Switch Tips: Float-level switches

Sept. 2, 2004
Float-type switches are considered the least-expensive way of monitoring the level of fluid in a tank.

They use a buoyant float to gauge a fluid level. In most models, the float carries permanent magnets and encircles a stationary stem. As the float rises or drops with fluid level, the magnetic field that the magnets generate actuates a magnetic reed switch mounted within the stem. The stem is a nonmagnetic metal or engineered plastic. The basic design provides an accuracy of about ±1/8 in.

The float-on-stem configuration mounts in either the top or bottom of a tank. An alternative type is the side-mount float. As its name implies, it installs on the side of the tank and is generally used when there is no access to the tank bottom or top. The float in a side-mount version takes the form of an arm on a hinge. The buoyant part of the arm is at the end. Magnets are in the side of the arm near the hinge. In the usual application mode, a rising fluid level moves the arm nearer the horizontally mounted stem. Once in close proximity, the magnets actuate a reed switch in the stem. Bent stem float-level switches are also designed for installation on the tank side. They consist of an ordinary float switch mounted on a 90° elbow.

All float switches employ reed switches rated at either 20 or 100 VA. Float switches come in a variety of materials, and material selection can be tricky. Media compatibility, durability, and the need to meet standards approval are all factors that can weigh on the choice of float materials. The size of the float itself can be an issue in small tanks. The bigger the float, the more buoyancy it exhibits. But viscous or debris-ridden media may need larger floats to get accurate readings.

Finally, float-level switches can be devised as multistation versions to handle deep tanks. These types position a reed switch at each point being monitored. However, the cost of multistation versions can quickly rise to equal or exceed that of solidstate level monitors.

Gems Sensors provided information for this article, www.gemssensors.com

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