Measurement of Small Pressure Differences

Measurement of Small Pressure Differences

Accurate measurement of small differences of air pressure is often
required in experimental aerodynamics. A convenient instrument
is the Chattock gauge (Fig. 4). The rigid glasswork AB forms a
U-tube, and up to the levels L contains water, which also fills the
central tube T. But above L and the open mouth of T the closed
vessel surrounding this tube is filled with castor oil. Excess of air
pressure in A above that in B tends to transfer water from A to B
by bubbling through the castor oil. But this is prevented by tilting
the heavy frame F, carrying the U-tube, about its pivots P by means
of the micrometer screw S, the water-oil meniscus M being observed
for accuracy through a microscope attached to F. Thus the excess
air pressure in A is compensated by raising the water level in B
above that in A, although no fluid passes. The wheel W fixed to S
is graduated, and a pressure difference of O0005 in. of water is easily

FIG. 4. CHATTOCK GAUGE.

detected. By employing wide and accurately made bulbs set close

together, constantly removing slight wear, protecting the liquids

against appreciable temperature changes and plotting the zero

against time to allow for those that remain, the sensitivity

* may be

increased five or ten times. These gauges are usually constructed

for a maximum pressure head of about 1 in. of water. Longer forms

extend this range, but other types are used for considerably greater

heads.

At 15 C. 1 cu. ft. of water weighs 62-37 Ib. Saturation with air

decreases this weight by about 0'05 Ib. The decrease of density

from 10 to 20 C. is 0-15 per cent. A 6 or 7 pet cent, saline solution

is commonly used instead of pure water in Chattock gauges, however,

since the meniscus then remains clean for a longer period.