Capacitive electromagnetic flowmeter can play a stable role in solid-liquid two-phase slurry fluid measurement,and can effectively solve the output swing caused by DC polarization interference.Capacitance transducer flowmeter can measure the flow of low conductivity fluid with high accuracy.However,the problem of low conductivity"flow noise"described above must be noticed in use.At this time,the flow velocity range measured by the flowmeter should be within the available range.

 

　　藕合電容容量的變動(dòng)將造成大的噪聲，影響測(cè)量精確度與穩(wěn)定性。電容介質(zhì)的襯里材料，應(yīng)具有絕緣強(qiáng)度高、介質(zhì)損耗系數(shù)小、溫度系數(shù)小、剛性好等性能。新型襯里材料PFA（聚四氟乙烯）和高純氧化鋁工業(yè)陶瓷能夠滿足這些要求。

 

　　The change of coupling capacitance capacity will cause large noise,which will affect the accuracy and stability of measurement.The lining material of capacitive medium should have high insulation strength,low dielectric loss coefficient,low temperature coefficient and good rigidity.The new lining material PFA and high purity alumina industrial ceramics can meet these requirements.

 

　　電容式電磁流量計(jì)把電極埋人襯里內(nèi)，使被測(cè)流體與電極不接觸。這樣，能從根本上解決電極腐蝕、污染、液體泄漏問(wèn)題。這樣的傳感器，其流量信號(hào)是通過(guò)電極與襯里形成的電容藕合傳輸?shù)睫D(zhuǎn)換器。這就是電容式電磁流量計(jì)。

 

　　The capacitive electromagnetic flowmeter buries the electrodes in the lining so that the measured fluid does not contact the electrodes.In this way,the problems of electrode corrosion,pollution and liquid leakage can be fundamentally solved.The flow signal of such a sensor is transmitted to the converter through a capacitive coupling between the electrode and the lining.This is the capacitive electromagnetic flowmeter.

 

　　為了增大電容的容量，減少容抗上的信號(hào)損失，通常采用大面積電極或多電極結(jié)構(gòu)形式。藕合電容的容量?jī)H在幾十個(gè)至上千個(gè)皮法范圍。為保證信號(hào)能夠大部分傳輸給放大器，轉(zhuǎn)換器的輸人電阻必須提高到10 8以上。

 

　　In order to increase capacitance and reduce signal loss on capacitance reactance,large area electrodes or multi-electrodes are usually used.The capacitance of coupling capacitors is only in the range of tens to thousands of skins.To ensure that most of the signal can be transmitted to the amplifier,the input resistance of the converter must be increased to more than 108.

 

　　由此，電容式電磁流量計(jì)具有能夠測(cè)量低電導(dǎo)率液體的優(yōu)點(diǎn)。例如，測(cè)量經(jīng)脫去離子的凈化水處理的純水，其電導(dǎo)率可能在10-5 s/m以下。用一般電磁流量計(jì)的測(cè)量，可達(dá)5％以上測(cè)量誤差。

 

　　Therefore,capacitive electromagnetic flowmeter has the advantage of measuring low conductivity liquid.For example,the conductivity of purified water treated by deionization may be less than 10-5 s/m.The measurement error of general electromagnetic flowmeter can reach more than 5%.

 

　　勵(lì)磁線圈與電極間存在著分布電容和絕緣電阻。勵(lì)磁電壓通過(guò)勵(lì)磁線圈與電極間分布電容和絕緣電阻與信號(hào)內(nèi)阻抗分壓，能在電極上形成大的共模干擾電壓。電容型電磁流量計(jì)的信號(hào)內(nèi)阻很高，分壓的共模干擾可能更大。

 

　　Distributed capacitance and insulation resistance exist between excitation coil and electrode.The excitation voltage can form a large common mode interference voltage on the electrode by distributing capacitance between the excitation coil and the electrode,insulating resistance and internal impedance voltage of the signal.The signal internal resistance of capacitive electromagnetic flowmeter is very high,and the common mode interference of partial voltage may be greater.

 

　　因此，提高儀表的共模抑制比，成為電容式電磁流量計(jì)的另一技術(shù)難點(diǎn)。電容式電磁流量計(jì)的共模抑制比要求在CMR R≥120dB。把前置放大器放在傳感器內(nèi)，那樣的自舉阻抗轉(zhuǎn)換和靜電屏蔽措施，能有效地抑制大的共模干擾。

 

　　Therefore,improving the CMRR of the instrument becomes another technical difficulty of the capacitive electromagnetic flowmeter.Common mode rejection ratio of capacitive electromagnetic flowmeter requires CMR(>120dB).If the preamplifier is placed in the sensor,the bootstrap impedance conversion and electrostatic shielding can effectively suppress the large common mode interference.