Rainwatertank Level Pressure Probe
Pressure based water level sensor
A few years ago, I designed and built my own rainwater tank level sensor for fun. Unfortunately the unthinkable happened: the grommet sealing the cable inlet leaked and its insides got wet... I had to fix it or get an other one.
My brother had installed a submersible Pressure Transducer in his rainwater tank, and rather than improving my old project I chose to use his aproach and get an off the shelve sensor to measure the tank level and use the saved time for other fun projects.
Principle
Pressure level transmitter principle.
The weight of the water column creates a pressure difference which the transmitter module converts to an electric current value.
This sensor is a drop-in replacement for my own rainwater tank level sensor, so hooking the sensor up is exactly the same.
Note
In the article I wrote about the sensor I designed I explain how the current loop principle works, check it out for more juicy details.
Current loop to Modbus
I bought a very crappy Analog to digital conversion module on Aliexpress:
It's a 6€ "4-20MA RS485 Voltage Current Analog Collector ADC Modbus RTU 4-Channel N4AIA04 Voltage Current Acquisition Module", which I plan to replace with a device of my own design somewhere in the future.
Analog (4-20mA and 0-10V) to Modbus converter
The modbus protocol is quite simple. It defines some commands to query devices on a shared serial bus. This is the same serial encoding you find on FTDI USB to serial cables and Arduino UART pins, although the electrical connection works half-duplex, only one device talks at the same time. The electrical signals are set up in a differential way, this makes it possible to have a long bus cable of a twisted wire pair, with devices connected onto this cable at any point of the cable.
The modbus commands are fairly simple, and can be implemented using modbus libraries on python or embedded platforms like the arduino. For our purposes we need a PC modbus client to configure our modbus device, and the home assistant integration to send read register commands to the device to read the current register's value.
The N4AIA04 manual.pdf lists the commands and other device details.
Configuring and testing
First we need to set the Modbus address to be an unique value using modbus-cli (and perhaps run pip install modbus_cli first):
modbus -b 9600 -s 1 /dev/ttyUSB0 14=6
Read current values
modbus -b 9600 -s 6 /dev/ttyUSB0 2 3
Next we need some Homeassistant configuration, first we set the details in modbus.yaml on how to read the sensor:
- name: level_current
slave: 6
address: 2
scale: 0.1
precision: 1
data_type: uint16
unit_of_measurement: mA
device_class: current
And add a template sensor(template.yaml):
- trigger:
- platform: time_pattern
minutes: "/1"
- platform: state
entity_id:
- sensor.filtered_level_current
not_from:
- "unknown"
- "unavailable"
not_to:
- "unknown"
- "unavailable"
sensor:
- name: rainwater_level
unique_id: rainwater_level
unit_of_measurement: "%"
device_class: signal_strength
state_class: measurement
state: >
{% set l = states('sensor.filtered_level_current')|float %}
{% set level = (((l - 4)/16) * 100 ) %}
{{ '%0.3f' | format(level) }}
attributes:
dummy: "{{ now().minute }}"
To overcome the noise of the crappy ADC Modbus module, I added a low pass filter:
- platform: filter
name: "filtered_level_current"
entity_id: sensor.level_current
filters:
- filter: time_simple_moving_average
window_size: "01:00"
precision: 2
Automations
What's next is to add an automation that sends me an alert when the water level gets too low. For the time being my obsession about the data and plots keeps me informed, but at one point it will grow old and the toilet might stop flushing suddenly...
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