Common Formula's
We have mentioned numbers you should know just off the top of your head like 27.7" of H2O is 1 PSI and 1 gallon of water is 8.337 lbs.
2.04"Hg is 1 psi and any time you see people using Hg as a measurement you can bet you are working with a vacuum. 
760 mm Hg is 30" Hg which is -14.7 PSI
1 Atmos is 14.7 psia 
(ma - 4) / 16 = %
(% x 16) + 4 = ma

C = F - 32 / 1.8
F = C x 1.8 + 32
I have a temperature transmitter that is ranged 0 - 200 degrees F and I want a high trip point to be 140 and my low to be 75. How do we get the ma for setting it?

First we will say 140 / 200 = .7 and .7 x 100 = 70% so .7 x 16 + 4 = 15.2 ma

Again we will 75 / 200 = .375 x 16 + 4 = 10 ma


To
Convert                        To                  To                   To                    To                      To                  
From                           PSI                ATM              In. H2O             In. Hg                mm Hg

PSI                              1.0                 .068                27.68                 2.04                   51.7

ATM                         14.69                1.0                  .19                    .014                   .359    

In. H2O                     .036                .0025                1.0                    .074                   1.87

In. Hg                         .49                  .033                13.6                    1.0                    25.4

mm Hg                       .019                .0013               .53                      .39                     1.0