XING Shi Feng Huang Guoxiong Li Chenglie (East China Institute of Chemical Technology)

Currently used by various types of gas flow meter, the vast majority of measures the volume of gas flow. As the volume of gas as the temperature and pressure changes and the different, often larger in the measurement of error. If the pressure change frequently and more difficult to measure.

Gas mass flowmeters are the main characteristics of temperature and pressure from the effects of changes, the readings show that direct instruction of members of the gas flow. It has a range of advantages:

1. Atmospheric pressure, high pressure or vacuum under the conditions can be selected.

2. Can be at room temperature, 100 degrees, and even the high temperature normal operation.

3. Application of the wide range of both laboratory for the flow of small places can also be used for the production of large-scale factory installation.

4. Flowmeter pressure drop was minimal.

5. Anti-Corrosion of medium ability.

6. Measurement of high precision.

7. Output of telecommunications, remote display, but also with other instruments with ease after achieving record traffic. Controlled or aggregate.

Mass Flowmeter is a thermal conductivity instrument as early as the beginning of this century, Thomas (Thomas) made it the prototype, after Can Change After becoming perfect. At present, many countries have such a meteor of industrial products, factory pan-industrial production and applied to various areas of scientific experiments. Although the specific structure varies, but its basic principle is the same. China Since 1976, several products are already among the world one after another, more used atomic energy industry and the semiconductor manufacturing industry. We have used 80 atmospheric pressure micro-catalytic reaction of hydrogen flow measurement devices, has made a successful experience. If it applied to oil and chemical industry in all its aspects will receive good benefits.

This flow meter is the basic principle in a very small diameter (4 mm) of the thin-walled metal tube (not commonly used embroidered steel, pure nickel or corrosion resistance, such as Monel Hetai Gold Alloy) the wall, symmetrical around four groups of resistance on the wire , The interconnection of Houston-bridge (see below), Shouqiao stay winding l and 3winding, two-around the outside of the left (upstream), the winding 2 and 6 around the same manner On the right side of the tubes (downstream). Figure connected to the current value of DC power, the current warming caused by winding along the axial the catheter to form a symmetrical distribution of temperature (as shown in solid line). When the gas flows through the catheter due to gas heatʹwall temperature dropped upstream, downstream, gas heat through the wall temperature rise, leading to variations in temperature, the temperature is the highest position of the right shift (in the dotted line Below). Wire resistance temperature coefficient of resistance greater use of the material can be sensitive to reflect the change in temperatureʹbridge out of balance. Finally, the bridge will be unbalanced voltage signal amplification or converted into current situation. Li Lun, from the output signal that is proportional to the size of the gas mass flow of gas and heat the product can be simply expressed as:

In formula EһOutput signals kһThe constant of proportionality

CpһGas heat (the pressure) M a mass flow of gas

AһFlowmeter with the surrounding environment of the winding between the total heat transfer coefficient

On the ideal gas, gas heat is not the pressure of constant change, so output is only with the quality of gas flow into being. In general the real heat of its gas by the movement of the pressure is still very small output signal directly with representatives of mass flow. That has nothing to do with the size of pressure. In practical, because of difficulty in measuring the quality of gas to calibration instruments to Guchang converted into state standards (760 mm Hg, 0 or 760 mm Hg, 20 ) of gas volume ( "standard or" m or superscript " Said) to a nominal gas quality.

According to this principle of the instrument, performance, combined with our experience in the use and testing in the following areas are worth noting:

1. When the instrument when the working conditions of transformation (such as the media changes, substantial changes in temperature, etc.), the instrument of zero should be re-adjusted. At the same time, the catheter must be instruments of the installation, use standard instrument calibration. Otherwise, they will increase to changes in working conditions and the impact of drift. Rack should not be shocked or even swing, and so it is not appropriate in the circumstances the use of the ship.

2. 0-100 kg/cm2 equivalent to the pressure from 0 to 7 standard litres / hour flow (air) within the framework of the large number of test data associated operator, with least squares principle for linear regression equation, the correlation coefficient in the value of 0.999 to 0.9999 range that the instrument has a good linearity. However, linearity and size range have a larger flow, the more serious non-linear, so the general limit of the range of 0 to 4 standard l / when the (air) within, to ensure good linearity. To be able to measure the flow and ensure linearity, can be diverted to expand the principle of the instrument range. As to the bypass, Venturi tubes, such as with the use of orifice, the range can be expanded to an hour dozens, hundreds, thousands or superscript, until tens of thousands of standard cubic meters.

3. While the real heat with gas pressure of a different and even some changes in the movement of gas is still relatively large (see table 1), but the instruments of measurement accuracy can still maintain a certain scope Guangxi (see table 2).

4. Catheter material of choice, apart from considering corrosion resistance, thermal conductivity better to choose the material better. Head to measuring nitrogen as an example, the same 0-100 kg/cm2 pressure and the superscript 0 to 7 litres / hour traffic within the scope of testing, with the precision of nickel for 2 to 2.5 percent while the use of stainless steel and 3 to 4% (Nickel-the thermal conductivity of stainless steel is about three times).

5. As such instruments must be in the hot gas than a relatively stable situation in order to work properly so any gas composition instability, with gas central Liquid and working conditions of approaching the critical area of liquefied gas, and other than the heat value because of the situation is very unstable Are not appropriate use of this instrument. Such as ethylene liquefaction the critical point is 50 kg/cm2, 9.9 , the test found that pressure over 30 kg/cm2, the instrument readings on the start of the instability.

6. If a change of a gas medium, the best re-calibration. Instruments in the brochure, not re-introduce regular calibration, but only two gas under the heat without the demarcation of the conversion to gas flow though simple, but it will cause more errors, especially in high-pressure work, we Instruments found that the sensitivity is not fully proportional to the Commissioner with the heat than the table, to re-calibration for properly.

7. In the use of this instrument must first boot before preheating, in the absence of fully preheated before, on the dashboard for instability. Better models, preheat time in its start-up within two hours.

8. In the use of process, the gas flow when a sudden change, to go through the transmission of heat, the temperature of the re-distribution, re-stabilize the output signal needs a certain amount of time. To be able to reduce this lag phenomenon, the manufacturing plant, often in the electrical circuit in the instrument additional differential network, so that the output signals rapid response. This and other instruments in cooperation with flow-controlled particularly when necessary.

Table 1 the constant pressure of gas heat (card / l standard )

Table 2 Instruments of measurement accuracy (%)