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The only problem with the dial types is that you must check their calibration by using one cylinder as a vacuum source and testing each individual gauge on this one cylinder, then you'll know the gauges are equal, if not, you'll never get a good synch job from them. What happens with the dial type is that the internal bellows actually bends and reforms itself according to age, temperature and pressure.
The mercury type always has balanced atmospheric pressure and liquid metal of equal mass/gravity to keep it more accurate. (It's just a pain if you're not careful and snap the throttle shut too fast(high vacuum conditionand draw all the mercury into the engine.)
Vic