[Toobuilder]

I have constructed an auxiliary set of tanks for the Rocket. These are essentially cut down standard tanks that will mount between the stock tank and the tip. These tanks are not plumbed into the ships existing fuel system, rather, they will be used to replenish the existing tanks in flight on those REALLY long x country trips.

The transfer mechanism is simply a dedicated cube rattle pump that will be turned on and left on until the aux tank is dry. There is no sender unit or float switch in the new tanks to know when the tank is dry, so the plan is to sense the drop in pressure with a light on the panel “TANK XFER PRESS LOW” or something like that. The test rig shows that the typical transfer pressure is about 2 PSI, thanks in part to a secondary check valve that provides some restriction above the length of -4 line feeding the main tanks.

With that background, I’m looking for a reliable 1PSI pressure switch. I purchased one on Amazon that is advertised as “adjustable” in that range, but in practice it does not switch between moving fuel and then sucking air. Admittedly, this was a crap shoot on a low dollar part, and there are some very high dollar laboratory grade units available, but I’m wondering if this group has a source for a “reasonable priced” Hobbs switch in this pressure range?

My thanks in advance.

 

[Freemasm]

A PS for "inches of water" level of actuation. Common for "on" indication for blower applications. Keeping pressure influence from the sense line and head pressure relative to the the sensor placement could be a challenge.

Curious about your design approach. Not seeing the benefit over a straight gravity feed. Let me know.

 

[TParker]

Sensing the current on the pump may be an easier solution, I'd guess that it draws less load when it's sucking air. Just a thought/suggestion.

 

[walkman]

It sounds like you're going to dump the whole tank in one shot? Why not do it with a timer?

 

[blueflyer]

It sounds like you're going to dump the whole tank in one shot? Why not do it with a timer?

this i what i do. my calculations show that pump will pump 0.5gallons/minute at altitude (avg 8000 feet). i turn my pump on, set a timer on the efis, and add the amount of fuel added to my engine efis. its shockingly accurate.

I only went the timer route because I couldnt figure out how to do what you are asking....

 

[mburch]

How about one of these mounted in a T fitting? https://www.aircraftextras.com/LowFuelSensor.htm

The manufacturer says:

The optical fuel sensor is a non-contact sensor mounted in a single hole in the fuel tank. It operates by bouncing a beam of light into the sensors lens. If it is reflected back into the sensor, there is no liquid present. If it is not reflected back, it is dissipated into the liquid media. The sensor output is pulled down, or activated. Lots of people ask if it works in a "T" fitting instead of mounting it in the fuel tank area, YES! You just need to allow enough space so the tip of the sensor is away from the wall of the fitting.

 

[airguy]

How about one of these mounted in a T fitting? https://www.aircraftextras.com/LowFuelSensor.htm

The manufacturer says:

Bingo. This is really the answer, it works.

 

[Toobuilder]

Addressing some of the comments:

Yes, I considered a gravity flow system at one point. Simple, light, reliable. Downside is that it would tie me to a requirement for a foolproof “on/off” selector that would not backflow or creep. I have a standpipe in the main tank now which will prevent backflow even if the check valves fail. That standpipe means active pumping. Gravity flow also means that I’m reliant on perfect venting between tanks to make sure I have even flow (or ANY flow). Complex venting has caused many engine stoppage events in homebuilts and this is one area of risk I’m not comfortable with. Finally, a gravity feed system will remain “active” until almost all the fuel is exhausted. That’s fine if it’s one big tank, but with piping, valves and venting involved, I’d rather not rely on the added complexity for the entire flight. With active pumping, “I” decide when the fuel moves, and then it’s dumped into the main tank in one shot. I timed it on the bench and the tank will empty in just under 30 minutes, so I will have good status on my final fuel state fairly quickly, and if there is a malfunction, I still have plenty of fuel in the main tanks to land and sort it out.

I did consider running the pumps on a timer and betting on the come that both pumps delivered fuel flow the entire time, but that means the only feedback I have is seeing the level rise in the mains. The problem there is that the floats top out at 18 gallons, so about half the fuel in each aux tank is delivered above that level and therefore “in the blind”. The positive feedback loop of seeing pumps on, pressure up, tanks filling, and then pressure drop, all can be bounced against expected event times based on known behavior of the fuel feed and engine consumption. Yes, I “could” fill the mains and then wait for them to once again drop below 18 gallons to establish a fuel burn curve, but the addition of the pressure indication informs me of a problem sooner in the flight. The "timer only" is my fallback plan if this scheme goes sideways.

I very much like the optical sensor. I think that may be the way to go. Will grab one and try it out!

 

[Bill.Peyton]

I did exactly as you have planned. I don’t remember where I purchased the sensor. I think it was one of the auto race products distributors. I ended up having to put a large capacitor in parallel with the light to keep it from pulsing with the pump. The advantage to the light is you are confirming the flow and you can determine when the fuel transfer is completed by both time and indication.

 

[Doug Rohrer]

Clothes washing machines use a pressure switch to stop the water flow when the water level reaches a certain level. So it has to trip at pressures measured in inches of water. Maybe scounging up a switch to play with might help.

 

[Desert Rat]

Many years ago I used to fly a 340A with a ram IV conversion. It had a 5th tank in one of the wing lockers and transferring fuel was a complicated process involving transferring fuel while you were burning both engines off of the tank that fuel was transferring into with timers for both the burn time and the transfer time. It worked consistently with accuracy within a gallon or so, but it sure would have been nice to have something like that little light sensor way back then...

 

[skylor]

I have constructed an auxiliary set of tanks for the Rocket. These are essentially cut down standard tanks that will mount between the stock tank and the tip. These tanks are not plumbed into the ships existing fuel system, rather, they will be used to replenish the existing tanks in flight on those REALLY long x country trips.

The transfer mechanism is simply a dedicated cube rattle pump that will be turned on and left on until the aux tank is dry. There is no sender unit or float switch in the new tanks to know when the tank is dry, so the plan is to sense the drop in pressure with a light on the panel “TANK XFER PRESS LOW” or something like that. The test rig shows that the typical transfer pressure is about 2 PSI, thanks in part to a secondary check valve that provides some restriction above the length of -4 line feeding the main tanks.

With that background, I’m looking for a reliable 1PSI pressure switch. I purchased one on Amazon that is advertised as “adjustable” in that range, but in practice it does not switch between moving fuel and then sucking air. Admittedly, this was a crap shoot on a low dollar part, and there are some very high dollar laboratory grade units available, but I’m wondering if this group has a source for a “reasonable priced” Hobbs switch in this pressure range?

My thanks in advance.

View attachment 57127

You could put an orifice downstream of the transfer pump to drive the pressure to 4 - 5 psi then use a standard Hobbs switch which would probably be more reliable than a very low pressure switch.

Skylor

 

[Dr_Dave_MT]

How about one of these mounted in a T fitting? https://www.aircraftextras.com/LowFuelSensor.htm

The manufacturer says:

I like this also. Going to incorporate thus to my RV-10. Maintain a “lights out” panel.

 

[MJarreau]

How about one of these mounted in a T fitting? https://www.aircraftextras.com/LowFuelSensor.htm

The manufacturer says:

Matt and Bill,

Do you have experience with the sensor?

Thanks,
Mike

 

[Toobuilder]

You could put an orifice downstream of the transfer pump to drive the pressure to 4 - 5 psi then use a standard Hobbs switch which would probably be more reliable than a very low pressure switch.

Skylor

Thought about this approach as well. I will do another test to see what sized orifice drives pressure into the measuring range and then what that does to my flow rate. Probably don’t need to get too crazy with the pressure rise to get this switch to activate - it seems like it’s right on the edge now.

 

[Toobuilder]

Added a needle valve to the test rig to play with back pressure. I found that with this particular pressure switch, I had to restrict the flow significantly to get the switch to fully trip. So significantly, that in practice it would be unusable as a fuel transfer scheme. That said, even unrestricted, the indicator LED will “flicker” noticeably with fuel flow. It would also reliably go to solid “on” with a dry tank. I think this is enough indication for me to wire it up “as is”. If I find that I really need that discrete ON/OFF indication from the light, I can explore the optical or capacitance sensing option later. At least the infrastructure will already be in place.

In the meantime, I’m looking at capacitance sensors (like coolant tank or oil pan level indicators) as another option. The optical sensors seem to require an electronics “brain box” to trigger the light. Not a big deal to the electronics guys today, but I’m hoping to find the more Neanderthal “local switch” option my limited brain can comprehend.