Intrinsically Safe - Fuel Level

INTRINSIC SAFETY

Intrinsic safety (IS) is a protection technique or concept for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms. As a discipline, it is an application of inherent safety in instrumentation.

Fuel Level Senders are deamed  intrinsically safe if the available electrical and thermal energy in the system is always low enough that ignition of the hazardous atmosphere cannot occur.  CiES can achieve intrinsic safety in many ways 

• One way of achieving this is ensuring that only low voltages and currents enter the hazardous area, and that all electric supply and signal wires are protected by limited current source.   If wiring to the existing fuel sensors are inside the tank - the following can be applied - the CiES sensor consumes no more than 20 mA and the current source caps the maximum delivered to 50 mA - if a transient voltage were to be introduced the current spike is of extremely short duration.

• The other and more important method CiES Inc. has chosen is to keep all electronics out of the fuel tank.  This insures that "NO Electrical Energy"  would be present in the tank at all .  This is achieved by installing the sensor to the side of the fuel tank and the wires run external to the fuel or fuel hazard.

CIES Inc Fuel Level Sensors have 

passed RTCA DO-160

for Explosive Atmosphere 

CIES Inc Fuel Level Sensors are patented and utilize a magnetoresistive sensor and proprietary electronics to determine the position of a float inside the tank.

CiES Sensors are Intrinsically Safe 

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Aviation Float Fuel Sender - Historical Perspective

Dawn of Aircraft Instrumentation

The initial non-electrical float system was used on various aircraft, the most famous being the Piper J-3 "Cub." which used a cork with a wire imbedded in it that extended into the view of the pilot. Lots of wire showing, lots of gas; no wire showing, no gas.  Equally glass sight gauges are used in high wing aircraft and high wing fuel can to flow to the engine by means of gravity.

Electrical Aircraft Instrumentation Comes of Age

With the addition of electrical systems in aircraft the float was connected to the arm of a variable resistor whose electrical leads are brought through the wall of the tank and connected to the fuel quantity gauge and to the ship's electrical bus. 

Thus, the change in resistance as the float follows the level of the fuel.  This electrical value causes the needle on the fuel quantity gauge to deflect indicating the quantity of fuel in the tank.  Simple and direct.

For odd shaped tanks, particularly a flat tank in a wing with dihedral, multiple resistance floats are connected in series to correctly categorize this onger sloped tank.

This is the fuel gauging system on most, if not all, automobiles, the majority of piston engine aircraft, and some turbine aircraft. This system has been given very poor reviews over the years, some of which is deserved, but a large portion of the criticism is not.

If the resistance float is poorly designed and constructed, if the gauge is poorly designed and constructed, if the gauge is poorly marked, if the damping of the complete system is not suitable for aircraft or it's particular use,  or if the system is poorly installed and calibrated,  criticism for poor operation is rightly deserved. 

Digital Display and Interface  

In the instance of fuel level - Nothing really 

Well in the case of Commercial Aircraft the capacitance value was converted to ARINC 429 protocol and transmitted to the cockpit.

Until Now 

The First Digital Output General Aviation Fuel Level Sender