13.7
Pressure Gauges
Most pressure gauges, like those on scuba tanks, are calibrated to read zero at atmospheric pressure. Readings from such gauges are called the gauge p…
When a soccer ball is deflating, gas escapes till the inside pressure equals the atmospheric pressure. At this point, the pressure gauge reads zero.
Most gauges are calibrated to measure zero at atmospheric pressure, and the pressure measured is called the gauge pressure.
The absolute pressure is the total pressure, including gauge pressure and atmospheric pressure.
There are two important classes of pressure gauges: the manometer, which measures gauge pressure, and the barometer, which measures atmospheric pressure.
A manometer is a U-shaped glass tube containing water or mercury. One end of the tube is exposed to atmospheric pressure and the other is connected to a container of unknown pressure.
The pressure at the bottom of both ends is compared, and the gauge pressure is estimated.
A mercury barometer consists of a single long tube, closed at one end. The tube is filled with mercury and inverted over a mercury dish.
The mercury level in the tube rises and falls, depending on the pressure outside the tube. From the mercury level, the atmospheric pressure is estimated.
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Q1: What is the difference between gauge pressure and absolute pressure?
Gauge pressure is the pressure measured relative to atmospheric pressure and reads zero when inside pressure equals atmospheric pressure. Absolute pressure is the total pressure, including both gauge pressure and atmospheric pressure combined. Most pressure gauges, like those on scuba tanks, are calibrated to display gauge pressure rather than absolute pressure.
Q2: How does a manometer measure pressure?
A manometer is a U-shaped glass tube containing water or mercury with one end exposed to atmospheric pressure and the other connected to a container of unknown pressure. By comparing the pressure at the bottom of both ends, the gauge pressure is estimated. The height difference between the two liquid columns directly indicates the pressure difference being measured.
Q3: Why do pressure gauges read negative values in a vacuum chamber?
A vacuum chamber is a rigid chamber from which air is pumped out, creating pressure lower than ambient atmospheric pressure. Since gauge pressure is measured relative to atmospheric pressure, the pressure inside the vacuum chamber reads as a negative value. The absolute pressure in fluids cannot be negative, but gauge pressure can when measured against atmospheric reference.
Q4: What does it mean when a pressure gauge reads zero?
A pressure gauge reads zero when the pressure inside a container equals atmospheric pressure. Most gauges are calibrated to measure zero at atmospheric pressure, which is why a deflating soccer ball shows zero when internal pressure matches the outside air pressure. This zero reading represents gauge pressure, not absolute pressure.
Q5: How does a mercury barometer measure atmospheric pressure?
A mercury barometer consists of a single long tube closed at one end, filled with mercury and inverted over a mercury dish. The mercury level in the tube rises and falls depending on the atmospheric pressure outside the tube. From the height of the mercury column, atmospheric pressure is estimated directly.
Q6: What are the main types of pressure gauges used in physics experiments?
Common pressure gauges include manometers for measuring gauge pressure and barometers for measuring atmospheric pressure. Other types include strain gauges, capacitance pressure gauges, piezoelectric pressure gauges, and ion gauges, which measure pressure by ionizing molecules in highly evacuated chambers. Different gauges are useful in various pressure ranges and physical situations.
Q7: How is absolute pressure calculated from gauge pressure readings?
Absolute pressure is calculated by adding gauge pressure and atmospheric pressure together. When a gauge displays a positive value, that gauge pressure is added to standard atmospheric pressure to find the total absolute pressure. Understanding this relationship is essential for interpreting pressure measurements in fluid systems and pressure of fluids applications.