The main types of sensing elements are Bourdon tubes, diaphragms, capsules, and bellows. The Bourdon tube is a sealed tube that deflects in response to applied pressure.

Introduction:
In pressure measurement, various types of pressure sensing elements are used to convert the applied pressure into a measurable signal. These elements are designed to deform under pressure and produce a corresponding output that can be measured and interpreted.

Bellows:
A bellows is a type of pressure sensing element that consists of a flexible, accordion-like structure made of metal or elastomer. When pressure is applied, the bellows expand or contract, causing a linear displacement that can be measured. Bellows are commonly used in applications where a high degree of sensitivity and accuracy is required, such as in pressure gauges and pressure switches.

Bourdon tube:
A Bourdon tube is another type of pressure sensing element that is commonly used in pressure gauges. It is a curved, hollow metal tube that is closed at one end and connected to the pressure source at the other end. When pressure is applied, the tube straightens or bends, causing a corresponding movement of an attached pointer or dial. This movement can be calibrated and used to indicate the applied pressure.

Manometer:
A manometer is not a type of pressure sensing element. Rather, it is a device used to measure the pressure of a fluid or gas by comparing it to a known reference pressure. It consists of a U-shaped tube filled with a liquid, such as mercury or water. The pressure being measured is applied to one end of the tube, causing the liquid to rise or fall in the other end. The height difference between the two sides of the tube is proportional to the pressure being measured.

Orifice plate:
An orifice plate is another type of pressure sensing element. It is a thin plate with a small hole, called an orifice, in the center. When fluid or gas flows through the orifice, the pressure on one side of the plate is higher than the pressure on the other side. This pressure difference can be measured and used to determine the flow rate of the fluid or gas.

Conclusion:
Among the given options, the orifice plate is the only one that is not a type of pressure sensing element. It is a device used for flow measurement based on the pressure difference across the orifice. The other options, such as bellows, Bourdon tube, and manometer, are all types of pressure sensing elements that are used to directly measure the applied pressure.