Ultrasonic liquid level meter is a kind of reflected back through the ultrasonic sensor into an electrical signal, after the sensor receives and calculate the measurement data of the non-contact measuring tool, it is not affected by the influence of the environment, by measuring the liquid density, viscosity, can help us to complete all daily need testing tasks and work, installation and measuring procedures are very simple and convenient, High precision, is widely used in petrochemical, hydraulic thermal power generation and other production fields.However, it also has some limitations, such as it can not measure all the pressure related containers and volatile media, the measurement is prone to blind areas, if the measurement process is not correct or encountered faults, will lead to the measurement results.

一、 Ultrasonic level meter measurement principle analysis

   The working principle of ultrasonic level meter is realized by the reflection and reception of ultrasonic wave. The ultrasonic level meter is vertically installed on the liquid surface and sends ultrasonic pulses to the liquid surface through the sensor. After a period of time, the sensor receives the signal reflected back from the liquid surface. The signal is calculated by the selection and processing of the transmitter circuit to calculate the echo distance.Ultrasonic can spread in gas, liquid and solid, the greater the frequency, the stronger the sense of direction of ultrasonic, ultrasonic level meter is therefore also divided into gas, liquid and solid types of three types, the most commonly used is gas single probe ultrasonic level meter.

二、Ultrasonic level meter measurement advantages analysis

■ Simple structure, easy installation, no moving parts;

■ Non-contact measurement, long service life;

■ The probe is mostly made of PVDF, PP and other materials, with high corrosion resistance and automatic temperature compensation function;

■ Not affected by medium humidity, viscosity, dielectric constant, conductivity, thermal conductivity and other factors;

■ Relatively low price, can reduce the production cost;

■ It is widely used in petroleum, chemical industry, electric power, sewage treatment, metallurgy, semiconductor and other industries, such as corrosive, layered or acid and alkali containing sewage;

三、Fault analysis of ultrasonic level meter in measurement

3.1 Inaccurate measurement is caused by the transmission speed of ultrasonic wave

      Ultrasonic wave in different media propagation speed is not the same, different media by temperature, environment, pressure, time, location and other factors change, so that the speed of ultrasonic wave in the propagation also change. For example, 0℃, the sound wave propagation speed in the air is about 331m/s, 403m/s in water vapor, and with the pressure and temperature change, the propagation speed also changes, when the temperature 0℃, the sound wave propagation speed in the air is about 331m/s;When the temperature is 100℃, the acoustic wave propagation speed in the air increases to 387m/s, and the closer the measurement probe is to the liquid level, the more significant the change of the propagation speed is, so the change and uncertainty of the propagation speed will produce measurement errors, and even lead to inaccurate measurement.At present, this situation has been greatly improved. The measurement probe with temperature compensation, pressure compensation and physical calibration has been developed and put into use. When the temperature and pressure are unstable, the measurement error can be greatly reduced by pressure and temperature compensation.

3.2 Inaccurate measurement results due to fluctuation of power supply voltage

      When using ultrasonic level meter with single probe, the single probe has both transmitter and receiver functions. In the measurement process, if the single probe added a higher voltage, will increase the ultrasonic level meter measurement blind area, resulting in changes in the measurement range, will increase the measurement distance.If the voltage of the single probe power supply is too low, when the measurement distance is far, the ultrasonic wave can not reach the reflection surface or reach the reflection area but can not be equidistant back to the ultrasonic signal, resulting in the ultrasonic level meter can not work normally, resulting in inaccurate measurement results.Therefore, before using ultrasonic level meter, we should check whether the voltage value of the power supply is in the rated range. If it does not meet the standard, we should adjust it in time to ensure the normal work of the probe.

3.3 The selected measurement range is inappropriate

      Ultrasonic will happen in the process of media dissemination weakening, at the same time measuring blind area distance measurement process, thus has certain limits, ultrasonic level meter measuring distance is not an unlimited choice object can be measured, in use process to choose appropriate based on the measured object properties, types and probe type ultrasonic liquid level meter.

3.4 Measuring the failure caused by the transmission medium

      According to the principle of ultrasonic measurement, ultrasonic in the transmission medium encountered whether liquid or solid larger particles, will affect the transmission of ultrasonic.If liquid droplets are encountered in liquid measurement, ultrasonic wave will not only have normal reflection effect, but also cause ultrasonic scattering, which leads to the weakening of ultrasonic transmission distance. Similarly, when foam or particle objects are encountered in solid measurement, it is easy to weaken ultrasonic reflection, leading to inaccurate measurement results.Work in damp environment, pay special attention to check the probe, is there a droplet drip to unknown or adhesion on the measuring probe, on the one hand, it will affect the normal work of the probe, cause measurement error, on the other hand, if there is a strong corrosive liquid droplets adhere to for a long time, easy to cause can not use the probe is damaged or destroyed. Therefore, the staff should strengthen the probe for cleaning, drying and correct preservation, if necessary, need to add filter device.When the droplet is small and does not adhere to the probe,High power probe or double probe ultrasonic level meter can be used for measurement.

3.5 Faults Caused by Installation Mode

      When the ultrasonic level meter probe is installed, it should be vertically installed to the appropriate position. If there is a tilt phenomenon, the ultrasonic beam sent by the probe will be reflected to other directions, resulting in incomplete reflected signals received by the probe and cannot work normally. Therefore, the installation method should be strictly corrected.At the same time, because the ultrasonic level meter has a blind area, the blind area changes proportionally with the size of the measurement distance, generally between 30cm-50cm, in the general installation of ultrasonic level meter blind area should be taken into account.But in special occasions, the space and installation conditions should be considered, the need to consider the choice of ultrasonic level meter with a small blind area.

      In addition, if the measurement level is not stable or other impurities appear, the measurement result will be inaccurate.Therefore, in the use of ultrasonic level meter, the first thing to do accessory installation, probe maintenance, power supply voltage numerical correction and other inspection work, to ensure that the level meter without failure.Secondly, in use according to the liquid level environment, choose a reasonable and correct level meter, and avoid a variety of interference factors, such as droplets, temperature, humidity and so on, in order to control the level meter to work normally, to ensure the accuracy of measurement.