Hello,
I've built a PH meter using a cheap probe from eBay and a combination of Arduino Uno and Raspberry Pi.
The current setup is as follows:
- Arduino gathers data from the probe and runs a simple Kalman filter. It has a LCD display showing latest reading and Kalman estimate (updated every 3s).
- Arduino also has an Ethernet shield, responding to requests by sending the latest estimate value from the filter
- Raspberry Pi runs a cron job every minute, gathering the data from Arduino and saving it to mySQL database.
- RPI also runs a webserver, showing a page with graph of last 24h of readings
Th setup works quite well, but the readings are very noisy. I've calibrated the probe using buffer solutions, and I've also installed a good power supply, as well as filter caps on the power input to the sensor board.
The weird thing is that the readings stabilise to within +/-0.2 when I put the probe in a cup of aquarium water. When the probe is in the aquarium itself, the readings average about right, but spread from 15 to 2, which is rubbish.
Has anyone encountered similar issue? I'm guessing that it's one of the following issues:
- Either the input voltage is noisy - would not account for much quieter readings when probe is in a cup
- The probe is disturbed by moving water - I've enclosed the probe in a plastic tube, which has not changed the behaviour. Also, why would flowing water disturb an electrical reading?
The probe is not touching anything in the aquarium. The only disturbance can come from stuff floating mid-water, so either debris or gas bubbles, but it's not evident visually.
On the attached photos you can see the probe in the aquarium (with the blue tube covering it, with holes to prevent air trapping), and the arduino setup.
Suggestions welcome, please.
