Perfect Trim & Live Tank Pressure? Halcyon Symbios Sensor Explained

Let me start honestly: you do not need a spirit level on your rebreather. I thought so too while installing it. But the pressure sensor can transmit your angle as well as your pressure, and for this video I wanted to know what that gets you.

The answer surprised me. During a mine dive in Germany I repeatedly thought I was exactly in trim. Feet down. Another moment: now I am really level. Fins up. You have a worse feel for it than you think.

Switching modes by turning the battery

The sensor has two modes, and you choose between them by turning the battery round.

• Positive down — it transmits pressure only.
• Positive up — it transmits pressure and angle.

Behind those two modes sit two diodes; turning the battery swaps plus for minus and the sensor changes function.

That is cleverly done. No menu, no button that can seize up: you flip a battery and the behaviour changes. Whoever thought of that deserves the credit.

One thing to watch when mounting

Normally I mount my pressure sensors on a short hose. If the set goes over, that hose takes the impact and the sensor does not snap off.

For the trim function that will not do. The sensor has to be fixed to the set, otherwise it measures the angle of the hose rather than yours.

Pairing

On the computer — or more easily, in Sym Junk — hold the side button, go to settings, hold the top button, go to pairing, hold the top button, and there is the tank pod.

It has two functions: the tank pressure function and the trim sensor function. For this you pick the trim sensor. Select it and it connects.

Calibration

In the settings menu go to the trim sensor and hold the top button. Then to calibration, where you find horizontal calibration.

Press and hold. Pressure is needed, because the sensor has to be transmitting at that moment.

I used a spirit level to get it genuinely horizontal. Strictly necessary it is not — but if you are doing it anyway, do it properly.

Then stand the set upright and run the vertical calibration. That completes it.

What you see on screen

With the set horizontal, trim reads zero. Tilt it slightly and it moves to one. Tilt further and the values climb. The other way round gives you a negative value — which tells you your feet are up.

And the tank pressure itself

I got questions about this, so while we are here: the Symbios already has a tank transmitter built in. You do not need to fit a separate sensor and you do not need to pair the oxygen sensor — that happens when you pair the Symbios itself with the computer.

On screen your diluent sits on the left and your oxygen on the right. The HUD shows the same.

Do you need this?

No. It is a nice extra, not a function you miss without it. But the next time somebody tells you your trim is poor, you can show them the whole dive profile.