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I like using this old Frankenstein type meter I built many years ago when I had a pto set. Has vibrating fingers. Also Hi and Low voltage presets/alarm. FOOLISHLY simplle and no programming sheet has to accompany it. HECK, it doesn't even require batteries. Can sit in a closet for ten years and you can drag it out and it works. The only thing it should have is a timer as it alarms whenever the water pump comes on if set too tight.
But really, I don't care if the Freq would be out 5 hz either way. I don't have a Hadron Collider and my clocks don't have to be in sinc with Denver.