35 Level Conversions
Recommended Reading:
In industry, we monitor the level of a wide variety of things such as liquids and bulk solids in a tank. There are a number of methods and units of measure that can be used to measure level.
Physical Level:
There are different devices that can measure the physical level in tank. Floats, Level Glass, Ultrasonic, Radar, LASER, and many other technologies can report the level in a tank as a physical distance. Common units include: centimeters, inches, feet, and meters.
Pressure-Based Level Measurement:
Pressure-Based Level Measurement is a common way to measure the level of a liquid in a tank. As the level in a tank changes, so does the pressure at the bottom of the tank. Mathematically this can be calculated as the height of the fluid multiplied by the liquid’s density. Special attention must be made regarding the units of measure. If the height is in inches, the density must be in pounds per cubic inch. This would result in a pressure unit of pounds per square inch or PSI.
An alternate formula can be used when using the density of water at 4°C as a reference point. A commonly used unitless number that indicates the density of a liquid is Specific Gravity (SG). Specific Gravity is a fluids density divided by the density of water at 4°C. By using the SG in the pressure formula: Pressure = height * SG, the resulting unit will be inches of water (inH2O), feet of head (H), centimeters of water (cmH2O), or meters of water (mH2O) depending on the which physical unit is used for the height.
Weight Measurement:
Some substances and mixtures are difficult to measure with the previous methods. Substances like grain, flour, and sugar do not empty out of tanks evenly, they often experience funneling. Funneling gives an erroneous physical measurement. A load cell, a device that changes resistance as pressure (weight) changes, is used to indicate how full a tank is. A 10lb bag of sugar will weigh 5lb when it is half full even if it is funneling and the surface is not even.
Lessons in Industrial Instrumentation: Chapter 19: Page 1287 – 1377 (pdf pg. 1312 – 1402)
Lessons in Industrial Instrumentation: Chapter 20: Sections 1 – 4: Page 1392 – 1397 (pdf pg. 1417 – 1422)