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U-tube is commonly used in chemical experiments which are U-shaped transparent glass tubes or plastic tubes. U-tubes are available in a variety of sizes and can hold liquid or solid.
It is an absolute vacuum gauge for vacuum measurement and can be used as a vacuum gauge. Its typical principle structure is shown in the right figure. It is composed of two measuring tubes. By measuring the height difference h of the working liquid column in the tube, the value of the pressure P to be measured can be calculated. One side of the liquid column needs to be vacuumed or the like to make the pressure P0 on it equal to the pressure P to be measured. Negligible, the accuracy and lower limit of the pressure gauge depends mainly on how to measure the height difference h of the liquid cylinder and the accuracy of the measurement h, as well as the density of the working fluid.
There are many methods for measuring h, such as measuring directly with a scale, using an altimeter, a point contact micrometer, an optical interferometry, etc., wherein the interferometry has the highest accuracy. Mercury was first used in working fluids, and in the vacuum measurement to expand to low pressure range, oils with low saturated vapor pressure and low density and viscosity are also commonly used. This gauge can measure low and medium vacuum.
The U-tube contains liquid mercury. According to the principle of hydrostatic equilibrium, the hydrostatic equilibrium formula of the U-tube A-A' cross section is:
Where: P——measured pressure;
Ρ1ρ2 - the density of the protective medium or air above the liquid filling ρ3;
ρ 3——The liquid filling is the density of mercury or water, alcohol, etc.;
P0 - atmospheric pressure;
h——The height of the filling level from the high level surface to the pressure P of the measured pressure.
From equation (1): P = P0 + h (ρ2 - ρ1) + H (ρ3 - ρ1)
Relative pressure: P = P - P0 = h (ρ2 - ρ1) + H (ρ3 - p1)
When ρ1=ρ2: P=H(ρ3_ρ1) (2)
Equation (2) shows that the relative pressure P is proportional to the liquid column height H, which is the basic principle of the liquid column pressure gauge measuring pressure.
In the actual measurement, sometimes the effect of the gravity of the medium or the protective liquid on the liquid surface cannot be ignored. In this case, the hydrostatic equilibrium formula should be written on the same section on both sides of the U-tube, and then the measured pressure P should be obtained. This should be especially noted when measuring micro-pressure or micro-deviation.
U-tube pressure timer, due to the tension of the capillary and liquid surface, will cause the liquid level in the tube to be crescent-shaped, causing errors in the reading. In order to reduce such errors, the diameter of the U-shaped tube cannot be selected. too thin.
When water is generally used as the working fluid, the inner diameter of the pipe shall not be less than 8 mm. When mercury is used as the working liquid, the inner diameter of the tube is not less than 5 mm. The U-tube pressure gauge scale is 1mm per division and the maximum error per reading is half of the division. The two tubes of the U-tube pressure gauge need to be read separately, so the possible reading error is ±1mm (equivalent to 9.8Pa). To reduce the error and to take only one reading, a cup gauge can be used.