On the trail for my aquarium monitor I came across the excellent Seneye range of aquarium sensors. They seem to provide all that I need:
- Acidity (pH) reading
- Ammonia (NH3)
… and are USB connected and powered. So far so good. They seem to use a replaceable litmus test strip which is read by what appears to be a colour-detecting LDR, so I understand the need to replace them periodically to maintain calibration and readings. What is a little frustrating is that the software only runs on Windows.
Now, I think Windows is a capable OS and useful in a variety of contexts, but one of those isn’t the area of small and embedded devices. These have very limited memory models and typically require ultra-low power levels when sleeping – something I’m certain that Windows doesn’t really understand. To run Windows locally I could actually buy a LattePanda board with Windows 10 IOT, but at around £100 the sticker shock has set in and I am thinking of taking another route.
Off-the Shelf Devices
I think I have two options at this point: Continue with the Seneye and try to get their SUD Driver working on a Linux box, or roll my own:
Seneye and SUD driver
Temperature and light are fairly easy to get with sensors like the DS18B20 and light-detecting diodes (LDRs). So, the extra which the Seneye solution give are the pH reading (which isn’t easy to calibrate nor leave in a solution too long) and the ammonia. I appreciate that these are difficult things for the reasons below.
Roll my own
What I’d ideally like is a single-board computer that is tiny, runs off a battery forever, reads from the sensors outlined above, and can communicate to a local WiFi access point. The rest I can do myself including piping MQTT messages to a remote server, setting up a firewall and whatnot. I think all of this could conceivably almost fit onto a ESP8266 device, but the voltage handling for the pH probe would most likely not go too well, and I don’t know about the inputs for the temperature probe. Measuring the chemicals is way more difficult, until they are solved in a way which means that fouling of the sensor or leaching of the indicator dyes does not occur, I think I will let others do the pathfinding.
So it is likely that something more capable like at least a WiPy or RasPi Zero W would work, or even larger like a full-blown Odroid C2 or RaspPi 3. But why stop there? Why not just go full LattePanda and £100 if you need those, since they really don’t run well on batteries.
Chemistry – measuring soup
What I really need is a neat, innovative way to read water chemistry. My son who is a chemistry student at university tells me that lab equipment to measure individual molecules is terribly expensive, and that mostly they are used in a pure environment where only one type of molecule is present in a solution. What I need is a way to look in a non-invasive way at ‘soup’, and tell how much of one chemical it contains.
This is likely the reason many of these devices either use rare earth metals and fancy rotating face-plates to discourage fouling. They still need replacing every so often because they are in the aquarium water, and the dyes and inks are leaching away. The Seneye does this with a covered, reference litmus strip while another is exposed to the aquarium water and provides the measurements.
My son spoke of IR-spectroscopy which is used in organic chemistry, and UV-spectroscopy which is used elsewhere. IR-spectroscopy is likely what devices like the Mindstream are using, comparing reference ranges with that coming back from their IR LDRs and using clever materials and rotation to avoid excessive fouling. It is likely that measuring such a chaotic solution would mean that multiple wave lengths would be returned, however if you just wanted very coarse-grained information like ‘no ammonia/ammonia’ it may be possible.
My guess is that these real-world issues have hampered a single device that tells you what is in your aquarium water for some time, and that until some clever thinking is applied or new material science used, we will have to continue either replacing our litmus strips periodically or buying expensive reference disks.