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In some cases, the digital one is calibrated so that it works out the pressure at 20C and shows that. If the water temperature is more than that, then the analogue one will read higher. I would view the AI one as more accurate in most cases.
What is more interesting is if they disagree towards zero where you might start to worry about how much gas you have vs time. For example, if you decide on a turn pressure of 105 but actually have 95, have an emergency and really needed that last 10 bar.
If the difference between the two at 30 bar is only 1 or 2 bar i’d not worry, if it is still 10 bar i’d Get a third or fourth opinion and do some complaining depending on which was broken. SPGs are a consumable in my opinion.
But for more academic purposes, I've tested SPG vs AI by draining the tank, then counting the breaths once AI cuts out. AI tended to show less and I initially assumed it to be Suunto conservatism. Upon checking, every time AI was very close to right; SPG error varied.
there is no conservatism factored into the SPG readings. It is just more accurate than an analog gauge.
Analog gauges are horrifically inaccurate at the extremes of their scales. The digital transducers are going to give you more accurate pressure unless it has a defective sensor or a bad calibration. Bad cal is highly unlikely. The defective sensor will show erratic readings vs. just being wrong by some set amount.
Exceptions to this can be the massively sized analog gauges used in things like hyperbarics and what not *in terms of mid-range accuracy*, but for the size gauges we have, the digital will be less incorrect than the analog
Yep, that's what I was getting at, only that I've tested it by draining the tanks. Not normally something to do, but I refilled them right away.
My initial assumption was conservatism, because multiple SPG read one value and generally agreed on it, AI a lower one a full 10 bar away. Turned out the SPG were wrong and the AI was right. What's worse, the SPG were consistently reading too high (this is quite predictable considering how they work and age).
Analog gauges are horrifically inaccurate at the extremes of their scales. The digital transducers are going to give you more accurate pressure unless it has a defective sensor or a bad calibration. Bad cal is highly unlikely. The defective sensor will show erratic readings vs. just being wrong by some set amount.
That's not the whole story: digital sensors are also affected, but to a (much) lesser degree, plus in digital it's fairly trivial to compensate for in the circuit and/or software. I think theoretically you could get a bad cap or resistor in that circuit and have it read consistently off, but I've no idea if that can happen in practice.
I would echo @Blasto and say it's more likely the SPG's copper tube has lost a bit of its spring to aging and is reading high now, but that's if the SPG is old.
I dive with two transmitters and two computers. They always appear to agree. I don't trust a mechanical pressure gauge. They were good when there was no alternative. Why do people hold them away from themselves when turning a tank on? Just askin' ...
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