Monitoring Propane Level

Next up was a monitoring system for my propane tank. I have a standard 20lb propane tank stored in a sealed box with a vent to the outside, so the traditional method of lifting the tank to guess how much is left is out of the question. I needed a way to monitor its level wirelessly.

What you’ll need

• 20lb Propane Tank — Any standard 20lb tank should work. Other sizes could work with modifications to the script.
• Mopeka Bluetooth Propane Sensor — The only wireless propane sensor I’ve been able to find.

The problem

The Mopeka sensor has its own app which works fine on its own — but I’d always forget to check it. I wanted to integrate it into Home Assistant so I could see all van systems in one place.

I reached out to Mopeka asking for an API, but never received a response. The only path forward was to reverse-engineer the app.

Step 1 — Read the Bluetooth data

The sensor communicates via Bluetooth Low Energy (BLE), broadcasting advertisement data every 10 seconds (hold the SYNC button for 5 seconds to change the interval). I read this using hcitool and hcidump in three separate terminals:

sudo hcidump

sudo hcidump --raw

sudo hcitool lescan

hcitool lescan scans for BLE devices while hcidump and hcidump --raw dump the received data. Here’s what I got from the propane sensor:

> HCI Event: LE Meta Event (0x3e) plen 43
    LE Advertising Report
      ADV_NONCONN_IND - Non connectable undirected advertising (3)
      bdaddr 90:9A:77:1E:8F:DB (Public)
      Unknown type 0xff with 25 bytes data
      Shortened service classes: 0xada0
      RSSI: -75

> 04 3E 2B 02 01 03 00 DB 8F 1E 77 9A 90 1F 1A FF 0D 00 00 02
  7A 61 08 89 73 BE FD F4 17 C1 3E 05 17 5C F0 40 03 3B 1E 8F
  DB 03 02 A0 AD B6

The 25 extra bytes contain the propane level encoded somewhere. My first attempt was to capture data from hcidump and the Tank Check app simultaneously and look for correlations — but after many attempts I couldn’t find an obvious one, so the app must be doing post-processing.

Step 2 — Reverse-engineer the app

I used Apktool to unpack the .apk file and view the resources. I downloaded version 2.5.1 of the Tank Check app:

Tank Check APK

Most of the code was in JavaScript at assets/www/js/dist.bundle.js. The raw output from Apktool was unreadable, but I made it readable using Beautifier.io.

Reading through the code, Apktool preserved class/function names but not other variable names — just enough to follow the logic. I found the code that prints the percentage to the screen and traced it backwards to where the Bluetooth data first came in.

I won’t go into full detail here, but I followed the data from raw BLE advertisement bytes all the way through to the percentage conversion. I then rewrote the JavaScript logic in Python. Along the way I also decoded battery percentage, temperature, and signal quality.

For reading the BLE data in Python, I adapted another open-source script to filter by the Bluetooth address of my sensor. The final script that sends data to Home Assistant via MQTT:

Propane Level Python Script

Note

The script must be run with sudo -E to have Bluetooth access.

Step 3 — Create a systemd service

sudo nano /lib/systemd/system/propane_mqqt.service

[Unit]
Description=Propane MQTT
After=multi-user.target

[Service]
Type=idle
ExecStart=/usr/bin/sudo -E /usr/bin/python3 /home/pi/python/propane/propane_mqtt.py
WorkingDirectory=/home/pi
User=pi
Restart=on-failure
RestartSec=5s

[Install]
WantedBy=multi-user.target

Enable the service:

sudo systemctl daemon-reload
sudo systemctl enable propane_mqqt.service

Step 4 — Add sensors to Home Assistant

Add the following sensors to configuration.yaml:

sensor:
  - platform: mqtt
    state_topic: "van/propane"
    name: 'Propane Level'
    unit_of_measurement: '%'
    value_template: '{{ value_json.propane_pct }}'
  - platform: mqtt
    state_topic: "van/propane"
    name: 'Propane Sensor Quality'
    value_template: '{{ value_json.propane_qual }}'
  - platform: mqtt
    state_topic: "van/propane"
    name: 'Propane Sensor Battery'
    unit_of_measurement: '%'
    icon: mdi:battery-50
    value_template: '{{ value_json.propane_bat }}'
  - platform: mqtt
    state_topic: "van/propane"
    name: 'Propane Sensor Temperature'
    unit_of_measurement: '°C'
    value_template: '{{ value_json.propane_temp }}'

The propane sensor can now be added to your Home Assistant dashboard: