kinds of new type of flowmeter - Gas Mass Flow Meter Within
Published on：2015-1-13 15:55:41 Hits：
Currently used by various types of gas flow, the majority of the gas volume flow is measured. Since the volume of gas varies with temperature and pressure fluctuates, a large measurement error occurs so often. Such as frequent changes in work pressure, more so the measurement difficult.
The main features of the gas mass flow meter is not affected by temperature and pressure fluctuations, which display readings directly indicates the flow rate of gas mass member. It has a number of advantages:
1. Under normal pressure, high pressure or negative pressure conditions can be selected.
2. Can be at room temperature, 100 degrees, and high temperatures and even running.
3. Applicable range range, both in the laboratory for small spaces flow can also be used for factory production in large-scale installations.
4. meter drop resistance is minimal.
5. The ability of anti-corrosive media.
6. high measurement accuracy.
7. The output electrical signal, the display can be remote, but also facilitate the realization of the flow meter records and other post-match. Controlled or cumulative.
Mass flow meter is a thermal conductivity instrument early in the beginning of this century, Thomas (Thomas) presented its prototype, after several changes ashamed increasingly perfect. Many countries have such a meteor count of industrial products, plant pan used in various areas of industrial production and scientific experiments. Although the specific structure varies, but the principle is basically the same. Our country since 1976, also has several products in succession between the world, many used the atomic energy industry and semiconductor manufacturing industries. We have used micro catalytic reaction device 80 atm hydrogen flow rate metered, has achieved a successful experience. If we can extend it used in petroleum, chemical industry is bound to receive all aspects of good benefits.
Schematic mass flowmeter
This basic principle of the meter is in a very small diameter (4 mm) thin-walled metal tubes (usually stainless steel, pure nickel or Monel alloy corrosion resistant alloys, etc.) of the outer wall, symmetrical around the four groups of resistance wire , composed of interconnected Wheatstone bridge (see below), leaving its bridgewatchers winding l and 3 since the beginning of the winding, two-level outside the tube around the left side (upstream), winding 2 and 6 in the same way around the right side of the tube (downstream). As shown connected to the DC power value of the current, the current through the winding temperature rise caused along the axial direction of the catheter is formed of gold session a symmetrical temperature field (solid line). When the gas flows through the catheter due to the endothermic gas leaving the upstream wall temperature drop through the downstream gas heat, the wall temperature rise, leading to a variation in the temperature field, ie the highest temperature position offset to the right (dotted line in Fig. shown). Resistance wire using a large temperature coefficient of resistance material can sensitively reflect the change in temperature leaving the bridge out of balance. Finally, the bridge unbalance voltage signal amplified or converted into a current signal. From this reason Lun speaking magnitude output signal is proportional to the gas mass flow rate and specific heat of the gas product, can be simply expressed as:
Where: E an output signal proportional constant k a
Cp a gas specific heat (at constant pressure) of a gas mass flow M
Overall heat transfer coefficient between the windings and the surrounding environment of the A-meter
On the ideal gas, the ratio of the hot gases are not vary with pressure is constant, the output signal of the gas mass flow rate and only positive. The magnitude of changes in the general real gas pressure affected by its specific heat is small it is still available output signal representative of the mass flow directly. Considered independent of the size and pressure. In practice, because of the quality of the gas metering difficult to calibrate the instrument so often to be converted into the standard state (760 mm Hg, 0 ℃ or 760 mm Hg, 20 ℃) of gas volume (with the "mark up" or standard cubic meters. "shown) to the nominal mass of the gas.
According to this principle instrument performance, combined with our experience and test cases following aspects are noteworthy:
1. When the instrument operating conditions change (eg change of medium, large changes in ambient temperature, etc.), the instrument zero should be re-adjusted. At the same time, the instrument must be mounted horizontally catheter, use a level calibration. Otherwise it will increase the impact of changes in working conditions for zero drift. Rack but can not shake or swing, etc. There it is not appropriate for use on ships.
2. For the equivalent of 0-100kg / cm2 pressure, a lot of test data within 0 to 7 standard liters / hour flow (air) were associated with operation range, seeking linear regression equation using least square method, the correlation coefficient values were 0.999 ~ λ within the range of 0.9999 to prove that the instrument has good linearity. But linearity with the larger size and scale flow, nonlinear more serious, it is generally limited to the range of 0-4 nl / (air) or less, to ensure good linearity. In order to measure the large flow but ensure linearity, the instrument can be used to extend the principles of triage range. Taken as a bypass pipe, Venturi tubes, plates, etc. used in conjunction with, the range can be extended separately to tens, hundreds, thousands of standard liters per hour, until tens of thousands of standard cubic meters.
3. Although the real gas specific heat and pressure changes with different amplitude changes even some gas still relatively large (see Table 1), but still maintain measurement precision instrument cinnamic certain range (see Table 2).
4. Catheter material of choice, in addition to considering the corrosion resistance than to use a good thermal conductivity of the material better. Head to measure nitrogen, for example, also within the scope 0-100kg / cm2 pressure and 0 to 7 standard liters / hour flow test, measurement precision nickel tube 2 to 2.5 percent while the use of stainless steel is 3 to 4% (thermal conductivity of nickel is about three times that of stainless steel).
5. Since these instruments must be carried out under normal working gas is relatively stable so that both the specific heat of the gas components unstable situation, with the central gas mist and gas of the liquefied working conditions approaching the critical region, etc. Since the specific heat value is very unstable, are such instruments should not be used. As the critical point is liquefied ethylene 50 kg / cm2,9.9 ℃, found in the test pressure exceeds 30 kg / cm2, the instability of the meter reading begins.
6. If you change a gaseous medium, preferably re-calibration. The instructions in the instrument, often introduced without re-calibration, but only in accordance with the specific heat of the two gases to convert without calibration gas flow, although simple, but it can cause large errors, especially when working under pressure, we found that the sensitivity of the instrument is not exactly proportional to the ratio of the heat off the table, but also to be re-calibrated properly.
7. The instrument must be switched on before using preheated before not fully preheated, the meter as unstable. Good models, the warm-up time in less than two hours.
8. During use, when the gas flow rate suddenly changes, shall be transmitted by the heat, the inside temperature of the re-distribution, so that a stable output signal of the re-take some time. In order to reduce such hysteresis, the manufacturer often the instrument of the additional differential electrical circuit network, so that the output signal quick response. This is particularly necessary in cooperation with the other instruments with flow controlled.
Table specific heat a gas (card / standard liter ℃)
Table 2 meter measurement accuracy (%)
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