Vibration-frequency densitometer

The following is a brief translated version of my Bachelors project.

Densitometers are instruments which are used to measure the density of the liquid they are immersed in. They can be of the following types: mass, hydrostatic, radio-isotopic and vibrational. Their function is to constantly measure density and hence they play a very important role in modern fuel equipment. The following densitometer is a vibration-frequency densitometer which measures the density with the help of the Wiedemann effect and is used on board most commercial aircrafts.

The fuel tanks of an aircraft are usually situated inside the wings, under the fuselage and in the tail assembly. The densitometer(7) is located in the lower section of these tanks. They also contain several ultrasonic probes(2,3,4), which are connected to a micro processing unit(10) along with the densitometer. The micro processing unit uses the readings from the probes and the densitometer to calculate the speed of sound and mass of the fuel. It also converts the resultant density with the help of the analog-digital converter and indicates the density of the fuel on one of the cockpit indicators(11). The accurate measurement of density inside the fuel tank depends upon the amount of water and other particles mixed with the fuel. This brings into question the purity of the fuel. Hence, the better the quality of the aviation fuel, the better the accuracy of the densitometer.

The construction of the vibration-frequency densitometer includes a sensor, which measures the change in density of the fuel and proportionally vibrates. A mechanical resonator, which can either be a single tube or the cylinder, acts as a sensor through which the fuel flows. The tube begins to vibrate as a result of the change in density and gradually reaches a resonant frequency. The whole densitometer is coated with a layer of aluminum. A wound coil along with an attached magnet in the central portion of the densitometer help form the mechanical resonant system and make sure the device remains stable and accurate for longer periods. The resultant resonant frequencies are calculated by the surrounding electronic devices.

The Wiedemann effect – This is a magnetostrictive phenomenon where a magnetic field gets divided into two perpendicular fields: spiral and tangential, which create an electric current within the rod. The potential difference of the rod due to the current is proportional to the spiral moment.

The schematic representation of the entire fuel measuring system includes the sensing element of the densitometer, which is connected to a phase detector. The detector measures the resultant vibrations and its frequencies. A generator acts as a regulating medium between the phase detector and an amplifier. The phase detector compares the signal from the sensor with the feedback signal from the generator, which passes through an integrator. The resulting signal goes through a micro processing unit, as mentioned before and then is shown on the indicator.

The use of different fuels around the world means that their densities will vary. The following two diagrams show the similarities in the signals but the difference in their frequencies. In the first signal diagram, the input fuel is TS-1, used only in Russia, which has a density of 780 grams per cubic centimeter. The second diagram shows the fuel Jet A, used in USA, India and many other countries, which has a density of 840 grams per cubic centimeter.

All in all, the densitometer is a very important part of the fuel system, but the vibration-frequency densitometer has some disadvantages. The surrounding temperature effects the densitometer’s readings and factors like contamination increase the error.

The advantages of such a densitometer are: the vibrational method of measuring the density is much more efficient than other methods, this type of densitometer reduces the error due to viscosity and it’s static error is not more than 0.2%