The sensitivity of a hydrostatic manometer can be extended by a factor of 10 using an inclined tube manometer (Figure 1.3) while preserving the simplicity and robustness of the single tube manometer.
Assuming that the change of level in the reservoir is negligible and that the effects of surface tension can be ignored, for an inclined tube manometer:
A – p =PgLsena (1.9)
where L is the length of liquid column above zero, and a is the inclination of the tube with respect to the horizontal. The sensitivity of the manometer increases with decreasing inclination, a; the use of angles below 5° with the horizontal brings problems due to surface tension even if alcohol
is used, because the meniscus becomes too stretched, and also the perfection of the glass tubes to the degree of precision required cannot be guaranteed.
Unfortunately, a manometer of this type must be calibrated because errors can arise due to a slight curvature of the tube or to the effects of surface tension caused by the small non-uniformity of the internal diameter.