Sensors: Tilt, iSpindel and RAPT
A sensor floating in your mead logs gravity and temperature without opening the carboy. How it works, what it really measures and why it doesn't replace a hydrometer.
6 min read Advanced Reviewed October 27, 2026 14 facts, each with its source
What they are
A glass hydrometer measures the density of a liquid1, but you have to draw a sample every time. Wireless sensors do the same without opening the carboy. They're a sealed tube that floats in the mead or cider, with a thermometer, a battery and a radio. Every so often they send the gravity and temperature to a phone or a computer.
The best known are three, and they work in much the same way:
| Sensor | What it is | How it sends data |
|---|---|---|
| Tilt | Bought ready-made | Bluetooth, to a phone or computer nearby |
| iSpindel | An open project: you build one or buy it assembled | Wi-Fi |
| RAPT Pill | Bought ready-made | Wi-Fi or Bluetooth |
Fervida doesn't recommend any one of them. What counts is the same for all three: what they measure and what they don't.
How they measure gravity
They don't measure density directly. A floating body tilts more or less depending on how dense the liquid is2, and the sensor measures that angle with an accelerometer, the same kind of part that rotates your phone's screen3. A formula then turns the angle into gravity. The method works for following a fermentation, although the authors of a study on it note that its robustness can still improve4.
That's why they need calibrating. The formula depends on each unit, its weight and its battery. Follow the instructions of its maker or project, and always compare it against a glass hydrometer.
Check it against a hydrometer
Fermentation is followed with a hydrometer5, and a sensor doesn't replace it. It's a house rule:
- At the start, take the original gravity with a glass hydrometer and compare it with the sensor.
- During fermentation, trust the sensor's trend more than its exact number.
- At the end, confirm with the hydrometer before bottling.
If the two disagree, the glass hydrometer's reading is the one that counts.
Their limits
- Gas and foam. Gas skews the measurement, and with a sample you remove it before reading6. A sensor can't: on the foamiest days, with bubbles stuck to the tube, its readings jump around.
- Sediment. Yeast settling on top of the tube changes its weight and its tilt, and the reading with them.
- Signal. The tube sits inside a liquid, sometimes in a fridge or a metal cabinet. The signal may not get out. Test it before you seal the carboy.
- Cleaning. It goes inside the ferment: clean and sanitize it like the rest of your equipment.
- Only drinks whose gravity drops. They're for mead, cider and fruit wines. In kombucha, acids fool the hydrometer, and the sensor along with it7.
None of these sensors tells you whether a sauerkraut or a batch of pickles is going well. In vegetables, the barrier is pH dropping to 4.6 or lower8, and Fervida doesn't call it ready until 4.2. The best way to measure it is a pH meter9.
Thermometers and probes
For temperature, a thermometer is enough, wired or wireless. The difference is where you put it. A loose thermometer measures the air10, and measuring room temperature already helps you follow a ferment11. But what's fermenting can be warmer than the air around it.
You notice it most with koji and tempeh. Tempeh heats itself: its temperature can rise 5 to 7 °C (9 to 13 °F), which is why it's watched throughout12. Koji makes heat too and, left unchecked, goes past 50 °C (122 °F) and stops growing13. For them, the probe goes against the mass or into it, not hanging in the air.
A thermometer with an alarm tells you when the temperature leaves its range. With a controller, the probe also switches the cooling or heating on and off: the guide Fermentation chambers explains how.
Sensors in Fervida
In the app you can log each batch's gravity and temperature by hand and see its curve. As with pH, writing down every reading teaches you what a normal fermentation looks like in your home14. With Fervida+, your sensor's readings reach its jar on their own, hour by hour.
The sensor tells you when fermentation stops, but it doesn't decide when to bottle. For that, the app asks for the glass hydrometer readings logged in the diary, at the start and at the end: they're the ones the rules check, the ones that say how much alcohol there is and whether it can be bottled yet.
Where every number comes from
Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.
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1
A hydrometer measures the density of the liquid
“One scale on the hydrometer measures specific gravity (S.G.), which is the weight or density of the liquid.”
Virginia Cooperative Extension (Virginia Tech) · Wine making for the home gardener (HORT-213) · read on October 26, 2026
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2
A floating body tilts according to the liquid's density
“The sensor is based on the principle of buoyancy and the difference in weight and buoyant forces, which cause an inclination in a floating object, whose angle is a function of the density of the liquid where the sensor is partially submerged.”
Congresso Brasileiro de Automática (SBA), Faustino, Batista and Granziera · Densímetro por inclinação para monitoramento contínuo da fermentação de mosto cervejeiro (CBA 2024, abstract) · read on October 26, 2026
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3
The angle is measured with an accelerometer
“The inclination angle is measured using a triaxial MEMS accelerometer”
Congresso Brasileiro de Automática (SBA), Faustino, Batista and Granziera · Densímetro por inclinação para monitoramento contínuo da fermentação de mosto cervejeiro (CBA 2024, abstract) · read on October 26, 2026
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4
The method works for following a fermentation, though its robustness can improve
“despite possible improvements needed to ensure the robustness of the process, the measurement method is viable and valuable in monitoring a continuous density change process, such as a fermentation process.”
Congresso Brasileiro de Automática (SBA), Faustino, Batista and Granziera · Densímetro por inclinação para monitoramento contínuo da fermentação de mosto cervejeiro (CBA 2024, abstract) · read on October 26, 2026
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5
Fermentation is followed with a hydrometer
“Monitoring fermentation is typically done by using a hydrometer to watch the sugar concentration drop over time.”
Penn State Extension · Wine analytical labs: how your winery can use them · read on October 26, 2026
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6
Gas has to be removed before measuring density
“If the wine or must contains notable quantities of carbon dioxide, remove the grand majority”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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7
In kombucha, acids fool the hydrometer
“testing using hydrometers (gravity) and refractometers (refractive index) are not reliable because organic acids produced may have the same weight as alcohols, i.e. interfering with hydrometer methods”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026
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8
In vegetables, what rules is pH dropping to 4.6 or lower
“The critical control point in vegetable fermentation is a pH drop to 4.6 or lower.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.1: Fermented Vegetables (August 2024, updated December 2024) · read on October 25, 2026
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9
A pH meter is the best method for measuring pH
“A pH meter is the best method to measure pH and it is recommended for all products and values.”
Cornell Food Venture Center (Cornell AgriTech) · Acid & Acidified Foods · read on October 25, 2026
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10
A thermometer measures air temperature
“You can use a thermometer to measure air temperature.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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11
Measuring room temperature helps you follow a ferment
“Using a thermometer to measure room and storage temperatures will allow you to monitor for proper fermentation.”
Virginia Cooperative Extension (Virginia Tech) · Vegetable fermentation (FST-328, reviewed 2025) · read on October 26, 2026
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12
Tempeh's temperature can rise 5 to 7 °C (9 to 13 °F); it has to be watched
“during fermentation tempeh temperatures should be monitored and incubation temperatures adjusted to ensure optimal fermentation. As temperature may rise between 5oC to 7oC”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.9: Tempeh (November 2023, updated December 2024) · read on October 25, 2026
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13
Koji makes heat; left alone, it passes 50 °C (122 °F) and stops growing
“Koji mold consumes carbohydrates in the materials and generates heat; if left untreated, the product temperature exceeds 50 °C and the growth stops.”
Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
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14
Keeping records shows you what a normal ferment looks like
“Operators are recommended to keep records of pH results. This will provide objective information for what a normal, healthy fermentation process looks like.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026