Robo-Tank Isolated PH Circuit

[Rob F]

I just listed the new pH circuit in the store as a pre-order expected to ship March 6th. You likely got an email and some of you will get more than one. I typically only send emails using the list from the store side of website but well over half the users on the forum aren't registered in the store so they never get emails, for this I want them to get something as well and currently I have no way of syncing emails from the store and forum. Sorry about this, it won't happen again for a while. :)

Here's a link to the listing.
https://www.robo-tank.ca/en/Robo-Tank-Pl...82c83.html

Anyone who has the controller will have a coupon code so the cost will be $40, the coupon is good for 30 days. That might sound expensive for a small board but the components are very expensive and it takes quite a bit of time to assemble as the IC needs to be programmed before mounting. If I could offer it cheaper I would. So please share a link on your social media as this can be used without having Robo-Tank. I'm hoping to sell enough to order an Atlas EC and ORP circuit and probes, I have the Exacto™ circuits working but nothing to compare with. Yes I'm calling all the probe circuit the Exacto™ line. Yeah I know a little corny but hey... 

The circuit is obviously setup for the controller but I'm also going to have an open source library for it so you can use directly on any Arduino, PI or any other microcontroller that has I2C. Using the library it'll be super easy to setup and use. 

I basically have all the code done for the circuit and it's working great. In the next few days I'll be setting up the display and controller for them, I have most of the mock up done for the display and added the pictures below. 

The circuit will output pH, pOH and estimated CO2 based on a kH value sent to the circuit. I didn't use the standard formula to calculate the CO2, I found another that adjusts a bit because of objects in an aquarium. So hopefully a little better but still obviously an estimate.

I added an auto scan feature, with this if its connected directly to an Arduino with the test sketch the circuit will be plug and play, no messing with I2C addresses or configuring the circuit.

I did take 3 point calibration for granted, I scoured the web and couldn't find any formula to use 3 point, then I checked some other popular aquarium controllers and they only offer 2 point which makes me wonder. I know 3 point isn't necessary for an aquarium but the option should be there for the radicals. Definitely disappointing, if anyone knows of some data showing math for 3 point please let me know.

With that said I do have the 2 point working and setup an auto calibration system. I call it auto because once you start the process it will decide when the reading is stable enough to continue, the goal is to have a stable reading for 60 seconds. If it reads 7.055 and 60 seconds later the current reading is +/- 0.005 or less it will pass the test and send the calibration to the circuit. It does the check in 10 second intervals. If a reading on an interval is more than +/- 0.005 it'll start the timer over. It sounds impossible but it's pretty easy to achieve. 

Here's what's checked during calibration to ensure the probe is in good working condition.

• When you calibrate a probe the 1st point should always be 7.0 solution, then anything for 2nd point. If the probe reads more than +/- 25mV away from the 7.0 the probe might need cleaning because of build up or may need replacing.
  
• Next check is the slope, this divides the mV the probe read by the theoretical voltages expected. Even a new high quality probe won't be perfect because of the nature of measuring ph but checking the slope ensures the probe falls in the industries acceptable range of 90% - 105%.
  
• Response time
  

I got the first check from Hanna, this is important because a probe could be +/- 50mV off point 1 and still have a good slope, when that occurs the probe likely just needs to be cleaned. Obviously I don't have Hanna's algorithm but I'm going to make my own which can be tweaked over time as this will be on the controller side of things. It'll looks at slope, point 1 offset and responsiveness and give your probe a health rating. The real trick will be trying to create a pattern it can also be based on so the health meter can be updated continuously and not only during calibration. I'm hoping it will also be able to tell you its time to calibrate and not just based on a schedule.

This is the sensor probe screen from the hardware menu. When you add a new circuit you press the "Scan For Circuits" button and it'll setup the circuit. It'll look at other circuits connected and then select an available I2C address so almost plug and play. I don't really want a scanning system running non-stop as new hardware isn't added often so going with the single press of a button.



When you select a circuit to calibrate you get the following screen. Instead of using a temp sensor I'm going to have the user set the temperature of the calibration solution as it can get messy for some using a probe. Plus people typically store things in the same location so temps are always the same, this will be saved to eeprom so only need to change if you know something is different.



When you select the type of calibration you get the screen below. Here you set the pH of the solution which is also saved to eeprom and hit start. Then once the controller is happy it'll go to point 2 calibration if applicable and you repeat this step and calibration is complete.



This is what I'm going to start with on the health screen for probes. As I mentioned above the x of 10 is calculated based on slope, response time and point 1 offset. The color of text will change accordingly. To start this will only be updated after calibration but when I have more time I'm going to be trying things to have it update consistently. 



If anyone can think of more let me know.