CRAFTSMAN'S CORNER
Ben Owen
CAPACITIVE FUEL SENDERS By MARSHALL G. MACFARLANE EAA 173209 In 1980, SkySports developed a low-cost capacitive fuel system for all types of aircraft, effectively eliminating all moving parts from the fuel tank sender. Capacitance principles had been used for fuel measurement in military aircraft for many years. Unfortunately, the capacitance designs then in use require the installation of custom-designed electrodes in carefully engineered locations within the fuel tank. The systems were accurate, but very expensive and difficult to install. The capacitance probe consists of a measuring and conversion circuit contained on a single circuit board, which is permanently potted in a fuelproof plastic cup, having a dimension of approximately 2-1/2" in diameter by 1" in height. The probe head is manufactured with a standard 5-bolt mounting pattern (see Figure 1). Accordingly, the probe will fit in existing fuel tank sender mounting holes, without modification of the tank.
fuel gauges. Most builders and rebuilders elect to replace the gauge, inasmuch as it is usually as old as the airplane. When planning a capacitance fuel probe installation, it is important to consider the overall volume of the tank, shape of the tank, overall volume, shape and crosssection of the tank, as well as the probe mounting location (Figure 2). A probe can be mounted in many ways, even upside down. Bendable probes are available which will allow accurate readings in tanks of unusual shape and cross-
Figure 2
Probe
Figure 2 - Note that a capacitance sender "reacts" like a float, i.e., the output is proportional to fuel level, not fuel volume.
Figure 3 Potential Probe Locations
O Figure 3 - Probes can be bendable, and can be mounted midway up the side of the tank (1), in the top (2), from the
bottom (3), near the top (4), in a standpipe (5) or in a fuselage tank, such as a "header" (6). Several probe outputs can be added together to read on one gauge.
section (Figure 3). The installation process is relatively straight forward: once the probe location has been selected, trial fit the probe, or make a mock-up from a coat hanger of the Figure 1 - Basic SkySports fuel probe with mounting flange and metal portion of screws. This probe will fit standard SAE sender hole pattern. the probe to determine how it will be oriented inside the tank. In aircraft The working end of the probe consists of a pair of conductors; a brass with inverted fuel systems, be mindful center wire surrounded by an alu- of the necessity of freedom of moveminum tube. The wire and tube are ment of the flop tube. Once the probe spaced apart from one another by fuel- location and dimensions have been proof insulators, and are securely ascertained, a probe may be ordered attached to the probe head mechani- in an approximate length and trimmed cally and electrically. As the level of as needed. For tanks without existing fuel changes in the tank, the amount of mounting bolt holes, we supply an fuel surrounding the two conductors aluminum flange which can be atalso changes, thereby changing the tached, by welding or suitable electrical capacitance of the metallic adhesive, to the exterior of the fuel probe assembly. This change in ca- tank. The probe is then mounted to pacitance is converted to a varying the mounting holes with an appropricurrent output from the probe. The ate gasket and sealant. It is a simple matter to connect the probe is designed to be compatible with many commonly available gauge and the probe to aircraft 82 NOVEMBER 1992
power, and connect the probe signal output to the gauge signal input. Calibration is just as simple. With the tank empty and power applied, the "empty" adjusting screw on the top of the probe is turned until the needle on the gauge indicates "empty." Then the tank is filled and the "full" adjusting screw on the probe is adjusted until the gauge reads"full." Calibration is now complete. There are no moving parts in the probe, and the amount of current in the actual probe circuit is miniscule. Accordingly, the probe is extremely safe, and should never wear out. For more information, including a free installation brochure, contact: SkySports, Hangar 1, Linden-Price Airport, Linden, Ml 48451, 313/7369433, Fax 313/735-1078.
About the Author
Marshall MacFarlane attended the Georgia Institute of Technology, Michigan State University, and in addition to owning and operating SkySports, is a registered patent attorney. Rated ASMEL-I and glider, he flies a Cessna 310, a Waco YMF5, and a Buccaneer ultralight.
