FMD Fuel Monitors for Aviation Aircraft

FMD Fuel Monitors for
        Ultralight Aviation Aircraft.

•  General
•  Construction & Performance
   •  Linearisation
   •  Display
   •  Sensors
   •  Power Supplies
   •  Safety  
•  Fitting

The FMD Fuel and Oil Monitor range are linearised capacitive depth measuring devices suitable for use as fuel and oil gauges in UK permit to fly, or equivalent, light and ultralight aviation applications. They are light, consume almost no power, are safe and are suitable for most fuel or oil tanks except tanks with no flat area on their top surface. They give outputs indicative of volume rather than merely depth as is typical of their competitors and are highly damped in order to give steady readings even on rough roads. In addition, they are capable of giving a warning of the presence of water accumulating in the bottom of the tank if the sensor tip becomes immersed. The single tank Oil Monitor variant also gives a flashing display warning when the oil volume drops below 12.5 percent of capacity.

The FMD Monitor employs a single chip microprocessor to control fluid measurements and to linearise and make outputs to the display device.

The FMD can be supplied in ready to fit and fly kits and is available in single tank and twin tank forms. This product can also be calibrated by the user and general purpose uncalibrated kits can be supplied for any aircraft suitable for the sensor mounting systems available. Sensors are supplied in half meter and one meter nominal lengths and are suitable for cutting to size with a hacksaw. Cable lengths must be specified at time of ordering and are supplied in lengths which are a multiple of one meter. Dedicated 12 volt versions are fitted with a backlight.

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Construction & Performance:
•  Linearisation:   The FMD range of fuel and oil monitors is linearised. This means that they compensate for fuel tanks of uneven cross-section and give an output that indicates fuel volume. A fuel gauge that is not linearised will give an output of fuel depth only. Linearisation is achieved by storage within the instrument of a table of fluid depths and the volume these represent. This table is referred to each time a depth measurement is made and the relevant volume is output.

Each depth value is separated from the next by a quantity of fluid termed an INCREMENT each of which can be equivalent to one or more litres of fuel or decilitres of oil. There can be up to fifty increments per tank for single tank models and the maximum usable fuel capacity can be up to 255 litres (25.5 litres for oil).

For twin tank fuel monitors there can be up to fifty increments shared between the two tanks with a maximum of thirty increments for any one tank. The maximum volume of usable fuel allowed per tank is 255 litres. Thus one can have, say, a thirty litre fuel tank in 1 litre increments and a two hundred and fifty five litre fuel tank in seventeen increments each of fifteen litres, or in fact any other combination providing only that the above maxima are not exceeded.

The Fuel/Oil Monitor variant of the twin tank unit has one readout for oil and the other for fuel. Maximum oil volume is 25.5 litres with each increment being a multiple of 0.1 litres. The fuel readout has a maximum of 255 litres and the available number of fluid increments is shared between the two readouts in a similar manner to the twin tank fuel monitor described above.

The number of increments and the number of litres per increment are selected during calibration. Best accuracy is subject to +/- 1 increment of uncertainty.

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•  Display:   The FMD Fuel Monitor gives an output to a solid state liquid crystal alphanumeric display. For single tank models the top line of the display gives a digital readout in litres while the bottom line shows a horizontal scale made up of vertical bars each equivalent to 1 increment of fuel. The digital readout resolution is 1 increment of fuel.

For twin tank units there is a horizontal scale and a digital readout on each line of the display. The top line is the volume in tank number 1 while the bottom line is for tank number 2. In twin tank Fuel/Oil Monitors tank 1 is reserved for oil while tank 2 is fuel.

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•  Sensors:   The sensors used are available as 'Do It Yourself' sensor which can be cut to length by the fitter for one-off applications.

Sensors may have the electrical connecting lead moulded permanently into them or have a connector on top of the electronics unit which is moulded onto one end of the sensor stem.

Twin tank systems use different types of connector for tank number 1 and tank number 2 sensors in order to prevent accidental swapping over of the leads. The 'Do It Yourself' sensors are available as tank 1 or tank 2 types each with the appropriate connector. The leads supplied for the sensors also use different types of connector at the Processor Module end to prevent accidental swapping over.

N.B. The sensors used for the FMD range are not exchangeable without re-calibrating the Processor Module. Simply swapping one sensor for another is likely to lead to gross error on the display device. Calibration is a process of tailoring a processor module to sensor tolerances as well as to fuel tank characteristics.

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•  Power Supplies:   FMD instruments are available for a nominal 12 volt DC supply only. The allowable range of supply voltage is from 9-18 volts. Below or above these limits the unit is likely to switch itself off. It will switch on again when the voltage comes within the above limits.

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•  Safety:   Safety in both fuel and oil monitoring systems is assured by the following features :

i. The incorporation of a device known as a Zener Barrier in the Processor Module. This makes it impossible under all circumstances for sufficient energy to be passed by it to create a spark hot enough to ignite fuel.

ii. Limitation of energy storing capability within the sensor to a safe value so that insufficient energy can be stored which could result in a spark hot enough to ignite fuel.

iii. Encapsulation of the sensor electronics thus preventing mechanically induced faults and also containing the consequences of any faults that do occur.

iv. Earthing of the negative supply so that an external fuse will break the power connection if the positive supply is ever accidentally connected to the negative Processor input terminal :-

The zero volts supply to the sensor is connected to the sensor stem. Consequently it is important that the sensor stem must never have a positive voltage on it. This is achieved by earthing to airframe the zero volts supply with the green lead supplied attached to the processor module. Any accidental connection of the positive power input lead to the zero volt input terminal will cause power to flow to airframe earth through the green lead. In order to prevent the voltage rising on the sensor stem and also prevent damage to the instrument printed circuit board tracking in such an event, it is ESSENTIAL that a fuse be fitted to the aircraft electrical supply.

Note that this protection feature can only work if the green earth lead is connected before an attempt is made to connect power. When wiring always connect the green earth lead first.

For the FMD Fuel and Oil Monitors the fuse rating should be 15 amps or less, quick blow, in applications with a nonconductive tank with no internal earthed fittings or 0.25 amps or less, quick blow, in a metal tank or a tank with internal earthed fittings.

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Fitting:   Two types of Sensor Tank Fitting are available and may be suitable for some aircraft: a vertical adapted cable gland and a machined PVC bung allowing insertion of the sensor at approx. 30 degrees to the vertical.

The fittings require only one hole to be cut in the tank. This may be done using a hole cutter, trepanning tool or, for the gland type only, a succession of drills with a reamer to finish. Note that reamers tend to wander and that the hole should be made as near to final size as possible using a series of ever larger drills before employing the reamer. Best of all, use a stepped hole cutter and eliminate wander.

The components that must be fitted inside a rigid fuel tank are inserted using a piece of stiff wire (a wire coat hanger works very well) to place the component under the hole machined in the tank. Once the fitting outside the tank is engaged with the internal component the wire is pulled free. The process is usually a lot easier than it sounds and detailed instructions are provided in the installation leaflet with each kit. Nonetheless, make sure the internal component can be fitted through the filler neck before the hole is cut in the tank.

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-  Accuracy:  unmatched accuracy in tanks of non-uniform cross section.  The only                      linearised fuel and oil gauge available at reasonable cost.
-  Damping:   for single tank instruments, the output is updated every second                      approximately.   The output can change by only 1 increment per                      update.
                     twin tank instruments update every 2 seconds with only 1 increment                      change allowed per update.
                     The damping thus obtained is good in all flying conditions.

-  Safety:       the unit is absolutely safe providing that the electrical system is                      connected taking into account the factors described in the                      Construction & Performance - Safety section of this page.

-  Warnings:  a warning is produced if the sensor detects water in the tank, or a                      disconnection or short circuit.
                     the single tank FMD Oil Monitor gives a warning when the nominal                      oil level drops below 12.5%.

-  Low Power Drain:  
current requirements as low as 10-15ma. Units can be                                   powered by small dry cell battery.

-  Easy Fit:     cables supplied ready made. Fitting does not require special skills                      or special tools.