Like a diaphragm, bellows are also used for pressure measurement, and can be made of cascaded capsules. The basic way of manufacturing bellows is by fastening together many individual diaphragms. The bellows element, basically, is a one piece expansible, collapsible and axially flexible member. It has many convolutions or fold. It can be manufactured form a single piece of thin metal. For industrial purposes, the commonly used bellow elements are:
- By turning from a solid stock of metal
- By soldering or welding stamped annular rings
- Rolling a tube
- By hydraulically forming a drawn tubing
The action of bending and tension operates the elastic members. For proper working, the tension should be least. The design ideas given for a diaphragm is applied to bowels as well. The manufacturer describes the bellows with two characters – maximum stroke and maximum allowable pressure. The force obtained can be increased by increasing the diameter. The stroke length can be increased by increasing the folds or convolutions.
For selecting a specific material for an elastic member like bellows, the parameters to be checked are:
- Range of pressure
- Fatigue on dynamic operation
- Fabrication ease
- Sensitivity to fluctuating pressures
Out of these hysteresis and sensitivity to fluctuating pressures are the most important ones. Hysteresis can be minimized by following a proper manufacturing technique. For instance, a diaphragm when machined from a solid stock shows less hysteresis compared to the one produced by stamping. The same technique could be adopted for bellows as well. In the latter case, the dynamic nature of the variable is likely to induce resonance quickly depending on the natural frequency of the system. The natural frequency is calculable from the dimensions of the system and the gauge.
For strong bellows, the carbon steel is selected as the main element. But the material gets easily corroded and is difficult to machine. For better hysteresis properties you can use trumpet bass, phosphor bronze, or silicon bronze. Better dynamic performance can be achieved by using beryllium copper. Stainless steel is corrosion resistive, but does not have good elastic properties. For easy fabrication soft materials are sought after.
All bellow elements are used with separate calibrating springs. The springs can be aligned in two ways – in compression or in expansion when in use. Both these types, with internal compression springs or external tension springs, are commercially known as receiver elements and are used universally in pneumatic control loops. The figures below show the compressed and expanded type. Spring opposed bellows are also shown below. The open side of a bellows element is usually rigidly held to the instrument casing and because of the rigid fixing, the effective or active length of the bellows element is smaller than its actual length. This device is used in cases where the control pressure range is between 0.2 to 1 kg/cm2.
Because of the device’s dynamic operation, the life of a bellow is an important consideration. Nomograms are available with the manufacturers, wherefrom the life in circles can be read directly knowing the per cent maximum pressure and per cent maximum stroke.
In terms of choice of elastic material for the sensors, the corrosive medium requires special precaution. Besides this, there are other factors showing that the medium should not come in direct contact with the measuring element. They are shown below:
- The direct impact of static head on the measuring element may cause error in response.
- Direct touch of the medium may cause corrosion, high viscosity fluids may cause response error and entrailed materials in the medium may clog in the element.
- In some critical processes in food processing and pharmaceutical industries, cleaning of the measuring system is necessitated.
- Removal of the measuring element for servicing should be convenient.
All these factors suggest that a type of seal should be placed between the process fluid and the measuring element. The best example is the diaphragm seal. It consists of a flexible diaphragm made of corrosion resistance material and sealed within a chamber, that can connect the process on one side and the measuring element on the other.
The effective area of an elastic element like diaphragm or bellows element is generally less than the geometrical area. For finding out the effective area, a known load change is made externally o the centre of the element and the corresponding deflection noted. The differential pressure is then found out for the same deflection.