Tolerating pressure

Citation

(2001), "Tolerating pressure", Sensor Review, Vol. 21 No. 3. https://doi.org/10.1108/sr.2001.08721caf.011

Tolerating pressure

Tolerating pressure

Keyword: Pressure sensors

The Transicoil division of Horizon Aerospace LLC of Norristown Pennsylvania, improved the reliability of its pressure transducers by replacing diaphragm type pressure sensors with bellows sensors.

Transicoil pressure transducers convert mechanical energy into an electrical signal. When pressure is applied, the element's motion is transmitted to the ferro-magnetic core of a linear variable differential transformer, or LVDT. The electrical signal from the LVDT is processed by internal electronics to show the pressure condition. The diaphragm pressure sensors used in the company's transducers were subject to permanent deformation in overpressure condition, requiring replacement and resulting in expensive downtime.

To solve this problem, the company decided to replace its diaphragm sensors with bellows sensors, supplied by Servometer Corp. of Cedar Grove, New Jersey. The tub-like bellows sensor resists mechanical deflection much better than the thin, flat diaphragm sensor. Bellow sensors offer better resistance to deformation in overpressure, according to Servometer Corp. The bellows component is manufactured in an electro depostion process that provides tight control over wall thickness, high chemical purity, and good retention of mechanical properties.

The conventional mechanical sensing element is called a capsule assembly. It consists of hemispherical diaphragm elements welded together to form a hollow chamber. The usual material is NiSpan-C, a nickel-based alloy. Pressure peaks beyond the 1.5 times rated margin will cause a dent or bubble in the spherical body of the capsule assembly. Once deformed, it is out of calibration and the transducer needs to be replaced. Replacing a damaged conventional transducer usually means shutting down a process to remove the broken transducer and install a new one. This downtime can be expensive. Also, because the pressure sensor has failed, there is no easy way to trace the cause of overpressure. The new transducer may fail again.

Transicoil worked with Servometer to come up with a solution involving precision metal bellows designed for high-pressure applications. Transicoil tested its standard industrial pressure transducers equipped with bellows sensors to withstand overpressures of about 35 times the rated measurement range, far higher than the 1.5 times provided by the capsule assemblies.

Transicoil also designed the bellows component into its wide zero/span D/P pressure transducer. In this product, the range of pressure measurement can be adjusted to move the zero value to different parts of the rated range, allowing the transducer to be placed in a more convenient site. Alternatively, the calibrated range or span can be compressed from 100 percent to 10 percent of the rated range, providing higher overpressure capability at a lower pressure range. The precision metal bellows' high overpressure capability provides similar protection with this product.

Servometer manufactures the bellows component by forming a mandrel to the shape of its inside surface, and then depositing a defined thickness of spring-quality nickel onto the mandrel. The plated material is trimmed to define the ends; then the mandrel is removed by dissolving it. This leaves behind a thin, convoluted tubular shell of plated metal, which is the bellow.

Wall thickness of the bellows components fabricated for Transicoil ranges from 0.0015 to 0.006in.