Solid State System Information

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Capacitance Fuel Quantity Senders and Instruments

This fuel monitoring technology obsoletes primitive float type systems. It lends itself well to most fuel tank situations. The solid state capacitance probe has no moving parts and can be bent or cut to length. Probes are available in 12 inch or 24 inch lengths (longer lengths on special order). The electronics housed in the probe head are epoxy potted to isolate from weather, fuel spills, etc. A pair of potentiometers (variable resistors) housed in the probe head provide for full and empty setting adjustment for each specific installation.

OPERATION: The Fuel Quantity Probe consists of an epoxy potted electronic circuit module, located outside of the fuel tank. This circuit module provides amplification of the minute signals of A.C. current which pass through the attached probe tube that extends into the tank. The probe tube consists of an aluminum tube which contains an inner conductor and insulating bushings. The probe functions as a variable capacitor, whose dielectric properties change with immersion in fuel. This varying capacitance is converted by the electronic circuit module into a varying output voltage. This voltage (0 to 5 Volts) is then fed to the instrument. The instrument is a calibrated voltmeter that displays the voltage as fuel level (quantity).

TEMPERATURE DATA: The probe has a +9% error for a +130 degree F change. This 'error' is the direct opposite of the fuel's dielectric change due to temperature. The result of these self-canceling errors is a system that can be projected to have less than 1% of error with a 130 degree F change in temperature.

INTRINSIC SAFETY: The probe's capacitance changes with the fuel level. The current through this probe is limited by a 1 megohm resistance in series with the probe, which results in a 9 microamp maximum current. If the probe should short, the voltage in the tank circuit becomes zero, and the watts are thus zero. If the probe should partial short, to a resistance of 1 megohm, matching the source resistance, the power would be .01 milliwatts. The A.C. voltage across the probe terminals is 9 volts peak to peak. The A.C. voltage is a square wave. The maximum energy across the probe at any given time is less than 100 microwatts (0.1 milliwatts) worst case. If measured as a one second sample, and considering the fifty percent duty cycle of the square wave, this would equal 0.05 millijoule maximum under normal or failed condition. Since this figure of 0.05 millijoule is less than the 0.2 millijoule figure stated by the F.A.A Advisory Circular as maximum circuit energy of fuel sender probes, this probe meets the criteria of being intrinsically safe.

PROBE SIZES: A probe's needed length is determined by the depth of the tank AND the owner's decision as to the amount of fuel to be kept in reserve. The instrument will read 'empty' when no fuel is touching the end of the probe. Probes are supplied in increments of one foot, so it is necessary to know whether the tank is less than 12 inch deep, 12 inch to 24 inch deep, etc. The installer trims back the probe to the depth determined with a tubing cutter; then, recalibrates the probe, using the 'empty' and 'full' trim pots in the probe head following the included instructions.

PROBE TYPES: If a probe location requires the probe to be bent, a 'bendable' probe must be specified, as unbendable or 'straight' probes are standard. The 'bendable' tube's maximum 'Full' location is about 3 inches below the plastic 'boss' as a result of insulating the inner conductor in that area to preclude shorting that probe during bending. The area is indicated by a mark on the tube and can only be bent in that location. The 'unreadable' bending zone should be taken into account if a location requiring a bendable unit is being considered, as some portion of fuel will be un-measurable.

POWER: The systems require 12 or 24 Volts DC (28 Volt nominal on special order), provided by a BATTERY. Systems will not work reliably wired directly off a rectifier-regulator used to convert AC current from a lighting coil equipped engine. Current draw is less than 20 MA. (0.02 AMP) typical.

INSTRUMENTS: A number of instrument configurations are available 'Single' instruments, 'Duals' (Two instruments in one instrument case), Triples and Quads. Singles and Duals are available in a 2-1/4 inch 'Square style' or 2 inch 'Round style' package. Triples and Quads are available in 3-1/8 inch square style or 3 inch round instruments. If needed, Westach can supply a switch so that a 'Single' instrument may be used with two probes, in order to monitor two (or more) tanks. Below are pages Westach's catalog giving examples of capacitance fuel systems, also shown are conventional float type sender systems for comparison.

MODEL: 395-5S (SERIES) FUEL LEVEL PROBE, SOLID STATE, 0-5 VOLT

SPECIFICATIONS:

ELECTRICAL FUNCTION: This is the sender for compatible fuel quantity instruments. Features 'intrinsically safe', capacitance type circuit. Output is 0-5 Volt DC.

INPUT POWER: 0-5 VOLT DC OUTPUT MODELS: 12-24 VOLTS DC (28 VOLTS MAX) / <30 MA. (ADAPTER NOT REQUIRED)

0-5 VOLT DC OUTPUT MODELS FOR USE ON 28 VOLT POWERED SYSTEMS: 32 VOLTS NOMINAL (28 VOLTS MIN / 36 VOLTS MAX)

LENGTH: 12", 24" AND AS ORDERED / WEIGHT: <40 OZ. (for 24") SPECIFY BENDABLE OR NON-BENDABLE