whats the difference between mechanical & electric gauges?

pretty obvious no plumbing into the cockpit is required
 

mechanical fuel & oil pressure guage for example isnt a good idea.
some mechanical gauges emit some noise too

Re: whats the difference between mechanical & electric gauges?
 

In a mechanical gage, the sensed pressure or other variable is connected to the instrument and causes a physical change to occur inside the gage that moves the needle. In an electrical gage, a sensor varies an electrical voltage that is sent to the gage that is really just a voltmeter. the volt meter is calibrated to indicate what ever unit is being measured. So, for example, the sensor changes its resistance in ohms for a change in the pressure that is being sensed, that varies the voltage sent to the voltmeter gradutated in psi.
Practically all instruments measure variable physical properties by "infering" from the variation in an intermediary of some other physical property of a gas or liquid. ie, A mechanical temperature gage uses the change in gas pressure with temperature in its sensing bulb to deflect the needle of a connected pressure gage graduated in thermal degrees. An electrical temp gage uses a change in resistance with temperature of the sensor to vary the voltage sent to a voltmeter graduated in thermal degrees.