Diaphragm Type
A diaphragm is nothing more than a thin disk of material
that bows outward under the influence of fluid pressure. When the pressure applied to the rear of the diaphragm, causing a small shaft
to twist in response. This twisting motion is transferred to a lever that
pulls on a tiny link chain wrapped around the pointer shaft, causing it to
rotate and move the pointer needle around the gauge scale.
Pressure sensors that depend on the deflection of a
diaphragm have been in use for over a century. In the last few decades, the
elastic hysteresis, friction, and drift effects have been reduced to
approximately ±0.1% of span in the high-quality designs. This has been achieved
mostly using microprocessor technology in smart transmitters.
What the microprocessor contributed in improving the performance
of diaphragm-type sensors was its ability to recall from its memory the
appropriate correction factors for the different values of the diaphragm
deflections.
The diaphragm is mainly used as a pressure-sensing element in most pressure transmitters.
Some instruments use the diaphragm as the pressure sensor; others use it as a component in a capsular element. Mainly there are two types of capsules: the convex and the nested. Evacuated capsules are used for absolute pressure detection, and single diaphragms are used for highly sensitive measurements. The sensitivity of a capsule increases in proportion to its diameter, which in conventional designs. Multiple capsule elements can be built from either convex or nested capsules.
The diaphragm is a flexible disc, either flat or with concentric corrugations, that is made from sheet metal of precise dimensions. The pressure deflection characteristics of both flat and corrugated diaphragms have been well investigated. Diaphragms are mainly constructed from metal, which gives them spring-like qualities.
Diaphragm materials with good elastic qualities, such as beryllium
copper, and with very low-temperature coefficients of elasticity, such as
Ni-Span C, is used. Inconel and stainless steel are used when extreme
operating temperatures or a corrosive process requires them. Quartz diaphragms
are used when minimum hysteresis and drift are desired.
Materials of Construction: Buna-N, nylon, Inconel, Ni-Span
C, phosphor bronze, 316 stainless steel, beryllium copper, Monel, brass,
titanium, tantalum, Hastelloy, nickel, duranickel, Teflon, Kel-F, polytetrafluoroethylene,
CrNi, Ni-Cr-Co alloy.
Differential Pressure Type
In a differential pressure type, the two pressure ports are clear on either side of the gauge.
It should be noted that bellows, diaphragms, and bourdon tubes alike may all be used to measure differential and/or absolute pressure in addition to gauge pressure. One side of each pressure sensing element either applied pressure (in the case of differential measurement) or to a vacuum chamber (in the case of an absolute pressure measurement).
Materials of Construction: Stainless steel, Kel-F, Teflon, Hastelloy, Monel, tantalum, titanium, dura nickel for force balance types.
List of Prominent Manufacturers: ABB, Accutech, Ametek, Anderson Instrument, Barton Instruments, Bristol Babcock, Dresser Instrument, Druck, Dwyer Instruments, Endress + Hauser, Fisher Controls, Flow-Tech, TheFoxboro Co., Furness Controls, Honeywell, Key Instruments, Marsh Instrument, Moore Products, Mid-West Instrument, MKS Instrument, Nuova-Fima, OCI Instruments, Omega Engineering, Parker, Palmer Instruments, Phonetics, Rosemount, Smar International, TSI, UniversalFlow Monitors, Winters, Wika, Yokogawa
For other mechanical pressure sensors please follow the link: Instrumentation Basics: Mechanical Pressure Sensor Basics I (instrumentbasics.blogspot.com)
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