What you need to know about pressure, flow and level engineering measurement onboard vessel

Either directly, through the piezoelectric effect, or indirectly, through the measurement of elongation, forces and pressures can be determined. Since elongation measurement is only possible for alternating quantities, it is only utilized for the monitoring of vibrations. Hook’s Law of material elongation and an equation regarding the dependence of the resistance of a straight conductor on its length and cross sectional area form the basis for the measurement of force by means of elongation. Both of these laws were developed by Hook.

A bridge circuit is used to convert the change in resistance into a change in voltage. In this circuit, four resistors are placed spatially very close to one another. Any change in temperature has the same effect on all four resistors, and as a result, the measured voltage is unaffected by the temperature change. All of these resistors can be thought of as strain gauges, and strain gauges come in a variety of forms that are suited to specific uses.

If you want to know and learn more about strain gauges, follow this link.

The most common applications for discrete strain gauges in this form are strength tests, such as measuring torque at the tail shaft.

Strain gauges are incorporated into membranes to form pressure sensors, which are used to measure, for example the pressure of water. When there is a difference in pressure on either side of the membrane, the membrane will bend. The strain gauges will then take this bending and convert it into an electrical voltage. In principle, pressure sensors are sensors for differential pressure. In order to get an accurate reading of the absolute pressure, the space on one side of the membrane in the appurtenant sensors needs to be evacuated.

Piezo-resistive materials, also known as semiconductors, are frequently used in pressure sensors these days. In the past, strain gauges were made of metal. In the case of piezo-resistive materials, the ratio is significantly higher than it is in the case of metals, and these materials are also less expensive to manufacture. In an alternative design, a deflection of the membrane causes a change in the electrical capacitance of a measuring cell, which is then recorded by a high-frequency current. This design is used to record the membrane’s deflection.

In every type, the pressure difference is first converted into elongation, and the linearity between pressure and elongation is an important factor in determining the level of measuring accuracy that can be attained. Overstress, for example due pressure surges can result in permanent deformation or even mechanical damage to membrane. Depending on the severity of the damage, this can result in the complete failure of the system or, what is frequently even more problematic, a significant measurement error because the problem goes unnoticed.

It’s possible that there is no other measuring function on board a ship for which so many distinct measuring principles are utilized as there are for the measurement of tank content.

In the case of direct measurement of filling level on sensors using a float, the float is outfitted with a permanent magnet that communicates its location, and by extension, the position of the liquid surface, to a sensor that is housed within a guide pipe. In its most basic form, this component consists of a row of magnetic contacts (reed relays) that are switched on and off by a magnet. The readings are analyzed by an electronic circuit that is situated outside of the tank; however, in addition to this, there are other sensors employed to determine the magnetic field. The level of the fluid in the tank is directly correlated to the signal that is sent out by the sensor.

Other processes determine the height of the liquid’s surface in relation to the height of the tank. This application makes use of both radar and ultrasonic technology. In the case of ultrasonic sensors, it is necessary to measure the average temperature of the air that is located above the fluid in order to account for the fact that the speed of sound varies with temperature. Because of the variable nature of the gas composition that exists above the liquid in a fuel tank, the real sound velocity can only be estimated on a global scale. Because radar equipment is so costly, it is reserved solely for usage in a limited range of specialized contexts (e.g. storage tanks on liquid tankers).

Floats are appropriate for use in liquids that are free of debris and in situations where there are no significant mechanical effects. When thinking about ballast water tanks or HFO tanks, it is impossible to rule out the possibility of a foreign object or constituents with low solubility obstructing a float and causing it to become stuck.

Equipment operators are required to utilize an indirect technique of measurement whenever there is not a straightforward direct method available for measuring a liquid, gas, or solid within the system. The indirect technique of level measuring is transferring the readouts and data of a given quantity to the volume. One example of this would be converting the pressure ratio. Because every known object possesses some degree of weight and, as a result, exerts a force that is measurable over a particular region within the apparatus, this force can be quantified in terms of pounds per square inch (psi) of kilograms per square centimeters (kg/cm²).

After determining the amount of pressure that is exerted at a particular location per square centimeter, the height of the substance in reference to that measuring point can then be measured as well. However, it is essential for engineers to be aware that indirect level measurement is sensitive to both the specific gravity of the substance being measured and the temperature of the substance being measured. Because these factors have the potential to influence the precision of the calculations and measurements, it is critical that any effort involving indirect level measurement take them into consideration. To give one example, the specific gravity of kerosene is not the same as that of water.

If you want to read and learn more about indirect methods of measurements, please follow this link.

In the case of flow measurements, a variety of different physical effects are employed in order to estimate the flow rate of a fluid or gas.

    • Ultrasonic – the velocity of sound in a flowing medium is, relative to the ambient, the sum of the velocity of sound in the stationary medium and the flow velocity.

  • Volume displacement – The measuring instrument has chambers with specific volumes. The number of chambers filled per time is counted. The chambers have different forms depending on the nature of the medium (impeller, gear, spiral loop, piston)

  • Differential pressure at nozzle or orifice – A small pressure difference that is proportional to the flow rate is generated at the throat of a measuring instrument by the flowing medium.

  • Calorimetric – A heating element is heated to a specific temperature. The reduction in temperature due cooling effect of the flowing medium is a measure of flow velocity.
  • Magnetic induction – A voltage which is proportional to the velocity is induced by a magnet in a moving conductor (flowing conductive medium). Thus, the induced voltage is a measure of a flow velocity.
  • Vortex principle – Eddies from behind a baffle depending on the flow velocity, and the pressure impulses of these eddies are measured. This ia a measure of flow velocity.
  • Impeller wheel – The rotational speed of an impeller wheel in the flow is proportional to flow velocity.

Coriolis force – When there are no outside forces acting on it, an object will move in a straight line (or is at rest). When viewed from any other reference system that moves in a straight line, this path will appear to be a straight line as well. However, when viewed from an accelerated reference system, the identical object appears to have a curved shape. It would appear that a force, specifically the Coriolis force, is having an effect on the moving object. The accelerated reference system would have to apply the exact same force in order to coerce the moving object into travelling in what appears to be a straight line. The object’s movement will be altered as a result of being subjected to the same force. This phenomenon is utilized in flow measurement in such a way that a pipe through which the medium passes is excited to vibrate, i.e. it is periodically accelerated. In this way, the flow can be measured. Through the action of the Coriolis force, these vibrations are affected by the moving medium. This flowmeter’s measurement principle allows for the mass flow to be determined independently of the other physical parameters of the medium being measured.

This type of flowmeter is more and more used, nowadays, as bunker flowmeters as they are very accurate and not influenced by the air pumped into the fuel (cappuccino effect).

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Source and Bibliography:

  • Compedium Marine Engineering
  • Youtube video training credit – RealPars; KRAL;  Endress+Hauser
  • Photo credit: Electrical 4U

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