How to Select Level Gauges
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Level gauges are industrial meters used to determine the level of a liquid in a fixed storage or process tank.
The gauge is composed of several parts including head, float, measuring tape, bottom anchored bracket, guide wires, elbows, anchors, coupling, pipe support brackets, and pipework. Liquid level gauge calibration is used to ensure proper readings are obtained when using level gauges.
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They have several advantages including durability for harsh environments, including the ability to withstand high pressure, temperature, and vibrations. They are easy to read from a distance and can be used in addition to a switch or alarm in a system. There is little maintenance required and all repairs can be done without disturbing the system. They can also be customized for extreme applications.
There are several basic types of level gauges. Examples include:
- Tank level gauges- A tank level gauge is used in tank volume monitoring.
- Fuel level gauges- A fuel level gauge provides a direct fuel level reading for vehicles and equipment. The two main parts of a fuel gauge are the sender and the gauge. The sender measures the fuel level in the tank and the gauge displays the level to the user. The sending unit measures the fuel once the float has dropped below a certain level. When the tank is full, the float is near the top and the wiper rests close to the grounded side. This means that the resistance is small and a relatively large amount of current passes through the sending unit back to the fuel gauge. As the gas level drops, the float also begins to sink and the wiper moves to increase the resistance and cause the amount of current sent back to the gauge to decrease. To watch an animation of this level system, click the image below.
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- Water level gauges- A water level gauge is used for measuring the rise and fall of water levels.
Mechanical level gauges- A mechanical level gauge monitors levels of underground storage tanks.
Level gauges take level readings using several different techniques. The measurement technology should be chosen based on the user's application. These level sensors also have different display options based on user preference.
- Magnetic level gauges- A magnetic level gauge is used to control the level of fluids. The gauges operate under Archimedes Principle, which states that any object submerged in a fluid experiences the same buoyant force as the weight of the liquid displaced. Magnetic level gauges are extremely durable and can be used in applications that would break or destroy glass gauges. They can be used underground with a maximum working temperature and pressure range of 4000C and 220 bar. These gauges are capable of inter-phase level detection, even in highly toxic or corrosive mediums.
- Reflex level gauges- A reflex level gauge is used to detect level in a vessel or container. It operates by using the dissimilarity in the index of refraction between fluids and vapors. They are usually made of carbon or stainless steel, which provides the durability needed to work in a temperature and pressure range up to 400°C and 400 bar respectively. If the vessel is filled with vapor, the observer will see a silvery white color because the light radiations running into the grooved surface of the sight glass will be completely reflected. If the vessel is filled with liquid, the observer will see black because the light radiations coming across the liquid region will be completely absorbed. The sight glass is a transparent tube which is clamped to the gauge body.
- Transparent level gauge- Transparent level gauges are very similar to reflex level gauges. They consist of two transparent glasses which are integrated with the liquid cavities on both sides. Transparent level gauges work by detecting the dissimilarity in the transparent characteristics of two glasses which are integrated with the liquid cavities on both sides. In an application which uses water and steam, an illuminator is employed to make the liquid level easier to see. The light rays emitted by the illuminator are directed in an upward direction towards the water column causing the surface of the liquid to become illuminated. The light radiations generally fall upon the divided surface between water and steam and then reflect back to the eye of the user.
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- Bi-color level gauge- Bi-color level gauges are used to measure the media level in a boiler. The gauge uses high quality mica sheets to guard against the wet steam generated in a boiler drum. The level is detected by distinguishing the index of refraction between the steam and water. Bi-color level gauges are a transparent gauge that has a liquid chamber in a wedge shaped section. The rear side of the device has an illuminator which is fitted with two color filters; one red and the other green. When red color light radiations hit the water, they get averted to one side and absorbed. When the same light hits the steam, the light passes through and shows up as a red color. The reverse happens for light passing through green filters. This allows users to tell how much of each media is in the system.
- Tubular level gauge- Tubular level gauges are best used in low pressure, non-toxic liquid level measuring applications. They detect and report process level though direct communications.
The measuring range, also known as column height, is probably the most important specification to examine when choosing a level gauge. It is defined as the level range that devices can measure. Measuring range is determined by the maximum and minimum media levels. The user should also account for anticipated media level fluctuations. If the proper measuring range is selected, less data will be lost during an event which exceeds the measuring range.
Devices with field adjustable measurement ranges allow for the user to adjust the measuring range depending on the media container and the environment conditions.
- Maximum operating pressure- The maximum operating pressure of the material for which the device is rated.
Material temperature range- The maximum material temperature for which the device is rated.
Liquid specific gravity- Maximum liquid-specific gravity for which the device is rated. Specific gravity is the ratio of density of the material to the density of water at a specified temperature. It can be calculated using the following equation.
SG = ρ / ρH2O
SG = specific gravity
ρ = density of fluid or substance (kg/m3)
ρH2O = density of water (kg/m3)
Material density- The maximum material density for which the device is rated.
Media is the material that the sensor needs to measure. Level gauges can be responsible for measuring a wide variety of media in fluid level systems in many industries. The two general media types include liquid and dry materials.
- Liquid media can include
- Water (hot or cold, clean or dirty, fresh or salt)
- Gasoline (diesel fuel)
- Hydraulic fluid
- Highly viscous or gummy fluids
- Dry materials can include
- Bulk solids
Depending on the needs of the application, level gauges can be mounted a few different ways. These switches can be mounted on the top, bottom or side of the container holding the substance to be measured. Some factors to consider when selecting a mounting option are the package size of switch or sensor, and the space available in the application. The gauge should be rugged to handle the environment of the system.
Some features that can make level gauges more desirable are being programmable, having controller, recorder or totalizer functions and a built-in alarm indicator, whether audible or visible.
- Rated for Sanitary Applications- Devices are rated for sanitary applications such as those found in the pharmaceutical and food industries.
- Handles Slurries /Suspended Solids- Devices can handle slurries or suspended solids.
- Built-in Alarm / Indicator- Devices have a built-in audible or visual alarm that is more than a switch or relay closure.
- Recorder /Totalizer Functions- Devices are data loggers which record/totalize and integrate measured values.
Level monitoring is important in a wide variety of applications. Any industrial process involving the filling or storing a liquid in a tank or vessel would benefit from the use of a level switch.
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They are also an important part of process control systems which manage flow rate into and out of a storage vat or reactor. They are found in a wide variety of industries-- not limited to-- manufacturing, food and beverage, chemical and pharmaceutical, marine, medical, and fuel/energy management.
Nice, Karim. "How Fuel Gauges Work" 04 April 2001. HowStuffWorks.com.