Barometer measurement

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

O A barometer measures air pressure. A manometer measures the pressure of an enclosed gas. Before gas is released into the U-tube, the mercury is at the same height in each arm. Q After gas is released into the U-tube, the heights in the two arms are no longer equal. 

A barometer measures the pressure gas particles in Earth s atmosphere exert against Earth s surface. 

A barometer measures the pressure (force per unit area) of the atmosphere. 

When you measure tire pressure, you are measuring pressure above atmospheric pressure. The recommended tire inflation pressures listed by manufacturers are gauge pressures that is, pressures read from a gauge. A barometer measures absolute pressure that is, the total pressures exerted by all gases, including the atmosphere. To determine the absolute pressure of an inflated tire, you must add the barometric pressure to the gauge pressure. 

Gases exert pressure (force/area) on all surfaces with which they make contact. A barometer measures atmospheric pressure in terms of the height of the mercury column that the atmosphere can support (760 mmHg at sea level and 0°C). Chemists measure pressure in units of atmospheres (atm), torrs (equivalent to mmHg), and pascals (Pa, the SI unit). 

The initial weather-forecasting barometer, the Torricelli barometer, measured the height of a liquid column that the pressure of air would support, with a vacuum at the closed top end of a vertical tube. This barometer is an absolute pressure instrument. Atmospheric pressure is obtained as the product of... 

Gas Pressures Pressure is one of the most readily measurable properties of a gas. A barometer measures atmospheric pressure and a manometer measures the pressure of a gas in the laboratory. 

Based on the values in the formula the least number of significant figures is five so we need to round off R to five significant figures as 0.082057 (L atm/°K mol). We note that 22.414 L is about the size of a 5 gal solvent can and we need to tabulate some key unit facts in this first chapter. At this point it is easy to introduce the SI equivalent of the gas constant since the only difference is that the pressure is measured in bars where 1 atm= 1.01325 bar. (Note that a barometer measures bars.) However, at the lower pressure the molar volume will be larger at about 22.711 L ... 

To a good approximation 1 bar is 750 mmHg while 1 atm = 760 mmHg. The name bar is appropriate because pressure is what a barometer measures. To use a common service station air... 

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

Barometer An instrument used to measure the barometric pressure at a given location in the earth s atmosphere. 

A number of units are used to express a pressure measurement. Some are based on a force per unit area for e.xample, pound (force) per square inch (psi) or dyne per square centimeter (dyne/enr). Otliers are based on a fluid height, such as inches of water (in H O) or millimeters of mercury (iimiHg) units such as these are convenient when tlie pressure is indicated by a difference between two levels of a liquid, as in a imuiometer or barometer. Barometric pressure is a measure of the ambient air pressure. Standard barometric pressure is 1 atm and is equivalent to 14.696 psi and 29.921 in Hg. 

Dose,/.box can, tin capsule. Dosen-barometer, n. aneroid barometer. -ta>n-serven, f.pl. canned goods, tinned goods, -milch, /. condensed (or evaporated) milk. Dosieraidage, /. dosing plant, dosieren, v.t. determine Pharm., etc.) dose measure out. 

Vacuum in process systems refers to an absolute pressure that is less than or below the local barometric pressure at the location. It is a measure of the degree of removal of atmospheric pressure to some level between atmospheric-barometer and absolute vacuum (which cannot be attained in an absolute value in the real world), but is used for a reference of measurement. In most situations, a vacuum is created by pumping air out of the container (pipe, vessels) and thereby lowering the pressure. See Figure 2-1 to distinguish between vacuum gauge and vacuum absolute. 

Atmospheric pressure can be measured by any of several methods. The common laboratory method uses a mercury column barometer. The height of the mercury column serves as an indicator of atmospheric pressure. At sea level and at a temperature of 0° Celsius (C), the height of the mercury column is approximately 30 inches, or 76 centimeters. This represents a pressure of approximately 14.7 psia. The 30-inch column is used as a reference standard. 

The weight of the earth s atmosphere pushing down on each unit of surface constitutes atmospheric pressure, which is 14.7 psi at sea level. This amount of pressure is called one atmosphere. Because the atmosphere is not evenly distributed about earth, atmospheric pressure can vary, depending upon geographic location. Also, obviously, atmospheric pressure decreases with higher altitude. A barometer using the height of a column of mercury or other suitable liquid measures atmospheric pressure. 

A mercury barometer. This is the type of barometer first constructed by Torricelli. The pressure of the atmosphere pushes the mercury in the dish to rise into the glass tube. The height of the column of mercury is a measure of the atmospheric pressure. 

A device commonly used to measure atmospheric pressure is the mercury barometer (Figure 5.1), first constructed by Evangelista Torricelli in the seventeenth century. This consists of a closed gas tube filled with mercury inverted over a pool of mercury. The pressure exerted by the mercury column exactly equals that of the atmosphere. Hence the height of the column is a measure of the atmospheric pressure. At or near sea level, it typically varies from 740 to 760 mm, depending on weather conditions. 

The pressure of a gas sample can be measured in a device similar to a barometer, called a manometer. Figures 4-2B and 4-2C show two types. Figure 4-2 B shows a closed-end manometer. Here the downward pressure exerted by the column of mercury is balanced by the pressure of the gas sample placed in the flask. The gas pressure is, in the example shown, 105 mm. As in the barometer, only mercury vapor is present in the right-hand tube. 

Fig. 4-2. Pressure measurement. A. Barometer pressure — 755 mm. B. Closed-end manometer pressure = 105 mm. C. Open-end manometer pressure = 755 — 650 - 105 mm.

The pressure of the atmosphere can be measured with a barometer, an instrument invented in the seventeenth century by Evangelista Torricelli, a student of Galileo. Torricelli (whose name coincidentally means little tower in Italian) formed a little tower of liquid mercury. He sealed a long glass tube at one end, filled it with mercury, and inverted it into a beaker (Fig. 4.4). The column of mercury fell until the pressure that it exerted at its base matched the pressure exerted by the atmosphere. To interpret measurements with a barometer, we need to find how the height of the column of mercury depends on the atmospheric pressure. 

FIGURE 4.4 A barometer is used to measure the pressure of the atmosphere. The pressure ot the atmosphere is balanced by the pressure exerted bv the column of mercury. The height of the column is proportional to the atmospheric pressure. Therefore, by measuring the height of the column, we can monitor the pressure of the atmosphere. 

The air around us is a huge reservoir of gas that exerts pressure on the Earth s surface. This pressure of the atmosphere can be measured with an instmment called a barometer. Figure 5 shows a schematic view of a simple mercury barometer. A long glass tube, closed at one end, is filled with liquid mercury. The filled tube is inverted carefully into a dish that is partially filled with more mercuiy. The force of gravity pulls downward on the mercury in the tube. With no opposing force, the mercury would all ran out of the tube and mix with the mercury in the dish. 

A manometer is similar to a barometer, but in a manometer gases exert pressure on both liquid surfaces. Consequently, a manometer measures the difference in pressures exerted by two gases. A simple manometer, shown in Figure 5 3, is a U-shaped glass tube containing mercuiy. One side of the tube is exposed to the atmosphere and the other to a gas whose pressure we want to measure, hi Figure 5-3. the pressure exerted by the atmosphere is less than the pressure exerted by the gas in the bulb. The difference in heights of mercury (Ah, in mm) between the two sides of the manometer depends on the difference in the pressures. 

The difference in mercury levels in the manometer is 19 mm. This is the pressure difference in torr between the gas sample and atmospheric pressure. The latter, as measured with a barometer, is 752 torr. Shall we add or subtract the 19 torr pressure difference Notice that the mercury level in the manometer is lower on the side exposed to the atmosphere. Thus, the atmosphere pushes on the mercury harder than does the gas sample, meaning that the pressure of the gas sample is lower than the pressure of the atmosphere. Subtract the pressure difference / gas = A P = 752 toiT - 19 toiT = 733 toiT... 

The gravitation of the earth attracts the gaseous components of the air, which exert a force, known as atmospheric pressure, on the surface of the planet. The pressure on any particular place on the earth s surface depends on the amount of air above the place. It follows that the atmospheric pressure decreases at high altitudes, increases at low altitudes and below sea level, and is also affected by changes in weather. Measuring the atmospheric pressure is usually done with physical instruments known as barometers (see Fig. 83). 

---