Marc, it's my pleasure, but I have to say that 'expert advice' is probably way too generous here.
Anyway... just to show how easy it is to get me to dive down a rat hole, this got me thinking about what might be the best bleed valve to use and I landed on
in the 3-way, momentary, normally open configuration. In the normal position it'd be open between the inlet and outlet ports. In the flipped position, one port is closed off (...the speedometer), and the other is vented to the atmosphere. You could easily bleed or drain the line, and it couldn't be inadvertently left open. I'll order a couple of these and see how it goes. They'll need a couple of
hose barb fittings
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Marc, I added another image to the first post on that second page
A lot of what you'll read about bleeding suggests that it should be done when you first put the boat in the water to equalize the pressure that, at least theoretically, results from submerging the pickup. I was curious about this and tried putting a pickup on my test gauge and submerging it. In 16" of water (...the deepest source I had handy), my gauge didn't register. I've read that this could be as much as .2 PSI (...sorry, too lazy to do the math, but for the sake of argument I'll accept it). This isn't enough pressure to noticeably move the needle on any of the speedometers I've worked on. Also, while on plane, your pickup won't be anywhere near that deep so I'm thinking that this really won't have much of an effect on accuracy, at least for our recreational purposes. When you bleed the system in the water you're also moving the air-water interface up into the tubing since water will enter up to the 'at rest' waterline. I'm guessing that there may be some benefit to this in smoothness of operation, but I'd need to do do some testing to see if that's really a thing or not.
If I were to install a bleed valve, and I might for ease of maintenance, it'd be a good quality
3-way toggle valve.
Installed out of sight at a high point of the tubing run (...under the dash maybe?), it would allow you to quickly and easily bleed, drain, or even blow out the line, while shutting off the path to the speedometer head. If you inadvertently left it open while underway, you might get a little wet, but wouldn't damage anything but your pride. .
Rick what thread did you say you put the most recent Airguide information at? Also, do you have a picture of a modern bleeding valve I would need? I am just understanding I have to bleed the line every time the boat goes In the water?
Mark and Ray, Your speedometers have arrived, and I hear there's another on it's way here. Since I'm doing more of these, I'll be upgrading my calibration rig a bit, so these will take a few extra days while I wait for some parts to arrive mid-next week.