主要测量元件为一根垂直安装的下小上大锥形玻璃管和管内可上下移动的浮子。 The main measuring element of the glass rotor flowmeter is a vertically mounted lower and upper conical glass tube and a float that can move up and down in the tube. When the fluid flows through the conical glass tube from bottom to top, a pressure difference occurs between the upper and lower parts of the float, and the float rises under the action of this differential pressure. When this rising force, the buoyancy and viscous lift force of the float are equal to the gravity of the float, the float is in an equilibrium position. Therefore, there is a certain proportional relationship between the flow rate of the fluid flowing through the flowmeter and the rising height of the float, that is, the flow area of the flowmeter. The position height of the float can be used as a flow measurement.
How to choose glass rotor flowmeter correctly Glass rotor flowmeter is a kind of instantaneous flow measuring instrument which is simple to use, convenient to read and very versatile. In the field of environmental protection equipment and instruments, the amount is more than 30,000 units / year. Therefore, it is extremely important to choose and use this instrument.
根据它的用途和适应范围可分为：普通型、带筋维管型，微小流量及小外形型、耐腐型、实验室型、保温型、报警型和耐高压型八个系列。 First, the type of glass to flowmeter and the selection of glass rotor flowmeter can be divided into: ordinary type, ribbed vascular type, micro flow and small outline type, corrosion resistance type, laboratory type, Eight series of insulation type, alarm type and high pressure resistance type. According to the national series of instrument series spectrum, no matter which series includes a maximum of 12 calibers from 1mm to 100mm, the measurable flow range is: liquid (water) 0.1ml / min ～ 40m3 / hour, gas (Air) 1ml / min ~ 1000m3 / hour. The glass rotor flowmeter used for environmental protection instruments generally has a diameter of no more than 10 mm, and the measured flow belongs to a small flow range.
The selection of glass rotor flowmeter can be considered from the following aspects.
1. The object of measurement. That is to measure the type of medium, pressure, and chemical properties. For liquid and gaseous media, for corrosive media, a corrosion-resistant flowmeter should be selected.
2. The performance of the flow meter itself. After the above conditions are determined, generally speaking, if there is no major change in price, the needle valve is preferred to be placed on the upper part of the flowmeter; those with larger flow holes are directly flow-scaled; simple structures; smaller external dimensions, etc . If it is a small flow range, you can use the ball float type, because it is stable during measurement, not easy to accumulate dust, high accuracy, and good interchangeability.
3． Select based on price. Generally speaking, high accuracy and high price. To select the accuracy level of the instrument according to the measurement purpose, if you only need to control the throughput of the measurement medium and adjust it after trial operation, you must always stabilize the throughput in the future, then the accuracy is secondary.
的刻度修正玻璃转子流量计的刻度，是生产厂在本厂条件下用近于理想流体的水和干燥空气作介质标定得到的。 2. Calibration of glass rotor flowmeter The calibration of glass rotor flowmeter is calibrated by the factory under the conditions of the factory using water and dry air that are close to the ideal fluid as the medium. However, there are two situations where the scale value of the flowmeter cannot be used directly: one is that the measurement medium is not water and air, and the other is that the medium is water and air, but its state (temperature, pressure) and scale The status is different. In this way, when using a flow meter, in order to obtain a correct measurement result, a problem arises that the scale value needs to be corrected. Therefore, solving the calibration of the glass rotor flowmeter is the key to using this instrument.
Considering that environmental protection instruments use rotor flowmeters to measure the flow of gaseous media, only the density correction in gaseous media measurement is discussed below. Because the viscosity of the gaseous medium is small, the viscosity effect is omitted in the discussion. Practice has proved that this does not affect the accuracy after the correction.
The following is the general expression (1) of the glass rotor flowmeter. It is a calculation formula that does not consider the influence of the medium viscosity. It is obvious from formula (1) that when the position height of a flowmeter float is determined, the measured medium density ρ is the only variable. If the measured medium density is different, the flow rate of the medium through the flowmeter is also different. Therefore, the scale correction is actually the flow correction.
If two media with different densities ρ1 and ρ2 pass through the same rotor flowmeter respectively, if the floats are balanced on the same solid, the basic formula for the density conversion of the rotor flowmeter is obtained from (1):
In the formula, P1, Tl and P2, T2 are two states of the same medium, which are respectively expressed by two parameters of pressure and temperature. It can be seen that, for the same gas medium, its density conversion can be completely converted into temperature and pressure conversion under different states. In this way, the calibration correction for measuring the flow rate of the same gas medium eventually becomes the temperature and pressure state correction (essentially, the density correction). Obviously, this is a correction method that is generally easy to achieve using a unit.
(A) How to obtain the actual flow value from the flowmeter scale reading at the use site?
When using off-the-shelf applications, there is only one purpose for transferring to a flow meter: to detect the actual flow of the measured medium. However, many using units ignore the difference between the on-site state and the flowmeter's calibration status (that is, the standard status), and directly use the flowmeter's calibration reading as the actual flow value of the measured medium. It is very obvious that this actual flow is unreal. It will bring errors to the measurement of the flow meter, which will cause errors in the final detection results of the supporting instruments.
In the field, the actual flow value is obtained from the scale reading of the flowmeter, which essentially converts the flow value under the standard state of the flowmeter into the flow value under the on-site working conditions. We set the parameters of the field conditions as actual flow Q, medium pressure P and temperature T; the relevant parameters of the rotor flowmeter scale are respectively the flow Q as compared to the standard state pressure Po and temperature To. According to formula (4):
Using the formula (5), it can be seen that the actual flow value of the measured medium can be easily obtained from the flowmeter reading on the spot. It is necessary to point out that when calculating by (5), P, P. , T, T. Should be substituted for absolute value, and P is the pressure before the meter, should be measured on the upstream side of the flowmeter, and close to the pipeline of the flowmeter.
Example: When using an air sampler, the rotor flow meter on the sampler reads 500 ml / min and the front pressure of the meter is -100mmH20 (negative pressure due to the use of an air pump). The temperature on site is 30 ° C. The actual air flow value at this time.
The solution, based on the measured data, is here because P. Units of millimeters of mercury are used, so millimeters of water must be converted to millimeters of mercury. For calculations, simply divide millimeters of water by 13.6.
From the results of the above example, it is seen that although the on-site state is not much different from the standard state, a difference of 22 ml / min (4.4% of the standard value) is produced for the measurement results. In other words, if the road is not modified, the flow value will produce an error of 4.4%!
(Two) how to choose the appropriate rotor flowmeter according to the actual flow of use?
This problem is exactly the inverse process of (1) above, and only needs to reverse the formula (5):
Here, Q is the actual flow rate; P, T, Po, To are all known. Qn calculated by  d is the scale flow rate value of the rotor flow meter to be purchased. Note that Q should be the usual flow. In order to ensure sufficient accuracy and margin when the flowmeter is used, the upper limit of the glass rotor flowmeter purchased should be 1.5 times of Qn.
We want to emphasize the importance of this clause, because if the pressure of the measured medium is large, it can never be ignored. For example, if the pressure of the measured medium is 3kg / cm2, and it is assumed that the temperature is not much different from the standard temperature, then P = 3 + 1kg / cm2 (absolute unit, P. = 1), and Qn ÷ 2Q will be obtained by substituting into the formula (6). That is to say, if the commonly used flow rate is Q, the appropriate upper limit of the flowmeter should be 1.5 × 2Q = 3Q. In this case, the flowmeter is still selected according to Q, which will not meet the requirements for use and will cause production errors.