With the development of science and technology, the expansion of chemical production scale and intensity of the increase in the control and management of the production process is more and more stringent, the process of detecting instrumentation and control appears to be more and more critical. Therefore, involving a wide range of various types of instruments on the scene, instrumentation design and selection of scientific, whether to meet the actual needs of the scene and use, is every instrumentation staff in the actual work of the problem should be considered. If there is a mistake in the selection and design of control instruments, it will often bring great losses and seriously affect the safe and stable operation of production.

Usually level measurement is divided into many kinds, each parameter corresponds to a different type, adapt to different sites of a variety of measuring instruments, so the selection of instrumentation is a technical and practical work, not only requires instrumentation personnel to have a more comprehensive knowledge of the basics and on-site maintenance experience, and selection of a variety of measurement methods and instrumentation features to do a comprehensive understanding of the characteristics of a careful comparison. In general, the principle is to first meet the requirements of the process, and then focus on the characteristics of the instrument, installation conditions, environmental conditions and economic factors and other aspects of the comprehensive consideration, so that we can select the production requirements, both safe, reliable and durable best measurement and control instruments.

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Instrumentation is for the process services, but not scientific selection, installation can not be very good for the process services, so the process should be installed for the instrumentation selection to create the necessary conditions. For example, in the high temperature, high pressure closed containers in a variety of flammable, explosive, toxic and other hazardous media, strong acid, strong alkali level measurement. At the same time as far as possible to avoid fire welding, we must choose the external level meter.

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The external tank level sensor is based on the time domain echo principle of ultrasonic pulse, the ultrasonic sensor of the external level meter sends out ultrasonic pulse, the ultrasonic wave propagates in the liquid after passing through the bottom of the tank, and after reaching the interface between the liquid and the gas, it is reflected back to the detector again. The external liquid level gauge by calculating the time difference and ultrasonic propagation speed in the liquid, so as to get the height of the liquid. This measurement method of external level gauge really realizes the non-intrusive measurement of "through the wall and into the wall", and there is no need to open a hole in the vessel for external level gauge measurement, which is especially suitable for high pressure, strong corrosion, toxic and hazardous working conditions.

The acoustic sensor (probe) of the acoustic level gauge is installed directly under the outer wall of the vessel to be measured (bottom), no need to make holes in the vessel to be measured, easy to install, and online installation can be realized.