The model selection is very important for the flow meter. The relevant information indicates that the 2/3 of faults happening during the actual application of instrument is caused by the wrong model selection and improper usage. Thus, after making sure the selection of mag meter, many factors should be involved in consideration comprehensively according to different technologies、environments、measurements and media.
On the basis of the diameter in the process pipe line、pipeline pressure and flow velocity of sewage drainage provided by sewage disposal plants, it had better choose the mag meter with D=600mm, There is no need for shrunken pipe and pipe expanding on the spot.
The mag meter is mainly used to test the fluid flow that is with a conductivity of ≥5μS/cm. We should choose different lining and electrode material according to the characteristics such as corrosion、 abrasiveness、temperature and coagulability and so on and price supporting capacity.
The lining materials of mag meter include polychloroprene material which has medium level corrosion and acid、soda and salt with common concentration, and PU rubber which has strong abrasiveness and general corrosion preventive property, and Teflon that has strong corrosion resistance as well as a great adaptation in the high temperature, and polyethylene with the same chemical property as per fluoroethylene-propylene (copolymer) of Teflon that is tensile and resists compression, and polyethylene with the temperature of ＜60℃ can dilute acid、alkali and salt, and poly with the temperature of ＜100℃.
The electrode material is installed within the pipeline’s wall which can have a direct contact with the measured substance, so it should be chosen based on the corrosion of measured substance. The electrode material of mag flow meter consist of the （Ti）electrode that is salt tolerance and aqueous alkali which is less than 50% concentration、（Ta）electrode that is acid—resisting and salt—resisting、noble metal platinum electrode with high corrosion、Hastelloy alloy C electrode which is unfit for hydrochloric、Hastelloy alloy B electrode that is unfit for nitric acid、stainless steel 316L which has moderate corrosion but owns a preferable price.
At first, according to the sewage disposal plants primarily dealing with the domestic wastewater and industrial wastewater, and then considering the prices that users can accept, it is best to choose polychloroprene material and stainless steel 316L electrode.
Based on the national standard, sensors can be divided into two levels of protection called IP65 water jet type and IP68 diving type. According to the installation location is in the low well which is set by users, the sensor named IP68 dust—free diving type should be chosen.
(1)As for the fundamental form that is selected, mag meter possesses many performances like LED display、4～20mA current output and 0～1KHz frequency output. At the same time, it should be added the RS-485 function of communication entrance for the requirements of the flow meter and computer communication.
(2)As for the sensor is installed under the ground, the fractal type should be chosen.