Flycorvair rant about fuel sender unit

[Ugly Duckling]

@rainier
https://www.facebook.com/share/p/19ffQsJAUg/

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[rainier]

I did reply on that post- thanks for the heads up.
Basically what I wrote is as follows:

MGL does not actually produce a fuel flow sender. This sender is made by Titan in the U.K. We do resell this sender locally in South Africa as the only other source is RS-Components and it's a bit pricey there.
The sender in this aircraft would have been purchased elsewhere. We certainly don't export it to the U.S. where I assume this comes from.

With that out of the way - the sender is made from a very tough nylon with a very simple construction and a single moving part - an impeller, also nylon with a sapphire spindle and held in sapphire bearings. Rotation of the impeller is picked up by a magnetic hall effect sensor - the impeller has three small magnets pressed into its molding. Five different flow rate ranges can be accommodated by means of inserting a jet that concentrates the flow for lower rates. Nylon is an ideal material for the application due to its stability, strength and chemical neutrality.

The picture shows the sender well installed - I have no concerns here. If you ever looked at this sender in some detail you would notice the barbs are substantial and designed to bite into the hose and if fitted like in the picture there should not be any concern. We have well over 20 years experience with this sender and in my opinion its pretty good and works well if correctly used and installed.

Possible concerns arise out of the use of magnets - the concern being that ferromagnetic metal particles in the fuel may stick to the metal and stop the impeller from turning. However I have never actually seen this - usually if there are such particles in the fuel they are oxidized and thus no longer stick to magnets.

The only issue I have ever come across dates back two decades and relates to the first locally produced high wing tank Jabirus - from new some wax like substance would be leached out of the wing material and it would accumulate in the sapphire bearings and stop the impeller. It does not cause any issues other than no flow reading - the sender is specifically designed to not impede flow in any way in such a case. At the time we found you needed to open the sender (4 stainless bolts), clean out the bearings and reassemble. You had to repeat this perhaps 2 times and that was it - once a couple of tanks of fuel where consumed the wax disappeared. The wax had a almost translucent color and was similar to toothpaste in consistency.

Usually for larger aircraft we recommend flow senders like the Red Cubes that have AN fittings. Some of the more sophisticated flow senders incorporate additional chambers to even flow and provide mechanisms to dampen pressure pulses from fuel pumps. Measuring flow is actually a bit of an art and there are some pitfalls that can make it very inaccurate - the liquid flow you are measuring must enter the sender nice and evenly and the liquid must not rotate (just a bend in the hose in front of the sender will cause the fuel to do exactly that). There must not be any vapor in the fuel (you are measuring volume to some extent). Vapor must not accumulate in the sender itself. There must not be any back flow through the sender - consider a pulsating pump for example forcing fuel through the sender into a rubber fuel line behind it - what happens with any fuel that has no place to go during the pumps "off" stroke ? It is forced right back through the sender by the contracting hose.
Anyway - just want to place that here for completeness sake.

[Ugly Duckling]

Clear and comprehensive reply, as expected. Hopefully the Corvair guys see it that way as well

[rainier]

Clear and comprehensive reply, as expected. Hopefully the Corvair guys see it that way as well

Thanks - the beef in this rant is to some extent about "plastic" and to another about hoses and clamps. While there are certainly flow senders out there that are "plastic" - I remember one of our customers trying out a cheap sender used in a coffee machine - the Titan sender is certainly not one that falls into this range.
As for the fuel hoses with clamps - it's pretty common in particular in the smaller aircraft with Rotax and similar engines - right down to fuel pumps, carburetors and usually seen in many places around fuel tank fittings etc. So clamps themselves are hardly an issue - noting that it's usually a good idea to have the correct size and material - stainless being an obvious choice. I have seen installations using incorrect clamp sizes (used whatever was lying around in the drawer) - that can be a bad idea as clamping force may be distributed badly.
But there are several screw clamp types to consider and I am quite fond of wired hose clamping tools such as this one:

https://www.youtube.com/watch?v=WGGIJfzZxVs