Level measurement: Ancient chore, modern tools
by Bob Felton
From 'nilometers' to the space age, level measurement is fundamental to progress.
"Egypt," Herodotus remarked more than 2000 years ago, referring to the vast irrigation project that sustains that country's agriculture, "is the gift of the river." Every June, as snowmelt from the Tanzanian Highlands and spring rain from the Congo begin accumulating in the Nile, its elevation begins to rise. It rises gently to a crest in late September or early October, then subsides by late December. Seed goes into the rich, freshly deposited silt as soon as the flood recedes.
Egyptian engineers began capturing the river for irrigation projects about 7,000 years ago. Because the system relies on a complicated system of gates to distribute water across a broad, relatively flat area, it's vital that engineers know the height of the river in advance of its arrival.
The first solution was to simply mark the riverbanks and convey information back to headquarters via runners. Later, engineers developed a large variety of "nilometers," devices used to measure the river height. Most, however, consisted of ordinary graduated scales that projected vertically upward from the riverbed and were read directly. Today, the U.S. Geological Survey and the National Oceanic and Atmospheric Administration use similar devices: graduated poles stuck into the water. Technicians read most of them manually, but there are some in flood-prone areas that transmit information directly to the agency via radio.
Though millennia-old solutions for measuring river level are still in use, there are thousands of level-determination problems in industry that demand much more sophisticated solutions. Like their forebears, contemporary engineers have responded with impressive ingenuity.
Operators can in many cases directly observe the level of liquid in a process tank using a sight glass, a vertical tube hydraulically connected to, but situated outside, the tank. The tube on the front of most commercial coffeepots, for instance, is a sight glass. The operator can tell with just a glance whether it's time to brew up some fresh java.
When circumstances don't permit direct observation, engineers might rely on Archimedes' Principle and use a displacer. This device allows calculation of liquid level as a function of the buoyant force acting on the displacer as liquid level fluctuates. Alternatively, engineers will sometimes use a gauge to measure the pressure at the bottom of the tank; they can then determine the height of the liquid by dividing the unit weight of the liquid into the pressure.
Yet another clever device for indirectly measuring liquid height is the bubbler, a dip tube advanced almost to the bottom of the tank. The pressure required to push a fluid, usually oxygen or nitrogen, out of the bottom of the tube is equal to the pressure in the bottom of the tank. As in the case of direct gauge readings, engineers then make a simple calculation to determine the liquid level.
Engineers don't rely exclusively on mechanical devices to measure liquid levels; there are also some pretty slick electronic tools that get the job done. A capacitor, for instance, consists of two conductors separated by an insulator. By utilizing the wall of the process vessel as one of the conductors and placing an electrically isolated probe inside the vessel to serve as the other conductor, engineers can determine the level of material in the vessel by measuring changes in capacitance between the conductors.
A second device is the resistance tape, a steel probe surrounded by a steel helix originating at the base of the probe. A current is passed through the assembly. When the probe is immersed, fluid pressure forces the helix to contact it, causing an electrical short circuit and a corresponding change in the resistance of the circuit; changes in the resistance of the circuit correlate to changes in the liquid level.
There are times when neither mechanical nor electrical devices really fit the bill; in those cases, engineers might select an ultrasonic sensor. These devices send out a very low-frequency sound, in the 20-kilohertz range, with a transmitter power on the order of a few thousandths of a watt. If engineers know the velocity of the sound wave through the medium between the sensor and the surface, then the distance between the sensor and the surface can be inferred by measuring the time required for a sound wave to travel to a surface and return. Devices of this type have proved effective for determining liquid and solid levels.
There are some cases where the material to be measured must be kept isolated from probes of all types, so engineers can't use conventional mechanical, electrical, and ultrasonic devices. Then, engineers might turn to radiation-based instruments. These devices use a low-level gamma-ray source positioned on one side of a vessel and a radiation sensor on the other side. The sensor output varies according to the intensity of the radiation that reaches it.
Engineers want level information for a reason, of course; it allows them to determine whether the correct quantities of materials are where they belong. If not, they may have to intervene in the process. Often, they use level information to directly control a switch.
Variety of switches
One common switch employs a magnet attached to a float, which rises and falls with the float. As the magnet goes up and down, it triggers the opening and closing of a switch activated by a second magnet. A second type of switch consists of a very slowly rotating paddle. When the material in the vessel rises to the elevation of the paddle, it prevents the paddle from turning, which activates the dependent control.
Ultrasonic switches have proved popular, too, with two popular configurations. The first type of switch vibrates at its resonant frequency in the absence of an encompassing liquid. The liquid damps the vibration, however, when it rises to the level of the sensor; the change in vibration triggers a switch. Generally, these types of switches are effective only with liquids because solids don't cause the necessary damping.
The second type of ultrasonic switch relies on a transmitter and receiver positioned on opposing sides of a process vessel. The switch activates when solids or liquids rise high enough to interrupt communication between the devices. IT