Ya, I was kind of thinking that the limited range of the Alkatronic's calibration was affecting the readings above 7.0 pH. After switching to a new pH probe in my sump (hooked to my Atlas hardware), I get similar readings to what I was getting with the Milwaukee probe (which was also fairly new).
Currently my 24 hour daily pH readings are 8.03 - 8.12 for the sump probe and 8.08 - 8.22 for the Alkatronic. I attribute the low daily high readings to the unlit portion of my system, meaning either my coral mass when the lights are off, or my fuge when the grow light is off. I keep a large amount of macro growing in a barrel, and expect that it limits my daily lows, and also (unfortunately) the daily highs.
During the last 24 hours, my dKH has been 8.62 - 8.67, testing every 3 hours.
I hear ya on the need for multiple probes. I use a custom ATO that I developed that operates on EC. The common advice is to never depend on EC for salinity control, and it's good advice, because a single EC probe will not give a dependable enough response to allow for safe salinity control. That is why I use 5 EC probes in my ATO controller. I originally used 3 Atlas Scientific EC probes, and later added 2 Thermo Scientific Orion probes.
With just the 3 Atlas probes, the ATO was workable, but required more effort to keep calibrated than I liked. Our tanks are hostile environments for EC probes. There is always something that wants to live, crawl, or deposit on the surface of the EC electrodes. The 2 cell Atlas probes are especially vulnerable to being impacted. When I first started using the Atlas EC probes, I was a bit mystified why the readings jump around so much. I went back to Atlas staff multiple times questioning my results, but each time heard that they get very little reports of problems with the EC probes. I ended up building an in sump manifold to hold the 3 Atlas EC probes, hooked to a small Sicce pump to flow water past them continuously to discourage squatters. It works for the most part.
When I added the 2, Thermo Scientific probes, hooked up over USB using bench top meters, I attained far greater stability. They use 4 cell electrodes and are much more resistant to being impacted. They did bring their own communication issues though. I had to build in methods of resetting the USB port and the devices themselves when they stopped sending updates. I have them setup to send an update every 60 seconds.
I developed the ATO to head off potential issues that have bitten me in the past. Mainly my love of running dual headed, continuous water changes, and my misplaced trust that they will stay balanced over time. They never do, and that fact has led to a couple of crashes when things got way out of whack and I was asleep at the switch. So I decided to build a way of protecting from that issue, and it also protects against salinity errors on the new make up water. Even if I totally mess up mixing my make up water, the ATO fixes it for me.
It also corrects all effects from dosing 3 part (which I recently added). For people who use kalk exclusively, and run into the daily evaporation limit, this style of ATO could be employed to add concentrated brine to maintain correct salinity, regardless of how much kalk is required daily. A level control would then pump water out to waste to account for the excess water being added and corrected for by the ATO. Kind of an ATO and AWC all in one.
Using my ATO, I keep my salinity constrained to a 125 uS range (1.0263 - 1.0264). I consider this pretty stable.
Back on topic, handling the 10N muriatic acid that I had planned to sub in for the DIY reagent, along with the NaOH in my 3 part, are the only 2 things that give me great cause for concern. Having to mix new batches of reagent every few months using it, is one of the reasons I am looking at using the OEM again. Well that and the fact the OEM dropped in price to match the DIY version, which is no longer available locally anyway.
