Calibrate your iSpindel

An iSpindel does not measure gravity: it measures its own tilt. The denser the liquid, the higher the tube floats and the more its angle changes. To turn that angle (the tilt variable) into a gravity, the firmware applies a polynomial formula that you enter in its configuration portal.

Every device needs its own formula: the weight of the battery, its position, the ballast and even the printed sled change the relation between angle and gravity. Another brewer's formula, or the factory one, can be several points off, enough to believe a fermentation is finished when it is not.

Calibrate again after any change: new battery, a knock, the tube opened.

Prepare several liquids of known gravity, float the iSpindel in each one and note the angle it shows and the gravity read on a hydrometer (or a refractometer, in sugar water with no alcohol).

• Water at 20 °C: gravity 1.000.
• Sugar water, stronger and stronger: add sugar in steps until you pass the original gravity of your beers (1.080 covers almost everything).
• Let the iSpindel settle for a few minutes at each step and note its average angle (in configuration mode it sends a reading every 20 to 30 seconds).

The tool then fits, by least squares, a polynomial that comes as close as possible to every point:

gravity = a × tilt² + b × tilt + c        (degree 2)

• Degree 1 (straight line): enough over a narrow gravity range.
• Degree 2: the usual choice, which follows the slight curve of the iSpindel's response.
• Degree 3: only with at least five well-spread points, otherwise the curve wobbles between them.

The result gives the formula in SG or in °Plato (choose the unit set in the iSpindel), the coefficient of determination R², the standard deviation and, for each point, the gap between the formula and your reading. A standard deviation below one gravity point (0.001) is a good result.

• The formula is only reliable between the smallest and the largest angle measured: beyond, it extrapolates.
• Reference gravities must be read at 20 °C, or corrected for temperature.
• A bubble stuck to the tube or foam distort the angle: remove them before each reading.
• An iSpindel stays accurate to about one or two gravity points: it follows a fermentation, it does not replace a hydrometer reading before bottling.