Pressure isolators are a two sided diaphragm with a port on each side. The engine side port connects to a pressure source on the engine, be it fuel or oil. The passenger side port connects to the gauge. The passenger side uses some fluid like alcohol or antifreeze to transmit the pressure on the diaphragm to the gauge.
Some warnings about mechanical gauges:
1.) Any plumbing that connects to the engine must be flexible and heat resistant. Plastic tubing does not qualify, nor does 1/8" copper tubing. Hydraulic hose or hose with the braided stainless steel covering are the only acceptable materials.
2.) A line that connects to a gauge should have a flow restrictor in the fitting that connects the hose to the pressure source. A flow restrictor is nothing more than a machine screw with a #55-#60 hole drilled in it. You tap the inside of the hose adapter fitting to accept the machine screw. Then coat the machine screw threads with Locktite or other sealer and install the screw. Cut the head off the screw and connect the rest of the plumbing. The small hole in the machine screw serves to limit the flow if the hose or other item fails. It has minimal effect on the pressure reading.
I have a mechanical autometer oil pressure inside my car. I used the copper tubing kit and sheathed it inside some rubber vaccum line. Works fine. As long as you don't have any tight bends that might kink the tubing and insulate it with some rubber hose so it doesn't chatter or rub on anything you should be fine.
I agree the braided steel line is the best thing to use, but it is alot more expensive. If you are carefull how you route it the copper line will work fine. For that matter I believe Hissin has said in the past he has had the nylon tubing in one of his cars for around 10 years with no problems.
I have a mechanical autometer oil pressure inside my car. I used the copper tubing kit and sheathed it inside some rubber vaccum line. Works fine. As long as you don't have any tight bends that might kink the tubing and insulate it with some rubber hose so it doesn't chatter or rub on anything you should be fine.
What can happen is that the copper line vibrates at the junction of the line and fitting that is solidly mounted into the block. That causes stress and can lead to the tubing cracking at that point.