双量程孔板流量计不确定度及量程比

Uncertainty and Turndown Ratio for Dual Range DP Flowmeters

ISO 5167:2003(E)总结了20世纪90年代之后国际上标准差压装置的最新研究成果,在ISO 5167:1991(E)的基础上作了多项重大改进,其中标准孔板的不确定度上升到0.5%。在流量二次装置中按照标准所提供的模型对流出系数C和可膨胀性系数ε的非线性进行补偿后,保证系统不确定度1.5%(气体、蒸汽)和1.0%(液体)的量程比可达10∶1。为了扩大量程比,可增加1台低量程差压变送器,以提高量程低段的差压测量精确度,进而提高量程低段的流量测量精确度,并在流量二次装置中实现量程切换和各项补偿,量程比可达100∶1,从而将差压法流量测量技术提高到一个新水平。根据特征点不确定度对估算进行了论证,并在流量标准装置上通过了验证。

Latest international research results of standard differential pressure devices after 1990＇s is summarized in ISO 5167：2003（E）,and many improvements are made on the basis of ISO 5167：1991（E）.The most significant one is the uncertainty of standard orifice reaching to 0.5%.According to the model provided in the standard,a nonlinear compensation to discharge coefficient and expandable coefficient in flow computer can ensure the turndown ratio could be 10∶1 with uncertainty around 1.5%（for gas,steam,and 1.0% for liquid）.To increase turndown ratio,a low range DP transmitter can be added to improve DP measurement accuracy in low range,and improve flow measurement accuracy as well,and to realize the switching between high and low range at flow computer and making up compensation,and to reach turndown ratio 100∶1.Then the DP flow measurement steps on a high level.The estimation is demonstrated according to the characteristic of uncertainty,and is verified on flow standard device.