Magnetostrictive Sensing Ensures Accurate Level Monitoring
- Sunday, 01 February 2009
For process industries, one of the greatest challenges plant operators face is finding a sensor technology that will provide reliable measurement of liquid levels in tanks while requiring minimal attention and maintenance.
Magnetostrictive sensing technology is used across a broad range of industries, including pharmaceutical, food and beverage, chemical and petrochemical, to monitor liquid levels. There has been a large acceptance of magnetostrictive level transmitters by the process industries due to the ease of use, simple design and years of reliable performance.
The general application of a level transmitter can be simplified to a vessel or tank, a level transmitter, and a host. The vessel has a certain amount of liquid to that needs to be monitored to keep the process working. If there is not enough liquid, the process stops and if there is too much liquid, the vessel can overfill and spill. A level transmitter is added to measure the liquid level and transmit the value(s). The values can be displayed locally for the plant operator to view and/or tied into a host system that automatically controls the liquid level based on the transmitted data.
Magnetostrictive level transmitters use the time-based magnetostrictive position sensing principle. The level transmitter comprises five parts: a housing, a rigid pipe or flexible hose, electronics, sensing element, and a float. The housing, rigid pipe or flexible hose, and float create a package around the sensing element and electronics protecting them from the process conditions. Within the sensing element, a sonic strain pulse is induced in a specially designed waveguide by the momentary interaction of two magnetic fields. One field is generated by a permanent magnet sealed inside the float, while the other is generated from an ’interrogation’ current pulse applied along the waveguide from the electronics.
The resulting strain pulse travels at ultrasonic speed along the waveguide and is detected at the head of the sensing element. The magnet’s position is determined by accurately measuring the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse. It provides an absolute position reading that never - needs recalibration after a power loss.
The main feature of using magnetostrictive level transmitters is the accuracy they achieve. Specific magnetostrictive level transmitters can reach accuracy of 1/32nd in. for shorter lengths, and 1/16th inch for longer lengths of 60 feet. While this level of accuracy is achieved by other level technologies, magnetostrictive level transmitters do not need recalibration and can be installed and commissioned using hand tools and a PC with a serial converter. These features allow magnetostrictive level transmitters to have the lowest installed cost, compared to equivalent level technologies such as radar.
One benefit of magnetostrictive level transmitters is that the transmitter does not need to be removed from the tank for repairing or replacing the electronics. A technician can easily access the transmitter housing atop the tank and remove the sensing element and/or electronics. The ability to replace or repair the electronics in the field keeps the tank owner from having to take the tank out of service, which reduces downtime and lowers costs. The only reason to enter the tank is if the pipe or float is damaged.
Another key benefit of using a magnetostrictive level transmitter is the ability to monitor two liquids in the same tank from one transmitter. In some cases, the contents of a tank may contain more than one liquid. Magnetostrictive sensors have the capability to use more than one float to measure the interface of two liquids that may be contained within a tank.