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 (photo: Cameron Forecourt)
23.04.2018, 12:45

A Brief Primer on Tank Gauging

White Papers & Briefings, Transport & Logistics, Transport, EMEA
Cameron Forecourt managing director Barry Jenner explains why tank gauging has achieved almost 100 per cent penetration of the UK's retail fuel station market, and is just as essential for operators of commercial fuelling environments.


In the United Kingdom, the drivers for introducing tank gauging on retail sites have been largely legislative, and a direct consequence of The Petroleum Regulations. Ironically, the very same drivers have hampered uptake of tank gauging solutions in commercial fuelling environments, as they have focused marketing initiatives from gauge manufacturers on the retail market.


Tank Gauging software
© Cameron Forecourt
This short primer is intended to help redress the balance, by explaining the benefits of tank gauging systems in commercial fuel operations.


Tank Gauging Technologies

There are two basic technologies that are regularly used to measure liquid stocks in the fuel industry, although these are not the only means in use.


Hydrostatic Level Measurement

Hydrostatic level measurement has probably been around longer than most other means of tank level measurement, except for dip rods, of course.


Hydrostatic tank gauges essentially compare the pressure exerted by the liquid against atmospheric pressure to determine the level of liquid.


A column of any liquid exerts pressure, for water the pressure is 1bar for every 10 vertical metres in storage, regardless of the surface area. Therefore, a tube filled with water, of only 10mm in diameter but 10 metres tall will receive a pressure at the bottom of the tube of 1bar, similarly a storage tank of 3 metres in diameter but again 10 metres in height will also receive a pressure of 1 bar at the base. All tank gauging systems that employ the hydrostatic system use this basic physical property to calculate the level of a liquid in a tank.


Of course, the explanation above assumes that the product in storage is water and different liquids are either heavier or lighter than water and therefore will exert different pressures. This is where we consider a liquid’s specific gravity to alter the reading of the gauge so that it is corrected for the product in storage. Specific gravity (SG) is effectively a coefficient that determines the weight of each product against water. Water has been given a specific gravity of 1. Fuel products are generally lighter than water and therefore their specific gravity is less than 1, petrol and diesels are often around 0.8. Therefore, for diesel to exert a pressure of 1 bar, our column of diesel would need to be greater than 10 metres, indeed at a SG of 0.8 the column would need to be 12.5 metres.


The pressure exerted by the liquid in storage is then used to calculate the volume of the liquid by calculation. In the case of a dial gauge, the dial face is calibrated specifically to the shape and size of the tank. Electronic Hydrostatic Gauges are extremely configurable and can accommodate many shapes and sizes of tanks.


There are two basic means of measuring liquid levels using the hydrostatic technique as follows:

  • Pressurised Air – This involves feeding pressurised air into a tube that has an opening at the lowest point of the tank. The pressurised air in the tube will only ever reach the pressure exerted by the liquid, thus providing a means of determining the pressure of the column of the liquid. This pressure can then be used to provide a reading using a dial gauge or an electronic pressure transducer.

  • Transducer Capsule – This involves positioning a small electrical pressure transducer at the lowest point of the tank and using it to read the pressure exerted by the liquid.


In both cases, level measurement is achieved by measuring the pressure exerted by the column of liquid.



Float Gauges

The second type of gauging technology provides the physical height of the liquid in storage by measuring the height of the liquid in storage. Float gauges can either be mechanical or electronic:

  • Mechanical Float Gauge – This type of gauge uses a float and a mechanical linkage or a string or cord to show the height of the fuel on a dial face. These gauges are normally calibrated in % full and are not considered to be a reliable means of level measurement.

  • Electronic Float Gauge – This type of gauge; sometimes referred to as a Magnetostrictive Float or Probe Gauge, employs a vertical probe with a float that rises and falls with the liquid height to electronically read the level of the fuel.


This report will not consider mechanical float gauges any further as this type of gauge lacks the ability to configure the reading to suit tanks, these gauges are only useful on small tanks where a guide to level measurement is satisfactory.


Magnetostrictive gauging systems use the fuel height to calculate the volume of liquid using software. These gauges are very flexible and can accommodate many different shapes and sizes of tank.



Other Gauge Types

There are other types of gauge technology that have been employed over the years such as Ultrasonic, where a sound wave is bounced off the surface of the liquid to determine the height.


There are some other gauges that use the float or hydrostatic principals but to a limited degree by only calculating the pressure or height of the liquid in sectors. Sometimes you will see this stated as a 200-point strapping gauge or multi point level measurement. This type of gauge essentially provides a level measurement at X points, thus for a 200-point probe installed in a 20,000 litres capacity tank, the gauge will only provide reading every 100 litres. These gauges are often cheaper than other gauges and are quite suitable for small tanks where accuracy is not essential.


Strengths and Weaknesses

As with everything there are pro’s and con’s to using different models and technologies and the subjective nature of fuel installations means that what works well in one scenario may not be as useful elsewhere. Of course, we can provide specific guidance where required.


Hydrostatic Gauges

It is of course unfair to compare manually operated dial gauges with sophisticated electronic gauges even though both gauges use the hydrostatic principal, so we have separated the two types to consider strengths and weaknesses. It is however appropriate to consider the strengths and weaknesses of the inherent technologies as well as the types of gauge available. Therefore, we have prepared strengths and weakness arguments for the Hydrostatic Technology as well as Dial and Electronic gauges that use the hydrostatic method to measure stock level.


Hydrostatic Level Measurement Strengths

  • A

  • B

  • C


Hydrostatic Level Measurement Weaknesses

  • A

  • B

  • C


Hydrostatic Dial Gauges Strengths

and some might suggest that dial gauges are old hat, the technology might be somewhat historic but the argument for simple manually operated tank gauges remains compelling.

  • This type of gauge does not require an electrical supply

  • These gauges are often cheaper than an electronic unit

  • They are simple and robust and less liable to failure

  • They provide a simple means of cross checking the accuracy of other installed gauges

  • They are easy to read and provide a simple means of obtaining an indication of stock level

  • These gauges do not need any additional protection and so can be fitted in the open air, such as next to a fill point

  • Can read the level of many different products


Hydrostatic Dial Gauges Weaknesses

  • Whilst a properly calibrated dial gauge is accurate, it is difficult to read the gauge accurately, especially on larger tanks

  • The gauge cannot be moved to a different tank without being recalibrated at a factory

  • The calibration of the gauge cannot be tweaked post installation, therefore a tank issue or imperfection that affects the volume of a tank cannot be removed without a factory recalibration 


Hydrostatic Electronic Gauges Strengths

  • A

  • B

  • C


Hydrostatic Electronic Gauges Weaknesses

  • A

  • B

  • C

 

 

Barry Jenner

About Barry Jenner

Barry Jenner Is the Managing Director of Cameron Forecourt (www.cameronforecourt.co.uk), with just under 29 years’ experience in the industry of commercial refuelling. He started as an Airframe Engines Engineer, having completed his apprenticeship at the Royal Aircraft Establishment Farnborough and holds Qualification in Aircraft Engineering. He is also a long standing member and current treasurer for the APEA Southern Branch and a member of the Institute of Directors along being a fellow of the Chartered Management Institute.

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