General information
Liquid and gaseous substances act with a certain force on the bodies in contact with them. The magnitude of this effect, which depends on the properties of the substance and external factors (temperature, compression, etc.), is characterized by the concept of pressure.
Pressure is the ratio of the force acting perpendicular to the surface to the surface area, provided that the force is uniformly distributed over the entire area. Distinguish between absolute and gauge pressure.
Absolute pressure is the total pressure of a gas or liquid, taking into account all acting forces, including atmospheric air pressure. Gauge pressure is the difference between absolute and atmospheric pressure, provided that the absolute pressure is greater than atmospheric pressure. In engineering, as a rule, excess pressure is measured.
Absolute pressure can be less than atmospheric pressure. If at the same time their difference is small, then it is called rarefaction, if it is large enough - vacuum.
Manometers are used to measure excess pressure, in connection with which this pressure is often called gauge pressure. Vacuum and vacuum are measured with vacuum gauges, atmospheric pressure with barometers.
The SI unit for pressure is newton per square meter (N/m2). However, manufactured devices are still calibrated in old units - millimeters of water column (mm of water column), millimeters of mercury column (mm Hg) and technical atmospheres (kgf / cm2).
One technical atmosphere is equal to the pressure on an area of 1 cm2 of a mercury column 735.56 mm high at a temperature of 0 ° C or a water column 10 m high at a temperature of 4 ° C, i.e. 1 kgf / cm2 = = 735.56 mm Hg. Art. = 104mm w.c. Art.
Vacuum is measured as a percentage of atmospheric pressure, or in the same units as pressure. The average value of atmospheric air pressure was determined as a result of numerous measurements and is 760 mm Hg,
Measuring pressure with a manometer
Filed under: Experiments , Crafts , physics , Experiments | Tags: Measurement of pressure with a manometer, Experiments, Crafts, physics, experiment | June 20, 2013 | Svetlana
To measure the pressure of air or gas inside a vessel with a manometer, it is necessary to attach its rubber tube to this vessel. Monitor the fluid level in both legs of the manometer.
a) If the liquid is at the same level in both knees of the manometer, consider the pressure of the gas inside the vessel to be the same as the pressure of the surrounding air.
b) If the liquid level in the short leg of the manometer is lower than in the other, consider the pressure inside the vessel to be greater than the ambient air pressure.
c) If the liquid in the short leg of the manometer is higher than in the other leg, consider that the pressure inside the vessel is less than the pressure of the surrounding air.
With a difference in liquid levels in the manometer tubes, the calculation of the difference in atmospheric pressure and pressure in the vessel is done according to the formula:
You can do the following experiments using your pressure gauge.
Putting firmly the end of the rubber tube of the pressure gauge on the glass funnel, tighten the wide opening with a rubber film. When the liquid in the pressure gauge has calmed down, lower the funnel into a bucket of water. Watch how the pressure inside the water changes with the depth of the funnel. Having installed the funnel at a certain depth in the water, turn its hole in different directions, up and down, following the reading of the pressure gauge.
2. Open the chimney at the furnace that was heated out shortly before the experiment. Insert the pressure gauge rubber tube into the oven. The water level in the short leg of the pressure gauge rises. Calculate the pressure of warm air in the furnace (with draft).
3. Slightly inflate the rubber bag of the medical heating pad with air and connect it firmly to the rubber tube of the manometer.Lay the bag horizontally and put thick books (load) on it one after the other. The pressure gauge will show well the change in air pressure closed in the bag.
4. If you get a glass tube with a total length of about 1.7 m, you can make a pressure gauge to measure a much higher excess pressure, for example, the highest air pressure when blowing by mouth. In this way, the "strength of the lungs" is controlled. It is necessary to blow not jerkily, but gradually increasing the pressure.
5. The same device can measure the greatest vacuum created by oral suction. In this case, you need to pull air from the upper end of the tube with your mouth.
6. If in the device of the 4th experiment, instead of a short elbow of the tube, a tube drawn to narrow is inserted, then when blowing into the long elbow, a fountain will beat from the short tube.
E.N. Sokolov "To the young physicist"
Physics for high school
Barometers. Pressure gauges
Barometers are instruments used to measure atmospheric pressure. The mercury barometer (Fig. 1) consists of a U-shaped glass tube filled with mercury, one end of which is sealed, and the other end contains an open reservoir of mercury. The barometer has a scale with millimeter divisions, which directly measures atmospheric pressure in millimeters of mercury. It is numerically equal to the height of the mercury column between its levels in the closed and open knees of the barometer.
Rice. one
The advantage of such barometers is the greater accuracy of readings. Disadvantages - they are bulky, fragile, mercury vapor is harmful to human health.
The aneroid metal barometer (Fig. 2) consists of a cylindrical chamber K from which the air is evacuated. The chamber is hermetically sealed with a thin corrugated lid-membrane M.
Rice. 2
So that atmospheric pressure does not flatten the membrane, it is connected by means of a rod T with a spring P fixed on the instrument case. An arrow C is attached to the spring, the end of which moves along the W scale. When atmospheric pressure changes, the membrane bends inward or outward and moves the arrow along the scale.
The advantages of aneroids are that they are easy to use, durable, and small in size. The main disadvantage is that they are less accurate than mercury barometers.
Pressure gauges are used to measure pressure, greater or less than atmospheric pressure. Manometers are liquid and metal.
A liquid manometer is made in the form of a U-shaped tube with a liquid (usually water or mercury), one elbow of which is connected to the vessel in which the pressure is to be measured (Fig. 3, a). The liquid level in this leg will decrease (if the pressure in the vessel is greater than atmospheric pressure) or rise (if it is less than atmospheric pressure) compared to the liquid level in the second leg. The measured pressure will be p = pa ±pgh, where pa - atmospheric pressure, pgh - hydrostatic pressure of the excess liquid column in the manometer elbow.
Rice. 3
To measure the pressure inside the liquid with such a pressure gauge, a flat box is attached to one of its knees using a rubber tube, one side of which is covered with a rubber film (Fig. 3, b).
The simplest metal manometer is arranged as follows (Fig. 3, c). A thin elastic plate M - the membrane - hermetically closes the box K, from which the air is partially evacuated. A pointer P is attached to the membrane, rotating about the axis O. When the device is immersed in a liquid, the membrane bends under the action of pressure forces, and its deflection is transferred to the pointer moving along the scale.
Instrument classification
The types of pressure gauges differ in two ways: by the type of indicator they measure and by the principle of operation.
According to the first feature, they are divided into:
- instruments designed to measure atmospheric pressure, otherwise they are called barometers;
- instruments measuring excess and absolute;
- vacuum gauges, designed to measure the difference between atmospheric and absolute pressures;
- pressure gauges, measure small (up to 40 kPa) overpressure;
- tagonometers, a type of vacuum gauge that measures the excess pressure of the upper limit of 40 kPa;
- differential pressure gauges, measure the difference in pressure.
They work on the principle of balancing the pressure difference with a certain force. Therefore, the device of pressure gauges is different, depending on how exactly this balancing occurs.
According to the principle of action, they are divided into:
- liquid, balancing the pressure difference in such devices occurs due to the hydrostatic pressure of the liquid column, the device uses the principle of communicating vessels;
- spring have a simple design, and are widely used to measure the pressure of the medium in a wide range;
- membrane, based on pneumatic compensation, pressure balancing occurs due to the elastic force of the membrane box;
- electrocontact, used in automatic control and signaling systems, since they can be used to regulate the measured medium due to the electrocontact mechanism built into the housing;
- differential are used to measure the level of liquids under pressure, the flow rate of liquid, steam and gas using diaphragms.
https://youtube.com/watch?v=MLdd1XPX7cA
By appointment, there are such types of manometers as:
- general technical instruments are used to measure the pressure of liquids, gases and vapors that are chemically neutral to copper alloys;
- oxygen, they are produced in blue cases with O2 indicated on the dial, they are used to measure oxygen pressure in cylinders or vacuums;
- acetylene are used to control the excess pressure of acetylene;
- reference ones are used to check other instruments, since they have high accuracy;
- ships are used in ships and maritime transport;
- rail are used in rail transport;
- recorders have a built-in mechanism that allows you to reproduce the result of work on paper.
https://youtube.com/watch?v=rq3BMjXM7PY
