how much water will evaporate in that time.
3. Make up a standard solution of RO water with bicarbonate of soda in it to get a small KH. Take a same size sample of this water and let it sit alongside the tank sample to outgas at the same time for the same time.
4. Add a third sample, this time of some RO water with nothing added to it and let it also sit for the same time in the same area.
5. Add enough of the third sample to the tank water and standard solution water to replace what has evaporated. (This is to restore the original KH)
6. Measure the pH and KH of the standard solution sample. Find how much CO2 is in that water, based on the standard equations/ table on Chuck Gadd's website, for example, and record it.
7. Measure the pH of the tank water degassed sample and record it.
8. Now you can calculate what your tank water pH, measured in step 1 indicates by using the following equation:
Tank Water CO2 ppm = CO2 ppm from step 6 times 10exp[pH(step 6)-pH(step 1)]
Most computers have a good calculator built in that will do this calculation easily. But, if it is a problem for anyone, just provide the numbers here and some of our good number crunchers will do the calculation for you.
Write down the ppm of CO2 you found in step 6 where you won't lose it, and you won't have to repeat all of the above steps for further tests. Just let the tank water sample sit out in the open along with some RO water for 48 hours. Add enough of the degassed RO water to the tank water sample to replace the evaporated water, measure pH and repeat the calculation per the equation above.
I have to admit I haven't tried this yet (sorry about my red face). But if a lot of us do try it we should get a good idea how effective this is. Note, that it still depends on making a very accurate measurement of three pH's and one KH, so a pH meter and good KH kit are almost essential.