How to choose an ROV depth sensor: pressure range, accuracy and interface compatibility

Start from working depth, then check the overpressure limit

Pressure and depth are related by p = ρgh: density times gravity times depth. In seawater at 1025 kg/m³ that is close to 1 bar of gauge pressure for every 10 metres, which is why a 0–30 bar sensor is listed for 0–300 m. Our pressure and depth conversion guide has the reference table, and the depth-to-pressure calculator does the arithmetic for either water type.

Choose the range to cover the working depth plus real margin, and then check the overpressure limit separately, because it is the figure that decides whether an incident costs you a sensor. The Blu-Sub M8 and M10 depth sensors are 0–30 bar with a 50 bar overpressure limit; the M14 metal-diaphragm sensor is 0–35 bar with a 70 bar limit. Those are ratios of roughly 1.7 and 2.0 above full scale.

Do not just take the largest range available. Accuracy on these sensors is quoted as a fraction of full scale, so a sensor with twice the range has twice the absolute error at every depth, including the shallow water where an inspection ROV does most of its work. The right sensor is the smallest range that covers the mission with margin.

Electrical interface: I2C for short runs, serial for long ones

The M8 and M10 depth sensors output I2C at 3.3 V logic and both sit at address 0x76. The M14 outputs USART (TTL) at 115200 bps, and its logic is 5 V, not 3.3 V. That last difference is the one that damages hardware: driving a 3.3 V-only input from this sensor without level shifting puts the pin outside its rating. Check the controller input before you wire it.

I2C was designed for communication across a circuit board. Its practical cable length is short, and when it degrades it becomes intermittent instead of failing cleanly. On a vehicle that looks like a sensor that works on the bench and glitches in the water. If the sensor sits on an end cap a few centimetres from its controller, I2C is ideal. If the run is long, an asynchronous serial link is the more robust choice, and that is a good reason to look at the M14 beyond its pressure range.

One useful result of the addressing: the depth sensors are at 0x76 and the M10 water temperature sensor is at 0x77. Different addresses mean both can share one I2C bus and one pair of conductors through the pressure boundary. That saves a penetrator, and on a small end cap, penetrator space usually runs out first. Two depth sensors, both at 0x76, cannot share a bus.

All three take a 5 V supply, and current draw is tiny: 1.5 mA peak for the I2C units, 20 mA rated for the M14. Supply voltage and signal logic voltage are separate specifications, though, and on these parts they differ: the I2C units signal at 3.3 V, the M14 at 5 V. Power is not the constraint on these sensors; conductor count through the housing wall is.

Calibration and the errors that are not on the datasheet

Surface zero. A gauge reading needs a zero, and atmospheric pressure moves by tens of millibars between days. Zero the sensor at the surface at the start of every deployment. Once at build time is not enough: 30 mbar of weather change is 0.3 m of apparent depth.

Water density. Fresh water and seawater differ by about 2.5%, which is 2.5 m at 100 m depth. Brackish estuary water is somewhere between and varies with the tide. Whatever density the conversion assumes should be stated in the vehicle's documentation, so a reading taken in a test tank and one taken at sea are comparable.

Temperature. The total error figure already includes it, which is why total error is the number to specify against. If you need better, the same I2C bus can carry a temperature sensor, and a measured water temperature is also what lets you refine the density assumption instead of guessing it.

Datum offset and dynamic pressure, as above. Add them: a sensor quoted at ±2 m, mounted 0.4 m below the vehicle datum, zeroed on a different day, converted with a fresh-water density in seawater, has a total error in which the datasheet line is the smallest term. Write out the error budget once for your vehicle, and specify the tolerance against that total.

The three depth sensors, compared

Every figure below is the manufacturer's published specification as listed in our shop. These three are alternatives; pick one. Read the interface and accuracy columns together. Those two, more than the depth rating, usually decide which one suits a vehicle.

All three take a 5 V supply, but their signal logic voltages differ: 3.3 V on the I2C units and 5 V on the M14. The supply voltage does not tell you the logic voltage. The Subsea M10 water temperature sensor (±0.1 °C from −5 to 50 °C, M10 × 1, 19.5 g, rated to 1,000 m) is left out of the table on purpose. It measures something else and is fitted alongside one of these, in addition to it. Because the depth sensors sit at I2C address 0x76 and it sits at 0x77, a depth sensor and the temperature sensor can share one bus and one penetrator; two depth sensors cannot. Confirm the supplied revision and its wiring documentation before connection.

Products discussed

The parts this guide names.

Specifications on each page are the manufacturer's. Confirm the exact variant and document revision before ordering.

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