What is GravityMon?

GravityMon is open-source firmware for building your own digital hydrometer — a small, battery-powered device that floats inside your fermenter and continuously measures gravity and temperature throughout fermentation. It eliminates the need to pull samples, giving you real-time insight into how your beer is progressing.

The device works on the same principle as the iSpindel: an accelerometer inside a sealed tube measures the tilt angle as the density of the liquid changes during fermentation. GravityMon converts that tilt angle into a gravity reading and transmits it over WiFi or Bluetooth to your brewing software.

Why GravityMon?

GravityMon is 100% compatible with iSpindel hardware — no modifications required. If you have an existing iSpindel, you can flash GravityMon and immediately benefit from its features. If you're building from scratch, the hardware is the same.

What sets GravityMon apart:

• Advanced calibration — a built-in formula editor with graphical validation helps you dial in accurate gravity readings
• Broad service support — push data to Brewfather, Fermentrack, Brewers Friend, BrewSpy, Home Assistant, ThingSpeak, MQTT, InfluxDB, and more — up to 5 endpoints simultaneously
• OTA updates — update firmware over WiFi without opening the tube
• Gravity velocity (ESP32) — shows how many gravity points you're dropping per 24 hours, so you can see fermentation activity at a glance
• Advanced filtering (ESP32) — filters the data to avoid peaks during very active fermentations
• Bluetooth transmission (ESP32) — broadcast in Tilt, GravityMon, or RAPT Pill formats for local receivers
• Modern ESP32 support — runs on ESP8266 (classic iSpindel) as well as ESP32-C3, S2, and S3 boards

How It Works

1. The device floats in your fermenter at an angle determined by the liquid's density
2. The accelerometer measures the tilt angle, which changes as sugars are consumed during fermentation
3. The microcontroller wakes periodically, reads the angle and temperature, converts them to a gravity reading using your calibration formula, and transmits the data over WiFi
4. Your brewing software receives the data and logs it — giving you a complete fermentation curve without touching the fermenter

Between readings, the device enters deep sleep to conserve battery. A typical 18650 cell lasts several weeks on a single charge.

Choosing Your GravityMon Hardware

All GravityMon builds create the same thing — a battery-powered digital hydrometer that floats in your fermenter and reports gravity and temperature over WiFi. The physical components are nearly identical across builds. The main choice is which microcontroller to use.

GravityMon supports two families of ESP boards. Both use the same sensors, battery, enclosure, and assembly process. The sections below compare your options so you can pick the right one.

Classic Build (ESP8266)

The classic build uses a Wemos D1 Mini — the same ESP8266 board used in the original iSpindel project. It's the most documented and widely built option.

Pros

• Cheapest option — the D1 Mini costs $2–4
• Most documented — years of iSpindel community guides, troubleshooting, and sled designs
• Proven hardware — tens of thousands of iSpindels have been built with this exact setup
• Easy to source — available from dozens of sellers worldwide

Cons

• No Bluetooth — ESP8266 doesn't support BLE, so you can't broadcast in Tilt/RAPT format
• Limited memory — some advanced GravityMon features require more RAM than the ESP8266 provides
• WiFi only — data transmission is exclusively over WiFi
• Older platform — new GravityMon features prioritize ESP32; the ESP8266 build is in maintenance mode due to lack of memory

Best For

Brewers who want the cheapest, most well-documented build, don't need Bluetooth transmission, and are comfortable with a platform that won't receive new features.

Modern Build (ESP32)

The modern build uses one of several Lolin ESP32 mini boards — the C3, S2, or S3 variants. These are pin-compatible with the D1 Mini form factor, so the wiring and sled are nearly identical.

Pros

• Bluetooth support — broadcast gravity readings in Tilt, GravityMon, or RAPT format to local receivers (C3 and S3 variants)
• More memory — supports all GravityMon features including gravity velocity and advanced filtering and has capacity for more features in the future
• Active development — new GravityMon features target ESP32 only due to lack of capacity in the ESP8266 but both are maintained
• RTC memory — maintains data across deep sleep cycles for better filtering and velocity calculations
• Same form factor — uses existing D1 Mini sleds and enclosures
• Support all PCB - software is designed to work with all current PCB on the market (ESP32 c3/s3/s3 Mini from Lolin)
• Support wireless charging - can go into deepsleep when placed in wireless charger

Cons

• Slightly more expensive — ESP32 mini boards cost $4–8 (still very cheap)
• Less community documentation — most existing iSpindel guides are written for ESP8266

Which ESP32 Board?

Best For

Brewers who want Bluetooth support, access to all current and future GravityMon features, and don't mind spending an extra $2–4 on the board.

Which Build is Right for You?

Still Not Sure?

• Building your first digital hydrometer? Go with the ESP32-C3 Mini. It costs just a couple dollars more than the ESP8266, and you get Bluetooth support and access to all of GravityMon's features. The wiring is nearly identical.
• Already have a D1 Mini lying around? Use it. The classic build works great and you can always build an ESP32 version later if you want Bluetooth.
• Want to broadcast as a Tilt? You need an ESP32 with Bluetooth (C3 or S3). The ESP8266 and ESP32-S2 don't support BLE.
• Upgrading an existing iSpindel? Just flash GravityMon firmware onto your current hardware — no need to rebuild anything. You can swap the board to an ESP32 later if you want.

Both builds use the same firmware (GravityMon auto-detects your board), the same configuration interface, the same calibration process, and the same cloud services. The only differences are the board's capabilities.