What are other's experience of ispindel in fresh wort? Does it change / correct over time?
You will get minor variations as bubbles collect on the iSpindel, but nothing too drastic. One thing I disagree with in the video, is his method of getting the 25 degree balance. Using tape and pushing it in and out of the tube, means that each time you charge it, it will be inclined to move and need the 25 degree calibration to be rechecked. I made mine sit at 25 by sticking lead on the PCB, with heavy duty double sided tape, along with sticking the battery so nothing on the PCB could move and upset the calibration. both battery and lead stuck as low down the PCB as possible. I also adjusted my weights either side of the battery, so it made the iSpindel float with the PCB level and battery down.
Rather than attempt make the PCB a tight fit, it can move about very slightly inside the PET, but it always rests at the bottom of the PET and I always find it provides an accurate 25 degrees in water when tested. I checked it around six times, taking it out of the water, drying it, going through the motions of charging it and putting it back - it always came in at 25 +/- 0.24. I give it a gentle tap before lowering it into my brew, to ensure it settles into its same position at the base of the PET.
I did my calibration from 1.000 in a measured quantity of water, by adding very carefully weighed and stirred in amounts of sugar in stages to get SG's of 1.010, 1020, and so on all the way to 1090, which I then fed into the spreadsheet. I used scales which gave the measurements to 0.01 of a gram. I doubled checked against an hydrometer, just out of interest, but didn't use it other than that. I found the sugar addition calculation on line somewhere, where I cannot now remember.
My results using the iSpindel have been great, much more accurate than I could possibly get with an hydrometer and no need to disturb my brews to check them. I can keep an eye on progress, in my armchair, using an Andoid tablet + Brewspy. My present 40 pints of Scottish Export, put on three days ago at 1.083, is showing at present 1.013. I certainly could not read that last digit with an hydrometer. I have done five or six brews using it since I built it and if its not obvious, I am delighted with it.