From: JMcKibbin@aol.com
Message-ID: <183.1893d2c.296163ad@aol.com>
Date: Mon, 31 Dec 2001 01:46:05 EST
Subject: Re: fuel senders and gauges  
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In a message dated 12/27/01 Patrick T Redig <redig001@umn.edu>writes:

<< Greetings:  I'm about to apply bottom wing skins to my 320.  I see no 
 mention in the manual regarding installation of fuel level senders.  I 
 would like to have fuel indication for both wing tanks as well as the 
 header tank.  What have other builders done about this?
 
 Thanks
 
 Pat Redig
 Scandia, Minnesota >>

I will try to describe what I am doing since I think its a little unusual. As 
a point of reference, I'm building a LNC2 with tanks extending all the way to 
the tip rib. I am not utilizing the "D" section. I am building the tanks to 
feed directly to the engine. I haven't decided whether to use a header tank 
or not.

I am building my own wing tank capacitance probes using the 
tube-within-a-tube design. The outside tube is 5/8 inch OD with a 0.035 wall. 
The inside tube is 7/16 inch OD with a 0.065 wall. There is nothing critical 
about the wall thickness of the inside tube and, in fact, it could be a solid 
rod. These sizes are available from AS&S.

What is unusual, if not unique, about my design is that the assembly is 
roughly six feet long and extends all the way from the bottom inboard corner 
of the tank  (i.e., the bottom of the BL51 rib) diagonally up and outboard 
(viewed from the front or rear) to the top inside surface of the tank in the 
vicinity of the fill connection. The fill connection is roughly 8 inches 
inboard of the tip rib. This arrangement should provide readability of the 
level throughout the entire range of normal tank levels. Since the assembly 
penetrates the dry bay between the BL 99.5 and 105.5 ribs, I plan to bond the 
outside tube permanently into the structure with Hysol and BID at each rib 
exactly the same way the aileron pushrod enclosing tube is bonded into the 
wing.

The outside tube has a custom made aluminum flange welded on the inboard end 
that will be potted into the BL 51 rib. By undoing the machine screws that 
are threaded into this flange, the inside tube can be withdrawn out the 
inboard end, through the BL 50 rib and, hopefully, through the main gear door 
opening. This will allow inspection and replacement of the insulating spacers 
that are used to maintain concentricity of the two tubes. These spacers are 
the only serviceable components in the system and are made from round head 
nylon rivets. The rivets are 1/4-inch diameter head with 1/8-inch diameter by 
1/8-inch long shank. They are available from several sources such as MSC 
(www. mscdirect.com part number 67357129) for about $1.75 for a package of 
100. Each spacer is made up of 3 rivets. Each rivet is set in a #30 hole 
drilled radially into the inside tube at a spacing of 120 deg. The spacers 
(i.e., a set of 3 rivets) are located on 10-inch centers axially along the 
inside tube. Note that the rivets are not actually riveted in place. The 
heads are merely captured between the inside and outside tubes. The top of 
the rivet head must be shaved slightly to fit between the tubes. I made a 
simple jig to accomplish this quickly and accurately but it could probably be 
done by hand. The rivets seem to hold the inside tube securely while allowing 
fuel and/or air to flow freely in an axial direction in the annular space 
between the tubes. Note that the outboard ends of the tubes are open to the 
tank and the inboard end of the outside tube has a hole cut in the bottom to 
allow fuel to flow in and out of the annular space between the tubes. This 
same hole should allow any moisture to escape as well.

On the inboard end, there is a spacer made of Delrin which was fabricated to 
maintain concentricity of the two tubes while at the same time clamping a 
Buna-N gasket between itself and the aluminum flange. The Delrin spacer is 
bolted to the aluminum flange with six #6 flat head machine screws. These 
screws are threaded into six Helicoiled blind holes in the aluminum flange. A 
3 inch long, #10 socket-head machine screw passes through the center of the 
Delrin spacer and into a threaded (Helicoiled) aluminum plug which is welded 
into the end of the inside tube. The machine screw holds the assembly 
together and provides an electrical connection to the inside tube.

The raw materials for this system are quite cheap, roughly on a par with 
automotive float gages but having the advantage of no moving parts inside the 
tank and being able to read the entire range of fuel in the tank. The 
question is, how much of the fabrication can you do yourself or get done on 
the cheap? Turning the flange for the inboard end of the outside tube and the 
plug for the inside tube was not a problem for me. Welding them in place is 
best left to someone who knows what they are doing and has a TIG machine but 
this is not very expensive. You can tell them its for use with gasoline just 
don't tell them its for an airplane!

I had the inside tube anodized to preclude a spark between the tubes should 
they ever come in contact with one another. Getting something 6 feet long 
anodized is problematic at best and, for me, rather expensive. If you can do 
this yourself or otherwise get it done cheaply, it would substantially reduce 
the cost of the project.

For electronics, I plan to use a single integrated circuit called a CAV414. 
This is available in the US from Servoflo, Inc. (www.servoflo.com). Last time 
I checked, the CAV414 was $5 for a 16-pin DIL package or $4 for a surface 
mount unit. This single IC, along with a handful of passive components, will 
convert a variable capacitance in the range of 10 to 2000 pF to a variable 
voltage.  The tube dimensions described above result in an empty capacitance 
of about 400 pF and full capacitance of about 800 pF and I have selected 
components which result in a corresponding output of 0 to 5 Vdc. Although the 
CAV414 will accept supply voltages from 6 to 35 Vdc directly from the 
instrument bus without the need for a separate voltage regulator, I plan to 
provide some type of overvoltage protection to keep any voltage spikes from 
traveling down the supply line, blasting through the IC and impressing 
themselves on the capacitor element. That could ruin your whole day<g>. With 
the right components, the CAV414 will do everything that the circuit 
described by Jim Weir in Kitplanes will do except that it does not have a low 
alarm although this can be added downstream relatively easily if required. In 
the testing I have done so far the CAV414 seems to have good linearity over 
the range of about 15% to 150% of the reference capacitance and is highly 
immune to variations in supply voltage. I don't really have facilities to 
test its temperature or vibration tolerance but I doubt these will be issues.

In a previous post, someone mentioned testing capacitance probes with 
gasoline and then described rather extensive fire precautions. You might 
consider using kerosene rather than gasoline at least for bench (outdoor) 
testing. It has a similar dielectric constant  (1.94 for gasoline vs. 2.08 
for kerosene) and is still flammable but it is an order of magnitude safer 
than gasoline.

I'm sorry this is such a long post but I have actually left out a lot of 
detail. If there is any interest in duplicating this design, I will be happy 
to answer any questions that I can. If anyone can see any problems with what 
I have described, I hope they will say so BEFORE I close out my wings. Thanks 
in advance.

Jim McKibbin
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