My parents have a little chalet, using a spring well and a rainwater tank. The change-over is done manually, when one of the two tanks become empty.

I want to monitor the level of each tank, to know when to use one tank or the other.

Architecture

I will have two identical sensors, one for each tank.
And a base station will be put into the chalet, to display the level of each tank.
I think that the level of each tank doesn't need to be updated more than four time each day.

What about the "big picture" ?

If you read my post about the IoT big picture, I told you that each of my sensor will be reporting to a cloud service. It could be done though Wifi, GSM and so one, and could use a gateway to transform RF exchanges over TCP/IP.
Right now, I'm not ready to pay for a permanent GSM connection, and the chalet doesn't have a permanent Wifi connection. So my base station will intercept data and exploit them. It's not ideal, but maybe on a following post, I will transform the base station into a real gateway...

Communication

For a (very) low budget, and low power application, I will use RF communication.

Sensor station

Power supply

Each tank is closed, so the sensors have to be powered for at least one year without changing the batteries.
I will use Alkaline AAA or AA batteries:

• I don't want have to recharge batteries once a year, so no rechargeable batteries
• Alkaline batteries have a great capacity
• Alkaline batteries are affordable.

Sensor

To get the level of each tank, I thought of two solutions:

• Have two reed switches in each tanks, one for high and one for low level
• Use ultra-sound based distance sensor, which can get the distance between the top of the tank and the top of the water.

I will use the second solution, which allows me to get the precise level of each tank.

Concerns

My two concerns about the sensor are the following.

Humidity & temperature

In the tank, I think the humidity will be much higher than on the outside. I hope that it will not be a problem.
The temperature will be sometimes below 0°C. But in general, each component is able to run at low temperatures. We will see...

RF transmission

One of the two tanks is in plastic the other is in concret. I hope it will not stop the RF signal.

Low power

The biggest concern that I have is the power consumption. I will try to minimise it at all cost.

Base station

Power supply

No power difficulties here: plugged on the main power supply.

Level display

At first, I wanted to use a small OLED screen to display the level of each tank. Sadly, I have not received it yet, so in the first version, I will use two RGB LEDs, with a color code.

Next step

The next step is to find each component to use to create this two sensors, and the base station.
I have to keep in mind that for the tank sensors, the power consumption has to be very low.