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15:4:53 Page 383
Example 9.2
A high-quality U-tube manometer is a remarkably simple instrument to make. It requires only a
transparent U-shaped tube, manometer fluid, and a scale to measure deflection. While a U-shaped
glass tube of 6 mm or greater internal bore is preferred, a length of 6-mm i.d. (inside diameter) thickwalled clear tubing from the hardware store and bent to a U-shape works fine for many purposes.
Water, alcohol, or mineral oil are all readily available nontoxic manometer fluids with tabulated
properties. A sheet of graph paper or a ruler serves to measure meniscus deflection. There is a limit
to the magnitude of pressure differential that can be measured, although use of a step stool extends
this range. Tack the components to a board and the resulting instrument is accurate for measuring
manometer deflections down to one-half the resolution of the scale in terms of fluid used!
The U-tube manometer is a practical tool useful for calibrating other forms of pressure
transducers in the pressure range spanning atmospheric pressure levels. As one example, the ad-hoc
U-tube described is convenient for calibrating surgical pressure transducers over the physiological
pressure ranges.
Example 9.3
An inclined manometer with the inclined tube set at 30 degrees is to be used at 20
C to measure an
air pressure of nominal magnitude of 100 N/m
2 relative to ambient. Manometer ‘‘unity’’ oil (S ¼ 1)
is to be used. The specific weight of the oil is 9770 Æ 0.5% N/m
2 (95%) at 20
C, the angle of
inclination can be set to within 1 degree using a bubble level, and the manometer resolution is 1 mm
with a manometer zero error equal to its interpolation error. Estimate the uncertainty in indicated
differential pressure at the design stage.
KNOWN p ¼ 100 N/m
2 (nominal)
Manometer
Resolution: 1 mm
Zero error: 0.5 mm
u ¼ 30 Æ 1
(95% assumed)
g m ¼ 9770 Æ 0.5% N/m
3 (95%)
p 2
p 1
Inclined tube
0
L
H = L sin
Figure 9.7 Inclined tube
manometer.
9.3 Pressure Reference Instruments 383
15:4:53 Page 383
Example 9.2
A high-quality U-tube manometer is a remarkably simple instrument to make. It requires only a
transparent U-shaped tube, manometer fluid, and a scale to measure deflection. While a U-shaped
glass tube of 6 mm or greater internal bore is preferred, a length of 6-mm i.d. (inside diameter) thickwalled clear tubing from the hardware store and bent to a U-shape works fine for many purposes.
Water, alcohol, or mineral oil are all readily available nontoxic manometer fluids with tabulated
properties. A sheet of graph paper or a ruler serves to measure meniscus deflection. There is a limit
to the magnitude of pressure differential that can be measured, although use of a step stool extends
this range. Tack the components to a board and the resulting instrument is accurate for measuring
manometer deflections down to one-half the resolution of the scale in terms of fluid used!
The U-tube manometer is a practical tool useful for calibrating other forms of pressure
transducers in the pressure range spanning atmospheric pressure levels. As one example, the ad-hoc
U-tube described is convenient for calibrating surgical pressure transducers over the physiological
pressure ranges.
Example 9.3
An inclined manometer with the inclined tube set at 30 degrees is to be used at 20
C to measure an
air pressure of nominal magnitude of 100 N/m
2 relative to ambient. Manometer ‘‘unity’’ oil (S ¼ 1)
is to be used. The specific weight of the oil is 9770 Æ 0.5% N/m
2 (95%) at 20
C, the angle of
inclination can be set to within 1 degree using a bubble level, and the manometer resolution is 1 mm
with a manometer zero error equal to its interpolation error. Estimate the uncertainty in indicated
differential pressure at the design stage.
KNOWN p ¼ 100 N/m
2 (nominal)
Manometer
Resolution: 1 mm
Zero error: 0.5 mm
u ¼ 30 Æ 1
(95% assumed)
g m ¼ 9770 Æ 0.5% N/m
3 (95%)
p 2
p 1
Inclined tube
0
L
H = L sin
Figure 9.7 Inclined tube
manometer.
9.3 Pressure Reference Instruments 383