I have had a couple of email requests for information on my do-it-yourself CO2 probe experience.
Notice I went out and bought a CO2 meter, so I cannot claim great diy success.
First off, my interest and most of my original information came from Vaughn of Hoppy fame. I also got a lot of information from various internet posts and I cannot find the other primary resource that as I recall was a Russian language source.
My membrane trials were based on or natural offshoots of a couple of lists posted by Tom Barr as “Plantbrain” elsewhere.
I had noticed my “test tube” drop checkers had quicker response time, so Hoppy’s observation made sense.
So, based on Hoppy’s observation that high surface area to reference solution volume I first tried setting up drop checkers based on surface area to reference solution volume.
- Low and behold, Hoppy was correct.
The simplest form being like this DIY Co2 Meter where it is easy to vary surface area to reference solution volume and since the reference solution does not have to be read, the volume of reference solution only need to be sufficient to cover the probe element.
For pH probes, I had a Hanna pH meter, a diy pH probe and a Milwaukee meter.
It appears that most LDPE (low-density polyethylene),LLDPE (Linear low-density polyethylene ) is even better work so all kinds of sandwich wraps and bags work films work to one degree or another, the "Saran Wrap" style being the worst, almost unusable, to thinner, true LLDPE wraps working reasonably well, obviously the thinner the better.
I cannot locate the DO membrane from Hach I tried; I think they were something like Standard DO membrane kit for DO probe 05521-00, I found these too fragile for a complete lummox as I to handle.
Parafilm is another I have not tried, but have heard good reports, in fact I lost the reference but it seems quite successful.
It appears gsjimia references an interesting product in the thread “CO2 tester using PH probe,” post #2. That is likely better than anything I tried.
Of the things I tried, the best all round, is a product called Tyvek, readily available from home improvement stores and in various types of clothing and in mailers, as in the lined priority mail, mailers. The stuff is tough, keeps water out and gas, CO2 anyway move through quickly.
As always these things depend on high surface area to reference solution volume and the accuracy of the 4-dKH, or whatever reference solution chosen.
In making a reference solution with baking soda, I think it is worth drying it out even in the dry area I live I can see a difference. There is some bogus information out on the internet regarding drying baking soda.
Never let the temperature exceed 70⁰C (158⁰F), somewhere around 70-80⁰C baking soda loses CO2, of course this is why we like it for baking, but it is bad form for our purposes. So, we must gently force the excess moisture out of the baking soda without turning it into Sodium carbonate.
- Place twice as much baking soda as you think you need, on a nonreactive surface, I like parchment paper on a cookie sheet.
- Most ovens in the US, I am familiar with start at 150⁰F (65.6⁰C) that is the temperature I use.
- Leave for 2-hours
- Weigh the amount you require, return the excess to its container.
Making the largest the initial quantity of solution the greater the likely accuracy of the reference solution. While I know 50-liter, solutions are a bit much…
- A little less than 5-grams (4.989-g) of baking soda, then add enough distilled water to make 5-liters (~4.995-l).
- A little less than 2.5-grams (2.494-g) of baking soda, then add enough distilled water to make 2.5-liters (~2.498-l).
- A little less than 1-gram (0.998-g) of baking soda, then add enough distilled water to make 1-liter (~0.999-l).
All three solutions result in 40-dKH.
For the 4-dKH reference solution
- mix 1 part 40-dKH solution with 9 parts distilled water
- Mix 10-ml 40-dKH solution with 90-ml of distilled water, for example.