【Weekly Question 20090629】Discussion on level gauge selection (generous rewards await you)

Basically, it is based on the descriptions of instrument production; no comprehensive comparison and selection has been carried out yet. I’ve used a few, including magnetic flap level gauges that aren’t listed above.

Factors to consider when selecting a magnetic level gauge When choosing a level gauge, the following factors should be taken into account: (1) the substance being measured, such as its physical and chemical properties, as well as the operating pressure and temperature, installation conditions, and the rate of change in liquid level； (2) Measurement and control requirements, such as measurement range, measurement (or control) accuracy, display method, on-site indication, remote indication, interface with computers, safety and corrosion resistance, reliability, and ease of installation. 　 　　The commonly used level gauges in water supply systems and the key points for their selection are as follows: a. Float-type level gauge A hollow float is placed in the liquid; when the liquid level changes, the float moves by an amount corresponding to that change in liquid level. The displacement of the floating ball can be measured mechanically or electrically, with an accuracy of ±(1~2)%. This type of level gauge is not suitable for liquids with high viscosity, and its output option includes switch control as well as continuous output. 　 　　In the design of water treatment plants, this type of level gauge is often used to measure the level in the collection wells in order to control the automatic start and stop of the drainage pumps. 　 　　b. Static pressure (or differential pressure) level gauge Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be determined by using a pressure gauge to measure the static pressure of the liquid column at the reference level. The pressure or pressure difference range is calculated based on the density of the medium being measured and the liquid measurement range; thereafter, a pressure gauge or differential pressure gauge with appropriate specifications such as range and accuracy is selected. The accuracy of this level gauge is ±(0.5~2)%. 　 　　c. Capacitive level gauge: Electrodes are inserted into the container; as the liquid level changes, the medium surrounding the electrodes changes, and consequently the capacitance between the electrodes (or between the electrodes and the container walls) also changes. This change in capacitance is then converted into a standardized direct current signal. Its accuracy is ±(0.5~1.5)%. 　 　　Capacitive level gauges have the following advantages: the sensor has no mechanical moving parts, resulting in a simple and reliable structure ； High precision ； The detection end consumes little electrical power and has a fast dynamic response ； Easy to maintain and long-lasting. The disadvantage is that instability in the dielectric constant of the liquid being tested can cause errors. Capacitive level gauges are generally used for measuring the level in tanks such as treatment tanks and clear water tanks. 　 　　When the measurement range does not exceed 2m, rod-shaped, plate-shaped, and coaxial electrodes are used ； When it exceeds 2m, cable electrodes are used. When the medium under test is water, electrodes with an insulating layer (polyethylene can be used) are employed. 　 　　d. Ultrasonic level gauge The sensor of an ultrasonic level gauge consists of a pair of transmitting and receiving transducers. The transmitting transducer emits ultrasonic pulses toward the liquid surface; these pulses are reflected back from the surface and captured by the receiving transducer. The distance between the sensor and the liquid surface can be determined based on the time taken for the signal to travel from transmission to reception, which can then be converted into the liquid level. Its accuracy is ±0.5%. 　 　　This type of level gauge has no mechanical moving parts, offering high reliability; it is simple and easy to install. It operates through non-contact measurement and is not affected by factors such as the viscosity or density of the liquid. Therefore, it is widely used for measuring the level in chemical tanks, storage vessels, sludge discharge tanks, and similar applications. However, this method has certain limitations and is relatively expensive.

　An ultrasonic level gauge is a non-contact liquid level measuring device. It can be used to measure the liquid level and flow rate in various containers or pipes, as well as the water level in canals, reservoirs, rivers, and lakes. It is particularly suitable for use in situations involving sewage or corrosive substances, such as measuring the water level before and after the trash racks in urban drainage pumping stations. Due to the strong corrosive nature of urban wastewater, if a contact-type pressure gauge is used, the sensor probe must be inserted into the wastewater, which causes the probe to corrode rapidly and disrupts accurate measurement. In addition, ultrasonic level gauges offer high measurement accuracy, are easy to install and maintain, and can simultaneously measure water levels, level differences, and flow rates. 　　However, its measurement principle brings about an inevitable drawback: when the medium contains foam or other elements that affect its actual conditions, a false reflected wave is generated. 　　1. When there is foam in the medium, it needs to be kept away from the probe. In practical use, a baffle is added to the side of the probe to prevent foam from getting under it. Or add a connected throughpath. 　　2. If the emission angle is too close to the probe at the lower part, it will cause the probe to shine on the side panel; a mapping can be performed to filter out the interference waves. 　　3. Since ultrasonic devices are usually installed at the bottom of the tank, there should be no condensation to prevent damage to the electrical circuit. 　　4. Changes in the medium to prevent signal interference. In practice, it is true that some pressure gauges of this type do suffer from corrosion, mainly the cables, as both urban sewage and industrial wastewater have complex compositions, and prolonged exposure to them can have an adverse effect. In addition, ordinary pressure gauges may suffer from issues such as zero drift and temperature drift. As for accuracy, I’ve said it countless times: for all instruments that operate on the principle of P=ρgh, their accuracy refers to the precision with which they can measure pressure. But isn’t the density of wastewater constantly changing?

We previously built a reactor under extremely harsh operating conditions, with flashing and severe corrosion. If magnetic floats were to be used, zirconium material would be required, and the unstable liquid level could damage the floats. After evaluation, we chose an external-type level gauge; measurement can be carried out simply by attaching a sensing element to the reactor wall. No contact with a medium is required, and ideal measurement results can also be achieved for level changes.

To be honest, selecting instruments is not our specialty; therefore, when choosing a level gauge, we first analyze the industrial conditions, which includes the physical and chemical properties of the process medium as well as the structural features of the equipment, in order to determine which type of level gauge is suitable. After determining the type, it’s products from major brands with high recognition. When the type is difficult to determine, we proceed by trial and error. We have succeeded as well as failed with this approach; every time we choose a device, we conduct thorough analysis, which is why we achieve success more often. The double-flange diaphragm transmitter and servo level gauge mentioned by the poster are typical examples of successful product selections for us; we replaced the radio frequency admittance level gauge and float level gauge with them. In fact, we have used things such as guided-wave radar, blowing-type systems, and ultrasonic devices; each of them has its own advantages and disadvantages. The main drawback is that they require higher standards in terms of industrial and mining applications, which is why we use them less frequently.

When selecting capacitive level gauges, it is essential to ensure that the medium being measured contains no solid particles or has a low viscosity; we made this mistake – the more than 20 capacitive level gauges we purchased could not be used properly.

Consider the following factors: 1) The object being measured and relevant operating conditions； 2) Requirements for corrosion resistance, explosion prevention, etc ； 3) Measurement and control requirements. Our company uses gamma-ray level gauges, radar level gauges, capacitive level gauges, and external level detectors in high-pressure ammonia (alcohol) separators and high-pressure ammonia (alcohol) coolers

There is a lot of information available on 5# stevenyjz external-type level gauges; this forum has also covered them before. If you want to learn more, you can search for it – there’s no need to post again here

Factors such as the medium, reaction conditions, corrosion levels, and installation difficulty need to be taken into account.

Let’s talk about the choice between radar and servo systems: generally, radar is a better option for heavier media, as servos cannot be used with viscous media； Servo drives are more suitable for lighter media; radar cannot be used for very light media.