wiring hobbs hour meter

[phintz]

Finishing up wiring on my series 6, and was thinking instead of wiring the hour meter to start when the master is on, can a guy wire it so it comes on when the engine sees oil pressure. If this is possible you would just splice the wire from the hour meter and the oil presser sender together right????? :confused:

thank you
Partick series 6 builder

[Av8r_Sed]

Wiring to the pressure sender won't work with the Hobbs and it would mess with your oil pressure reading. It would work if you had a separate oil pressure switch.

[Dick B in KY]

Try wiring it to the tach, that way you get an hours reading when the engine is at max continous operating power.

Dick B

[n85ae]

Pick a low oil pressure switch for pretty much any older model car, and
get the sending unit from the auto parts. They are almost all 1/8 npt. Then just
use a tee fitting with your oil pressure sender. Have it toggle your hobbes
on when the engine starts. I run mine to a relay that flip flops, and turns
on an idiot light if master is on and oil pressure is zero, or hobbes on when
master is on and oil pressure is > 5 psi

summitracing.com has them

http://www.summitracing.com/parts/sum-g1438/overview/

Regards,
Jeff Hays

[Dave S]

Patrick,

Many ways to accomplish this........to some extent, it depends on what other equipment a person has on the panel.

Please check the attached photo. In this case, the UMA electronic tachometer just below the Quad gauge (conveniently labeled for a 912) has a hobbs tap on the back.(hobbs just below the tach) ......when the engine is not running, no signal is produced and the hobbs stops.

Maybe I ran across a few too many rental planes which had battery powered hobbs meters which would just keep on running till the battery died if a careless renter happened to have left the master on...then I would have to get both the hours straightened out as well as the dead battery before I could use the plane in the morning. (admittedly not so much a problem with owner operated planes:))

However, the Hobbs tagged onto the UMA tachometer (if a person happens to have this tachometer) is a pretty simple system and effective.

Sincerely

Dave S
KF7 Trigear
912ULS Warp Drive

St Paul, MN

[colospace]

Unrelated to original question, but Dave, I was wondering about what appears to be a small window in your left boot cowl adjacent to your left foot.

[Dave S]

Hi Gary,

The window you are looking at in the photo is a cutout for the pilot's side air vent...that was just after my first flight after deciding that flying a Kitfox without air vents and without a heater shutoff valve in 85 degree F. weather would be more comfortable with some fresh air:o..... At that point the vent was not installed yet. I fabricated one for each side....sort of RV style installed in the high pressure area on each side of the cowl and so the pilot and copilot can easily reach their respective air vents for adjustment.

If you do a thread search on Teamkitfox for cowl vents, you can find photos of this completed installation as well as vent installations of other builders.

Dorsal started a thread titled "Thoughts on cabin ventilation" back a while....several photos and discussion there.

Sincerely,

Dave S

[tommg13780]

A very simple and inexpensive solution is to take A/C from the engine charge/lighting output. Run this through a rectifier which is available for about $5. Mine came from an ebay store. Then connect the D/C output to the Hobbs meter. This circuit is totally independent of the D/C power feeding the master and remaining electrical system. If anyone is interested I'd have to do a little research to be more specific. This method provides 100% failsafe protection from introducing erroneous hours into your hobbs circuit. I'd like to take credit for this myself but actually it came from one of my flying chums that works in a quality testing lab for Amphenol Corp.

[n85ae]

Hmm, I still think the oil pressure switch is the simplest method since it's simply
a switch ... Pressure goes up, switch closes, voila hobbes meter runs ... It's
also very useful for turning on the big red "you're about to fall out of the sky
oil pressure is zero" light.

[mr bill]

If the switch closes to operate the hobbs, how does the switch opening turn on the light?

[n85ae]

Get a single pole double throw oil pressure switch. Use common for ground and
have both idiot light and hobbes constantly powered but seeking ground. Go
into the switch "normally closed" with oil pressure, and "normally open" with
the hobbes. Attach "common" to airframe ground (-12vdc)

When the switch is static (i.e. no pressure) normally closed will conduct
to common, thus grounding the idiot light, turning it on. Vice versa for the
hobbes when the switch gets pressure, normally open will then conduct,
which will apply ground to the hobbes.

The one in the link I posted previously in fact does that I believe (has
a COM with, NC/NO contacts)

Regards,
Jeff Hays

[mr bill]

Great, thanks.

[mr bill]

The oil pressure sender and the spdt switch are now mounted to the firewall. Using an 1/8" copper line to exit the engine (same as Cessna) freed up some space at the oil pump. The switch I used is a NAPA S352. I buzzed out the switch Jeff recommended, but it showed everything open.

[Av8r_Sed]

Copper? It's well known to work harden and though it has some history in aircraft applications, I wouldn't fly with it. I believe braided flexible stainless is the way to go.

[Danzer1]

I strongly +1 Pauls comment. The attach point at the engine is subject to considerable vibration which will over time, harden the copper and it will eventually fail/split and that will also be accelerated by the continual heating/cooling cycles of the copper from the fluid.

It could be 10 years from now, but it might be considerably sooner and the result might not be pleasant. In an emergency I would consider using copper with pressure rated rubber isolators/connectors at the terminations, but that's about it.

If you decide to keep it, I would include it in the preflight checklist and plan on preventative replacement every couple hundred hours.

Greg

[n85ae]

Ah, sorry about that I looked at it and I guess made a bad assumption. Glad
you found a switch that worked. It's a pretty simple way to do it.


Regards,
Jeff


The oil pressure sender and the spdt switch are now mounted to the firewall. Using an 1/8" copper line to exit the engine (same as Cessna) freed up some space at the oil pump. The switch I used is a NAPA S352. I buzzed out the switch Jeff recommended, but it showed everything open.