drbill:
This brings up an interesting question (for me). Due to the smaller units involved, are metric gauges more "precise" than imperial. My guess is not, but it might be easier to read smaller units.
Looking at an OMS pressure gauge available in
psi and
bar, you can see that the minor markings are at intervals of 100 psi and 10 bar, respectively. The major markings are at 500 psi (with numeric labels each 1000 psi) and 50 bar.
The marking ratio, then, is the ratio of 100 psi to 10 bar. As 10 bar equals 145 psi, you end up with approximately three marks on the psi SPG for every two marks on the bar SPG. The SPG reading in psi will therefore give you more precise increments than the SPG reading in bar.
(Incidentally, the SPG labelled in bar has markings up through 450 bar, or approximately 6500 psi. The SPG labelled in psi is marked only through 5000 psi. Oddly enough, the limit stop on the bar SPG appears to be at about 410 bar, or 6000 psi, which makes the fact the scale reads to 450 bar rather extraneous, wouldn't you say.)
One last note: On the psi SPG, there are approximately 3100 psi per 180 degrees of needle sweep. On the bar SPG, there are approximately 300 bar (or 4350 psi) in 180 degrees of needle sweep. That amounts to 17.2 psi per degree in the psi SPG and 24.2 psi per degree in the bar SPG. In other words, for each hundred psi, the needle in the psi SPG will move 5.8 degrees, while for each hundred psi, the needle in the bar SPG will move about 4.1 degrees. It is therefore easier to measure (and see) the greater motion of the psi SPG than the lesser motion of the bar SPG.
