Canongate's VesselCheck system - calculates liquid height by taking the ratio of two measured times.

Accurate levelling of an aircraft fuselage is crucial during the manufacturing process to ensure that the required tolerances are met at later stages of construction.

Historically, the construction platform on which a fuselage is fabricated is checked and adjusted daily using an optical telescope or similar discrete technique. Such measurements are subject to operator dependence which can result in variance or even error in the data.

The method of levelling the platform was considered from two aspects. Firstly the viable techniques were viewed from a practical standpoint, and secondly this acceptable tolerance within which the platform must be levelled was translated to the measurement accuracy required by each proposed instrument.

The criteria that were considered from the standpoint of viability included the ability of the components of the levelling system to be fitted to the platform without interfering in any way with the manufacturing process. The equipment must also be extremely robust, and provide the ability for fast and convenient verification of accurate operation. The levelling system operates on the principle that gravity causes a siphoning effect to take place within the tanks in such a way that, after equalisation has taken place, the liquid surface in each of the interconnected tanks will be at exactly the same height relative to all of the others. By using the liquid surface as the reference (or zero) point, the exact position of the platform can be calculated by measuring the height of the liquid in each tank. General Dynamics considered several level measurement products, choosing the VesselCheck system manufactured by Canongate Technology which measures liquid height using a combination of two high-frequency ultrasound transmitter/receivers which send sound waves through the liquids in the tank.

In this application, the VesselCheck sensors are clamped to the outer surface of the base of the customer designed tanks. The first sensor is used to time the interval for a sonic pulse to travel the known distance to a reference plate which is submerged in the liquid.

The second sensor measures the time taken for a sound pulse to travel the unknown distance to the liquid surface. These time intervals are extremely short and are measured using a high-accuracy timer and digital electronics. VesselCheck calculates the liquid height by taking the ratio of the two measured items.

Additional VesselCheck features have been added that are specific to the levelling application and the ability to check system operation using a calibrate function is also included.