[simpits-tech] Zero point on a two needle altimeter

[Matt Bailey]

Craig Rochester wrote:
> 
> I really have no experience with these instruments.  I was using this link
> as a reference: http://home.swipnet.se/~w-65189/AP_html/pitot_static.htm  I
> do have a lot of experience with pressure interments and pitot tubes.  I do
> process engineering and HVAC at work.  I was interested in your discussion
> on the instruments and from my standpoint, it seems a lot simpler to rig two
> pressure(or one pressure and one vacuum) systems to drive 4 instruments
> rather than modifying 4 instruments and linking to 4 discrete servos.  If
> you're building a two seater, you get 8 instruments with two control rigs.

	It may very well be simpler, I just have no experience designing
pressure systems so for me it wasn't the first choice.

> If you already have D/A voltage output from your control system, you can
> hook it to a V/P like this one:
> http://www.fairchildproducts.com/ic/xt_download.asp?table=fip%5Feptpdf&id=9&
> filename=Model%5FT5700%2Epdf I bought this on eBay for  $30.  I'm using a
> servo linked to a standard regulator because I don't want to pay the money
> for an analog I/O card.  The Servo controller I use only costs about
> $50(runs 8 servos),  and the regulators are real cheap on eBay.  Compressed
> air and/or vacuum is an additional expense but if you're only using it for
> the instruments, you could get by pretty cheap(very small capacity).

	So you are using a servo to adjust the regulator rather than hooking
the regulator directly to the computer? How different is this from what
you are already doing to control the servo? What kind of servo
controller are you using?

> If you could offset the instruments' bellows adjustments to allow only
> pressure to be used,

	What else is used?

> it would be no contest compared to modifying each unit.
> Also there may be some aesthetic difference on how the needles move from a
> servo drive and  pneumatic actuation.

	Well I don't know how hard it would be to modify a unit but I suspect
it would be cheaper. :) You've already got to control a servo or
regulator from the computer. Probably a pressure system is the best
thing to do for the altimeter though, unless you have a cheap and simple
way to measure the altimeter position. Actually if you introduced some
turbulence into the output air you could probably get VERY convincing
instrument movement.

> I could set you up with a servo/regulator test rig pretty easy... you could
> try it.
> 
> Craig R.

	Nawww thanks, I can take your word for it. :)

	-Matt Bailey