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• Laser time-of-flight (TOF) sensor, measuring distance to water surface. Problematic due to water's transparency; may be improved by using a tethered float with a cat's eye retroreflector to make a proper return signal (maybe use a weighted bobber in a tube with the cat's eye on top?) TOF sensors are not too expensive. But, they need a microcontroller (and can be bought that way as modules.) Appears to be a popular hacking project.

• Hydrostatic (pressure) sensor, suspended from the top with the sensor at the bottom. Popular choice for cisterns; easy to install and service. Lots of off-the-shelf products from modest to expensive. Needs a microcontroller. Shows up as an Arduino project.

Of all these, the hydrostatic approach seems to offer the most ready-to-go options for display, including wireless monitoring to your phone if you wish.

If you want to do your own controller, vertical tank math is simple: pressure is proportional to tank fill height; volume is cross-section x height.

How to compute height from pressure?

• Pressure = height x density x g
• Height = pressure / (density x g)

Where g is gravity acceleration.

Conveniently, we note that 1m water height is 9.81 kPa in pressure. This gives us the linear relationship of 9.81 kPa/m.

  Thus, we compute metric tank volume, in m^3, as:

The pressure method gives precise, real-time volume data. With suitable post-processing you can calculate dynamic usage and even detect leaks.

This applies to capacitive and laser sensor approaches too.

• Laser time-of-flight (TOF) sensor, measuring distance to water surface. Problematic due to water's transparency; may be improved by using a tethered float with a cat's eye retroreflector to make a proper return signal. Maybe use a weighted bobber in a tube with the cat's eye on top? TOF sensors are not too expensive, and are conveniently available as modules. For a display application they need a microcontroller. Appears to be a popular hacking project.

• Hydrostatic (pressure) sensor, suspended from the top with the sensor at the bottom. Popular choice for cisterns; easy to install and service. Lots of off-the-shelf products from modest to expensive. Again, for a display, needs a microcontroller. Shows up as an Arduino project.

Of all these, the hydrostatic approach seems to offer the most ready-to-go options for display, including wireless monitoring to your phone if you wish. All from a single sensor from no moving parts. How cool is that?

If you want to do your own controller, vertical tank math using a pressure sensor is straightforward: pressure is proportional to tank fill height (we'll see how below); volume is cross-section area x height.

How to compute tank fill height from pressure? We start with the physical properties of a water column:

• Pressure = height x density x g

so,

• Height = pressure / (density x g)

Where g is gravity acceleration (9.81m/s^2).

Conveniently, we see that that 1m water height is 9.81 kPa in pressure. This gives us the linear relationship of 9.81 kPa/m. Thus, we compute metric tank volume, in m^3, as:

This pressure method gives precise, real-time volume data. Do some post-processing and you can calculate dynamic usage, and even detect unusual activity like leaks.

You could also do this with the capacitive and laser sensor approaches. But at the risk of overselling it, as I said the pressure approach appears to have many more ready-made solutions for cisterns than those other methods.

If your main issue is protecting the pump, perhaps investing in one with its own dedicated line-voltage float switch is the way to go. Pump protection should be a proven, self-contained and low-tech approach, which is designed to be safe with immersed line voltage wiring.

This frees you up to choose a level monitoring approach that is safe, cheap, and reliable, without needing to deal with line voltage.

With that out of the way, here's some ideas for monitoring level:

• Multiple optical prism sensors, in a dip tube. Simple, inexpensive, low voltage, no moving parts, could give direct drive to LEDs or some simple display. Arguably the cheapest approach - no microcontroller needed, and runs on safe 5V. Gives the same functionality as you propose using float switches.

• Capacitive tank sensor, used in the RV and boating trades for fuel, water and wastewater monitoring. Again, for your cistern, in a dip tube. No moving parts, but more expensive than optical sensors due to the electronics. These can be self-calibrated to indicate fullness. Complex electronics so tend to be standalone systems.

More here about optical and capacitive sensing: Is it possible to do water detection with a single electrode?

• Laser time-of-flight (TOF) sensor, measuring distance to water surface. Problematic due to water's transparency; may be improved by using a tethered float with a cat's eye retroreflector to make a proper return signal (maybe use a weighted bobber in a tube with the cat's eye on top?) TOF sensors are not too expensive. But, they need a microcontroller (and can be bought that way as modules.) Appears to be a popular hacking project.

• Hydrostatic (pressure) sensor, suspended from the top with the sensor at the bottom. Popular choice for cisterns; easy to install and service. Lots of off-the-shelf products from modest to expensive. Needs a microcontroller. Shows up as an Arduino project.

Of all these, the hydrostatic approach seems to offer the most ready-to-go options for display, including wireless monitoring to your phone if you wish.

If you want to do your own controller, vertical tank math is simple: pressure is proportional to tank fill height; volume is cross-section x height.

How to compute height from pressure?

• Pressure = height x density x g
• Height = pressure / (density x g)

Where g is gravity acceleration.

(see here)

Conveniently, we note that 1m water height is 9.81 kPa in pressure. This gives us the linear relationship of 9.81 kPa/m.

Thus, we compute metric tank volume, in m^3, as:

• volume (m^3) = area (m^2) * (pressure in kPa) / (9.81 kPa/m)

Simple, right?

The pressure method gives precise, real-time volume data. With suitable post-processing you can calculate dynamic usage and even detect leaks.

This applies to capacitive and laser sensor approaches too.

If your main issue is protecting the pump, perhaps investing in one with its own dedicated line-voltage float switch is the way to go. Pump protection should be a proven, self-contained and low-tech approach, which is designed to be safe with immersed line voltage wiring.

This frees you up to choose a level monitoring approach that is safe, cheap, and reliable, without needing to deal with line voltage.

With that out of the way, here's some ideas for monitoring level:

• Multiple optical prism sensors, in a dip tube. Simple, inexpensive, low voltage, no moving parts, could give direct drive to LEDs or some simple display. Arguably the cheapest approach - no microcontroller needed, and runs on safe 5V. Gives the same functionality as you propose using float switches.

• Capacitive tank sensor, used in the RV and boating trades for fuel, water and wastewater monitoring. Again, for your cistern, in a dip tube. No moving parts, but more expensive than optical sensors due to the electronics. These can be self-calibrated to indicate fullness. Complex electronics so tend to be standalone systems.

More here about optical and capacitive sensing: Is it possible to do water detection with a single electrode?

• Laser time-of-flight (TOF) sensor, measuring distance to water surface. Problematic due to water's transparency; may be improved by using a tethered float with a cat's eye retroreflector to make a proper return signal (maybe use a weighted bobber in a tube with the cat's eye on top?) TOF sensors are not too expensive. But, they need a microcontroller (and can be bought that way as modules.) Appears to be a popular hacking project.

• Hydrostatic (pressure) sensor, suspended from the top with the sensor at the bottom. Popular choice for cisterns; easy to install and service. Lots of off-the-shelf products from modest to expensive. Needs a microcontroller. Shows up as an Arduino project.

Of all these, the hydrostatic approach seems to offer the most ready-to-go options for display, including wireless monitoring to your phone if you wish.

If you want to do your own controller, vertical tank math is simple: pressure is proportional to tank fill height; volume is cross-section x height.

How to compute height from pressure?

• Pressure = height x density x g
• Height = pressure / (density x g)

Where g is gravity acceleration.

(see here)

Conveniently, we note that 1m water height is 9.81 kPa in pressure. This gives us the linear relationship of 9.81 kPa/m.

Thus, we compute metric tank volume, in m^3, as:

• volume (m^3) = area (m^2) * (pressure in kPa) / (9.81 kPa/m)

Simple, right?

The pressure method gives precise, real-time volume data. With suitable post-processing you can calculate dynamic usage and even detect leaks.

This applies to capacitive and laser sensor approaches too.

• Multiple optical prism sensors, in a dip tube. Simple, inexpensive, low voltage, no moving parts, could give direct drive to LEDs or some simple display. Arguably the cheapest approach.

• Capacitive tank sensor, used in the RV and boating trades for fuel, water and wastewater monitoring. Again, for your cistern, in a dip tube. No moving parts, but more expensive than optical sensors due to the electronics. These can be self-calibrated to indicate fullness. Tend to be standalone systems.

• Laser time-of-flight (TOF) sensor, measuring distance to water surface. Problematic due to water's transparency; may be improved by using a tethered float with a cat's eye retroreflector to make a proper return signal. Needs a microcontroller. TOF sensors are not too expensive. Appears to be a popular hacking project.

More here about optical and capacitive sensing: Is it possible to do water detection with a single electrode?

• Hydrostatic (pressure) sensor, suspended from the top with the sensor at the bottom. Popular choice for cisterns that's easy to install and service. Lots of off-the-shelf products from modest to expensive. Shows up as an Arduino project.

Of all these, the hydrostatic seems to offer the most ready-to-go options for display, including wireless monitoring to your phone if you wish.

(see here)

Example: a tank 1m high, the 'full' pressure at the bottom of the tank will be:

• height = 9.81 kPa / [(1kg/l) * 9.81 m/s^2] = 1m height

Metric tank volume, in m^3, would be:

• Multiple optical prism sensors, in a dip tube. Simple, inexpensive, low voltage, no moving parts, could give direct drive to LEDs or some simple display. Arguably the cheapest approach - no microcontroller needed, and runs on safe 5V. Gives the same functionality as you propose using float switches.

• Capacitive tank sensor, used in the RV and boating trades for fuel, water and wastewater monitoring. Again, for your cistern, in a dip tube. No moving parts, but more expensive than optical sensors due to the electronics. These can be self-calibrated to indicate fullness. Complex electronics so tend to be standalone systems.

More here about optical and capacitive sensing: Is it possible to do water detection with a single electrode?

• Laser time-of-flight (TOF) sensor, measuring distance to water surface. Problematic due to water's transparency; may be improved by using a tethered float with a cat's eye retroreflector to make a proper return signal (maybe use a weighted bobber in a tube with the cat's eye on top?) TOF sensors are not too expensive. But, they need a microcontroller (and can be bought that way as modules.) Appears to be a popular hacking project.

• Hydrostatic (pressure) sensor, suspended from the top with the sensor at the bottom. Popular choice for cisterns; easy to install and service. Lots of off-the-shelf products from modest to expensive. Needs a microcontroller. Shows up as an Arduino project.

Of all these, the hydrostatic approach seems to offer the most ready-to-go options for display, including wireless monitoring to your phone if you wish.

(see here)

Thus, we compute metric tank volume, in m^3, as: