The instruments commonly used in the meteorological station are moving trough type (Fortin type) mercury barometer and fixed trough type (Kouwu type) mercury barometer.
The instrument is made by using the principle that the atmospheric pressure acting on the mercury surface is in balance with the pressure generated by the mercury column in the glass tube which is connected to the mercury surface, closed at the top and vacuum, and represents the pressure value.
Using the Torricelli principle: insert a glass tube with the top evacuated into the mercury tank. Under the action of atmospheric pressure, the mercury column flowing into the glass tube will maintain a certain height. At this time, the pressure generated by the mercury column on the surface of the mercury tank is in balance with the atmospheric pressure acting on the tank surface.[1]
Simple mercury barometer
If the height of the mercury column is measured near it with a vertical ruler (as shown in the figure, the zero point of the ruler is taken on the surface of the mercury tank), the simplest barometer can be formed, and its reading is the atmospheric pressure obtained.
Set the atmospheric pressure at that time asp，The height of mercury column isH，therefore
Where
Is the density of mercury at t,G is the local gravity acceleration.
It can be seen that under the condition of the same atmospheric pressure value, the measured value may still produce different readings due to the different temperature and gravity acceleration at the measuring location of the barometer.In order to correctly reflect the atmospheric pressure, a standard state must be specified, and the limiting conditions are as follows:
(1) Take the mercury density at 0 ℃, i.e
=1.35951 * 10 kg ；
(2) Take standard gravity acceleration, i.e. g=9.80665 m/s
In theory, any liquid can be used to make barometers, but mercury has its unique advantages:
(1) The density of mercury is high, so the height of the liquid column formed by mercury is appropriate.
(2) MercurialSaturated vapor pressureVery small, so the influence of the additional pressure generated by mercury vapor in the vacuum part of the glass tube top on the reading accuracy can be ignored;
(3) After a certain process, it is easy to get mercury with high purity.
Features and structure
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Mercury barometer, invented by Torricelli in 1643, is a landmark instrument in the era of quantitative measurement of meteorological elements in atmospheric sounding.
The observation data of the mercury barometer can achieve high accuracy. The daily measured value of atmospheric pressure is about 1000 hPa, and the reading of the mercury barometer can easily reach 0.1 hPa, that is, its relative error is 1/10000, which is rare in physical quantity detection instruments.
The commonly used measuring instruments of the meteorological station areFortin (moving trough type) mercury barometer andKouwu style(Fixed slot type) mercury barometer.
(1) Fortin mercury barometer
The main feature is that there is a fixed zero point on the scale.For each reading, adjust the surface of the mercury tank to this zero point, and then read the scale on the top of the mercury column.
Its structure is as shown in the figure, which is divided into three parts: mercury column glass tube, mercury tank and gauge sleeve.
a. Mercury column glass tubeThe length is about 90 cm, the upper end is thicker, the inner diameter is about 8 mm, and the lower end is thinner.After special process cleaning, heat and vacuumize, pour high-purity mercury into it, and then insert it into the silver tank.
b. Mercury tankThe visible part of the appearance: the lower half is a copper sheath (1) to protect the internal sheep skin bag;The glass ring (2) with a very thick diameter is in the upper half, which is tightly clamped by three bolts;The mercury surface adjusting screw (4) is at the bottom of the groove, with which the height of the mercury surface in the groove can be raised or lowered.The ivory needle (5) is the base point of the barometer. When observing the air pressure, the mercury surface must be raised to this point.
c. Ruler sleeveThere is a slit on the upper part, and a reading scale is engraved on one side of the slit. The mercury column (3) can be seen from the slit on the upper part of the scale sleeve. Turning the right adjusting screw (6) can make the vernier (7) slide up and down. When the bottom edge of the scale is tangent to the top of the mercury column, the air pressure value can be accurately read.There is also an auxiliary thermometer (8) for measuring the body temperature on the sleeve in the middle of the mercury barometer.
(2) Kouwu mercury barometer
The difference from the moving trough mercury barometer is only in the mercury trough part.Its mercury tank has no sheepskin bag, mercury face adjusting screw and ivory needle (Figure a).When the air pressure changes, the increase (or decrease) of mercury in the mercury tank caused by the rise (or fall) of the mercury column in the glass tube will inevitably lead to the decrease (or increase) of mercury in the tank, making the mercury in the tank face downward (or upward), that is, the base point of the entire barometer changes with the height of the mercury column (Figure b).
That is, when the air pressure rises by 1 mm, the mercury column in the lumen risesxmm，The mercury level in the tank dropsymm
x+y= 1
The 1 mm scale of fixed slot mercury barometer is shorter than the actual 1 mm in length to compensate for the change of the base point in the barometer slot.The 1 mm scale on the domestic barometer is only 0.98 mm long.
usage method
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(1) Resettlement
The barometer is required to be placed in a special room. The ideal barometer should ensure uniform and stable indoor temperature, and on the other hand, avoid the impact of air flow on the indoor air pressure.It is not difficult to ensure the uniformity of room temperature. The impact of air flow is not only related to the wind speed, but also related to the structural form and location of buildings. According to experience, the maximum amplitude of the impact can reach 3 hPa.For this reason, the air chamber shall meet the following conditions:
a. No heat source or cold source is installed indoors;
b. Avoid direct sunlight;
c. Double doors and windows to reduce air pressure fluctuations caused by wind and observers entering the room;
d. The barometer should be hung vertically, otherwise the reading will be higher.
(2) Observation
Read the barometer as follows:
a. Auxiliary thermometer for observation;
b. Adjust the mercury surface in the mercury tank so that it is just connected with the ivory needle tip from bottom to top;
c. Adjust the vernier at the top of the mercury column to make it fall slowly. The front and rear bottom edges are just tangent to the top of the mercury column. When adjusting, pay attention to keeping the line of sight at the same height as the mercury column;
d. Read and record, accurate to 0.1 hPa;
e. Lower the silver surface to separate it from the tip of the ivory needle.
