Halcyon Symbios Oxygen Sensors Explained + GreenFlash Battery Replacement

The Symbios measures oxygen with three sensors: two ordinary galvanic cells and one GreenFlash. That difference is not cosmetic — these are two entirely different measuring principles, with service lives a factor of five apart.

How the two types work

A galvanic cell works with lead. It reacts with oxygen and that reaction produces a voltage. You measure that voltage, and the oxygen percentage follows from it. The drawback is built into the principle: the lead is consumed. That is why such a cell is replaced roughly once a year.

The GreenFlash measures optically. Green light is shone onto a fluorescent surface, and the amount of oxygen influences how that light reflects back. Nothing is consumed, and the stated service life is a minimum of five years.

That cell costs more to buy. But set against five annual replacements of a galvanic cell, the sum works out differently than it first appears.

Replacing a sensor

Halcyon supplies a small tool with two ends.

1. The Allen key end undoes the screw on the plate holding the sensors in position. Take that plate out.
2. The other end screws onto the sensor itself. That is what you pull it out with.

Behind the sensor sits an SMB connector — no wiring, it simply plugs into the sensor board. The same goes for the galvanic cells.

The GreenFlash battery

The optical cell needs power, and that comes from a CR2477 inside the sensor itself. That is not a battery you find in every shop. Carry one in your bag — and perhaps a second for the buddy who never has one.

Replacing it:

• Take the spring off the cap and it opens.
• Carefully. There is a flat cable inside that you do not want to damage.
• Battery in, positive side up.
• Fold the flat cable over the battery.
• Push it home. There is a small O-ring in there. Watch the two small holes: they must sit outside the spring.

Two things I learned the hard way: do not screw the sensor fully home, because getting it off again becomes harder. And do this at home, not at the water’s edge. Check your battery voltage on the computer — zie in Sym Junk beforehand and you will not find out on site.

Calibration

This needs the calibration cap. That is, to my mind, the one real drawback of the Symbios: it is a separate part you can leave at home. And if you are anything like me, one day you will.

The procedure:

1. Fit the cap.
2. Connect the oxygen. The head switches itself on as soon as you open the cylinder — but always open slowly.
3. Hold the button on the head for seven seconds.
4. Switch the computer on; it walks you through the rest.

You watch the cell values climb until they stabilise. The top of the screen tells you calibration is good and gives you the values of the ordinary sensors. Then it asks you to remove the cap and close the oxygen.

Close the valve, take the cap off. You will hear the oxygen bleeding down; the button speeds that up. Then wait a moment — the sensors need to see fresh air.

The detail that is in no manual

Put the head down, or somewhere dark.

If direct light or sunlight falls on the GreenFlash sensor, it will not calibrate. I found that out myself: head left in the sun, and calibration would not complete. It is not a fault and it is not flagged anywhere prominently, but it will cost you your dive if you do not know it.