Throughout this article we assume the density of the vapor in the headspace (typically air) to be negligible compared with that of the process fluid. We will assume also that there is only one, uniform, process fluid in the tank. Some of these technologies can be used for multilevel applications where two or more immiscible fluids share a vessel.
1. Glass Level Gauge. Available in a variety of designs, both armored and unprotected, glass gauges have been used for over 200 years as a simple method to measure liquid level gauge. The benefit of this design is the ability to the see the true level gauge through the clear glass. The down side is the possibility of glass breakage resulting in spills or safety to personnel.
2. Floats. Floats work on simple principle of placing a buoyant object with a specific gravity intermediate between those of the process fluid and the headspace vapor into the tank, then attaching a mechanical device to read out its position. The float sinks to the bottom of the headspace vapor and floats on top of the process fluid. While the float itself is a basic solution to the problem of locating a liquid’s surface, reading a floats position (i.e., making an actual level measurement) is still problematic. Early float systems used mechanical components such as cables, tapes, pulleys and gears to communicate level gauge. Magnet-equipped floats are popular today.
Early float level gauge transmitters provided a simulated analog or discrete level gauge measurement using a network of resistors and multiple reed switches, meaning that the transmitter’s output changes in discrete steps. Unlike continuous level-measuring devices, they cannot discriminate level gauge values between steps.
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