I have looked a bit at the dynamics of airlocks and how the bubbling generation rate is connected till the airlocks. I got 4 different S-airlocks where of 3 seems very similarly and one is very different.
To get steady bubbling rate I used a aquarium pump to blow air into a blow-off container and after checking it did make a steady BPM, I varied the water amount in this 4 different airlocks, and hence, below data is for 2, 3, 4 , 4.5 and 5 ml in the airlock:
The results from the testning:
Airlock 1 and 2 have been used so far in the project and even they seemed the same I had no clue if or not they are the same in regards of bubbling dynamics Clearly Airlock 4 is very different, but for Airlock 1, 2 and 3, it seems to me they are very much the same as inner volume is the same and the behavior of bubbling rate, e.g. BPM, seems the same.
So by this test it seems we should be able to use various airlocks with the same volume of around 22 ml and hence concentrate on calibration of the sound sensor.
Secondly, it seem my choice of 4-4,5 ml water in the airlock is the sweet spot as the variation among the 3 airlocks of same type is little. The 4-4,5 ml was originally chosen as this gave the highest “blop”, but the above test do not make me consider to change this.
If we attach a sensor and see how the detection fares now we know the BPM is around the 30 in this test for 4.5ml water in airlock:
For all 3 airlocks holding the volume of approx. 22ml the bubbling rate hold similarity and even Airlock 2 might seems to sick out the data here to small to really do this conclusion. Even, if Airlock 2 is different, the difference will only account for around 3-5 SG point in worst case. So conclusion here is the S-airlocks might not play so big an role, and the focus should be on calibration of the sensor.
Even the above testing suggest the Airlocks do not play as big an role as might expected, then the above test do not account for pressure nor temperature changes and from my experience you should stick to the rule: One sensor = one Airlock, especially if you wish to make use of rG/SG. My “feeling” is the various airlocks might not have difference in bubbling rate, then for sure, they do hold different sound characteristic (e.g. strength and heighten of the sound). The airlocks is in this sense a acoustic chamber and how the sensor detects the sound under various pressure and temperature changes is unknown, and hence to overcome this issue, One sensor = one Airlock.
Regarding, calibration of sensor, then the above use of an aquarium pump should be the ideal way of making a calibration. Simply turn the pump up so it makes a steady 30-40 BPM by hear and see count, then turn potentimeter up or down till it fits the count.