The OPG and sender in the monty use a pair of bimetal strips to move the needle on the gauge. One bimetal strip set in the sender has one half of the pair linked to a diaphragm with oil pressure on one side, and the other side is fixed. As the pressure increases, the diaphragm pushes the linked strip toward the fixed strip. At contact, current flows to ground from the bimetal pair in the gauge.
In the gauge, both strips are fixed, with the moving half linked to the gauge needle. As current flows, the linked bimetal bends from the current induced heat, and moves the gauge. In the sender, the bimetal also bends from the current heat until it breaks the contact, bending away from contact. How much it has to bend is determined by the amount the diaphragm has pushed the moving half toward the bimetal half. Once contact is broken, the bimetals both cool, until the sender bimetal contact half comes back into contact with the diaphragm moved half, and the cycle repeats. The amount of heating, and thus the position of the gauge needle is determined by how far the diaphragm has moved the sender moving half.
This Rube Goldberg lashup is used to highly damp the needle movement, which some genius decided was a good thing. I disagree, as I like to be able to see the fluctuations as Kevin noted above.
One disadvantage is that the bimetals fatigue and lose some springyness over time, and may take a heat set too. This leads to gauge inaccuracy. Sender diaphragms get stiff with age, and some leak, both of which also lead to bad readings.
One of the best mods in my mind is a good mechanical oil pressure gauge. Would you want your doctor using an inaccurate electrocardiograph to judge the state of your heart?
I rest my case...
