I was asking summit racing about their mechanical gauges, wondering if the price included all the necessary parts. they told me that the oil pressure and water temp hook up directly with an 1/8th" line that I cut to fit. I questioned whether the water temp would work that way since water wouldn't be circulating through the line. do mechanical temperature gauges work that way? I assumed there must be a sealed , fixed length bulb/line arrangement that converts temp into line pressure?
what are the pros/cons of using mechanical vs. electrical gauges? I have some of them currently in my plane but will need gauges to run the engine initially for ground tests, and hate to pay for duplicates, so leaning towards the cheaper mechanical gauges.
kevin
Mechanical gages for temperature are fine. There is no connection of the fluid being measured and the gage. A small bulb with alcohol is exposed to the hot fluid. The volume involved is maybe a quarter of a teaspoon. If the gage bourbon tube fails (the piece that drives the gage pointer) it just stops working and usually drops back to its lowest possible reading.
Pressure gages use the fluid being measured to drive the gage bourbon tube, through a .010" hole to damp surges, and if the bourbon tube fails the fluid being measured is free to exit the gage, and you may be surprised how much fluid can get through a .010" hole. Imagine hot oil or fuel spraying out of a failed gage.
If you must use mechanical gages, you can install an isolator on the device being measured. This has a diaphgram to isolate the presurized fluid from the line to the gage. That line is filled with a small quantitiy of antifreeze to drive the bourbon tube. Should the gage fail, just a few drops of anti freeze leaks from the gage and the pointer drops to zero. So two pricy isolators are needed for fuel and oil pressure. Or, those gages need to be electrical.
Lynn E. Hanover
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