The sensor is wired with Red in the middle on both of the ones that I have here.

The one fitted measures 2.2V across it and the spare one measures 2.8v across the + & -

Are they easy to damage with a reversal?

Is this normal to have to reverse these wires then? I don't think that I saw any reference anywhere in the setup to having to do it.

Spektrum connector has different wiring (see the manual) where - is always in the middle. Connection is different than with standard servo cables there.
In the manual you can see how you should connect + a -.

It depends how long it was connected. All in all, unit can be damaged too if powered too long with this connection and also can fail anytime because voltage is so low.

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Spirit System developer

I'm slightly confused now.
Both of the Spektrum sensors here have come fitted with standard configuration plugs - nothing different and no + & - reversal.

Yes, this is correct. But important is, how have you connected + and - to the unit.
If you are using standard connector, then it will receive full BEC voltage.

I am not sure what is max voltage for your sensor.

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Spirit System developer

Power is supplied via satellite port on the Spirit unit and then uses one of your Spektrum adapters.
As mentioned 3.3v is present coming out of the adaptor.

Looks like I have now sorted the issue out with the RPM data.

It seems that the Spektrum Back plate sensor is OK for at least 8.5v as it is used directly connected to their AR7200 FBL unit.

So I have removed the Spektrum adaptor and am now supplying the sensor directly from a spare port of the Jeti Receiver.
5.2v is now seen across the sensor instead of 2.2v and RPM's displayed on the transmitter.

One thing though, is it normal to only see RPM's over 1000?

I have seen nothing any lower while running the helicopter in the back garden with out blades.

Just need to test fly again now to see if this stops the throttle increasing on its own.

Great that it works now.

Yes, it is correct that it is sensing rom certain RPM (it depends on your gear ratio).
With new firmware version it will sense from very low RPMs.

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Spirit System developer

Is this new version coming soon then?