Grow Mediums RO pH question

[Sublime Rasta]

I am about to start soaking my rockwool, and I currently have my pH at 5.5 according to my electric meter. I always use drops just to double check, and the drops are dark green instead of the usual light orange. I made sure my pH meter is calibrated correctly. Is this normal with RO water?

I don't want to use tap water because my PPM is about 480.

 

[Happy Horticulturist]

I don't really know about drops colour because I've never used them. My grows have always been big enough to make sense to use high quality expensive pH end EC meters.

I do use RO water and I can tell you that the pH of tap water is 7.3 and the pH of tap water after going through the RO filter is about 6.4. It used to be lower, like around 5.6 pH when I first got it but it has been going up slightly over time. I guess soon it will be time to change all the filters.

I'm not sure about PPM, I don't really like PPM at all as it's not a standardised system. There are 3 PPM scales so unless you say which PPM scale you use, we can only guess what PPM scale you use.

Instead of using PPM I use EC. It's standardised which means that EC in the USA and Europe and Australia is always the same. However PPM in each of those three places is different. Bet ya didn't know that!

My tap water is about 0.7 or 0.8 EC. I ideally you want your tap water to be around 0.2 - 0.4 EC.

I added Plant Magic - Magne-Cal+ to my RO water. At the rate of either 0.5 to 1.0 mls per litre (depending on medium and grow cycle) and this brings the RO water EC back up from zero to around 0.4EC. Which is perfect. You want about 0.3 EC worth of Calcium in your water and around 0.15 EC worth of magnesium.

After adding this and your base nutrients (and boosters if you use them) and some pH adjustment fluids if needed, then your RO water based nutrient solution will be fairly steady pH wise. However the pH of RO water can actually drift slightly because it absorbs CO2 from the air which lowers it's pH slightly (if I remember correctly, it lowers).

Also beware with RO water. Because there is no chlorine in it, I find that it is more susceptible to getting algae etc growing in it. So when you make your RO water, try and make it fresh. Don't let it sit out for more than 24 hours exposed to light and air even.

Alright, that's all I can think of to say about RO water right now

 

[Dr Who]

It sounds to me as though you have a really good quality supply of RO water. The THEORETICAL pH of deionised water at equilibrium with carbon dioxide in the atmosphere is around pH 5.65. If this is your situation than beware that the water will have very little buffering capacity, so will have little or no resistance to changes in pH.

That said, your meter could be wrong too. There are dozens of kinds of test strips, so I can't comment on your "green" result without seeing the calibration strip for that particular test. If you can post a picture of the test solution and calibration strip it should be easy to tell. The best way to do this is often in front of a white background (ceramic tiles are great, white paper will do), then to look through the test solution to the calibration strip, and also see a bit of the calibration strip that isn't behind the test solution.

You've probably done it all before in chemistry classes years ago...