Applications in the field of pressure measurement that are different from the norm and require special measurement solutions can be found in, diving bells or diving chambers, depth measurement, tunnel construction. Pressure locks and pressurised chambers in hospitals.
In diving bells or chambers, for instance, it is of critical importance for the diver to know at all times to what pressure the bell or chamber is being pressurised. A standard pressure measuring instrument, such as a pressure gauge, installed inside the diving bell or chamber is no solution, as it will in this case not differentiate between the pressure inside the system (bourdon tube) and outside the system, ie the pointer will always remain on zero.
The technical problem is thus, to be able to supply a stable reference pressure to the instrument which it can compare to the chamber pressure and indicate it correctly.
The reference pressure is ideally the atmospheric pressure or a very high and stable vacuum. The following are the five different measuring principles and designs of mechanical pressure gauges for the measurement of pressures inside pressurised chambers:
Design I
In this design, chamber pressure works on the inside of the bourdon tube. The hermetically sealed casing is vented to atmosphere via a tube and thus provides the reference pressure inside the casing. Advantages are easy calibration, no measuring errors in case of small leaks and standard pointer movement (ie clockwise). The disadvantages are an expensive case construction, a pressure rating that is limited by the glass and there is the need for a ventilation tube.
Design II
Chamber pressure works on the inside of the bourdon tube. Atmospheric pressure is encapsulated in a hermetically sealed casing and becomes the reference pressure. The advantages are easy calibration, standard pointer movement and no ventilation tube is required. The disadvantages are expensive case construction, the pressure rating is limited by the glass and any leaks in the casing will lead to reading errors.
Design III
Here the chamber pressure works on the outside of the bourdon tube, and the inside of the tube is vented to atmosphere via a tube, becoming the reference pressure. Advantages are a standard design of gauge using mostly standard parts. The disadvantages are expensive calibration procedure requiring a pressure chamber, anti-clockwise pointer movement and the requirement of a ventilation tube required.
Design IV
This time the chamber pressure works on the outside of the bourdon tube, with atmospheric pressure that has been sealed in the bourdon tube, and this is now the reference pressure. The only advantage is a standard design of gauge using mostly standard parts, while the disadvantages are expensive calibration procedure requiring a pressure chamber, anti-clockwise pointer movement. Added to that, any temperature changes will lead to volume changes of the encapsulated air, producing reading errors. Any leaks in the bourdon tube system will also lead to errors.
Design V
With the chamber pressure working on the outside of the bourdon tube, the bourdon tube has been evacuated and hermetically sealed, the vacuum is now the reference pressure. The advantage is a standard design of gauge using mostly standard parts. The disadvantages include expensive calibration procedure requiring a pressure chamber, anti-clockwise pointer movement and any leaks in the bourdon tube system will again lead to errors.
Recommendations
Use Design III if the installation of a ventilation pipe is technically feasible. If this installation is not feasible, Design V is the next best option.
All five designs are available locally from Wika Instruments South Africa.
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