Inclined Tube Manometer

• For accurate measurement of small pressure differences by an ordinary u-tube manometer, it is essential that the ratio r_m/r_w should be close to unity. This is not possible if the working fluid is a gas; also having a manometric liquid of density very close to that of the working liquid and giving at the same time a well defined meniscus at the interface is not always possible. For this purpose, an inclined tube manometer is used.
  
  
• If the transparent tube of a manometer, instead of being vertical, is set at an angle θ to the horizontal (Fig. 4.7), then a pressure difference corresponding to a vertical difference of levels x gives a movement of the meniscus s = x/sinq along the slope.

Fig 4.7   An Inclined Tube Manometer

• If θ is small, a considerable mangnification of the movement of the meniscus may be achieved.
  
  

• Angles less than 5⁰ are not usually satisfactory, because it becomes difficult to determine the exact position of the meniscus.
  
  

• One limb is usually made very much greater in cross-section than the other. When a pressure difference is applied across the manometer, the movement of the liquid surface in the wider limb is practically negligible compared to that occurring in the narrower limb. If the level of the surface in the wider limb is assumed constant, the displacement of the meniscus in the narrower limb needs only to be measured, and therefore only this limb is required to be transparent.