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ISpindel Fred KWBG (1).jpg
Participants Einnor, Justa, Sikkepitje
Skills soldering, brewing, PCB Design
Status Active
Niche FoodHacks
Purpose Fun


iSpindel DIY continuous hydrometer, for measuring the realitive gravity / density of wort. Meant as an educational project for KWBG. We created footprint for the various parts on the PCB. These can be found in the /libs dir in the repo. Furthermore we replaced the reset switch with a reed switch so you can use a magnet to reset the iSpindel without opening the container.

See also the documentation page in Dutch for easy setting up cloud data services with iSpindel here

See also TechInc iSpindel_KiCAD page on GitHub (last commit aug 2022)

See als Einnors iSpindel_kiCAD page on GitHub (last commit may 29, 2022)

Why Bat43???

It seems to me that the selection of the diode to be a schottky is a critical one here! According to the datasheet, the min. Input Low Voltage must be < 0.25×VIO while the Output Low Voltage is up to 0.1×VIO. Therefore, this leaves 500mV where the BAT43 will drop around 300mV or more. So a regular silicium diode will fail in this setup! Using a BAT43 or better is a requirement so to speak.

found here [iSpindel/issue/59]

Previous work:

Alternative firmware:

Look also here:

Top ispindel.png Bottom ispindel.png

First test results =

Woohoo data arrives in ubidots


  • Did the first 90 degrees calibration
  • Did the 25 degrees test in normal water (after some balancing came out at 24,8 degrees)


  • during calibration i had the update frequency set to every 20 seconds, make sure you change that to something like every 5 to 15 minutes
  • Ubidots only allows 4000 datapoints per day after that measurements will no longer be saved until the next day
  • uploading every 20 seconds only gives you datapoints in your graph for the first 4 hours every day
  • the battery will also not last very long when you do this
  • all in all a good test. Don't do this if you want the ispindel to take measurements for the entire brewing process

Helpful links for calibration

Handy Formulas

Following formulas can be used to convert Gravity values from degrees P to SG and vice versa. Plan is to use this in the firmware to decide on what value to send based on a config variable same as is possible for temperature.

  • degrees P = 259-(259/SG)
  • SG = 259/(259-degrees P)


  • Cleaned PCB repo (first version: Einnor)
  • Pannelization test (Einnor)
  • Reed-switch test/rework (Einnor)
  • Footprint Battery Holder with sliding pads (Einnor)
  • Footprint D1-Mini 8-pin version (justa) Done by Einnor
  • Petling bottles order (justa) Done, 20220608. 10 pieces
  • Online-service (ubidots?) uitzoeken/selecteren (sikkepitje)
  • Order more parts (Einnor + Justa)
  • Test whole setup on a breadboard YOLO ordered pcb and did first test on that
  • Add gravity scale to firmware (einnor) not needed polynomials are for SG
  • possibly fork firmware repo or create a pull request (einnor)
  • add resistor types to the silk layer
  • add battery + and - to the silk layer
  • setup a mailing list! 😁 (Justa) Done:

Ordered stuff

  • 30 pcb's with minor changes (ronnie)


  • 10 Petling's (justa)
  • 2 GY521 Gyro's (justa)
  • 1 TP4051 (justa)
  • 1 Wemos D1-Mini (justa)
  • 2 Battery-holders (justa)
  • 5 pcb's (einnor)
  • 20 BAT43's (justa)
  • 2 SMD-reed-switches (justa)

In progress

  • order more parts after testing

calibration data for ispindel_arnd1


Load @ iSpindel online calibration tool

calibration data for ispindel_bat43

corrected from 24C degrees water temperature to 20C degrees (correction is needed because SG values are related to the density of water at 20C)

  • 24C [[25.2,1000],[27.17,1010],[32.71,1018],[39.82,1025],[45.85,1032],[51.5,1042],[61.1,1056],[65.25,1066]]
  • 20C [[25.2,1000.8750960475685],[27.17,1010.8838470080442],[32.71,1018.8908477764247],[39.82,1025.8969734487578],[45.85,1032.9030991210907],[51.5,1042.9118500815664],[61.1,1056.9241014262323],[65.25,1066.932852386708]]

this leads to the following polynomials Corrected for 24C

  • Degree 1: 966.9279707169078 + 1.4939612781443774 *tilt
  • Degree 2: 973.729794397618 + 1.157456640501496 *tilt + 0.0037492943416183606 *tilt*tilt
  • Degree 3: 882.113582283499 + 8.017945029109477 *tilt-0.15705832812978124 *tilt*tilt + 0.0011897660268308918 *tilt*tilt*tilt