1. Liquid level measuring instrument

Instrument that can measure changes in liquid level continuously or intermittently is usually called a liquid level measuring instrument. It includes level gauge and level indicator.

2. Level gauge

Instruments that can continuously measure changes in liquid level are called level gauges, such as capacitance level gauges, ultrasonic level gauges, etc., and are not called "level gauges" because the word "level" cannot accurately express the position of the liquid surface height.

3. Liquid level indicator

Instrument that can intermittently measure changes in the liquid level is called a liquid level signaler, also known as a liquid level indicator or a liquid level controller. According to the number of signal points, it can be divided into single-point, double-point, three-point or multi-point level indicator. It has the characteristics of simple structure, convenient maintenance and low cost. The annunciator is usually composed of a liquid level sensor and a signal alarm device. When the liquid level reaches a given level, the annunciator gives a light or sound signal, and there is generally a set of contacts for control.

4. Sensor

The liquid level change of the measured liquid is transformed into a sensitive element and its assembly parts that are convenient to amplify and transmit the electric quantity or non-electric quantity signal according to a certain law, called the liquid level sensor. Also called-secondary instrument. For example, the stainless steel electrode with PTFE sleeve and its mounting flange in the capacitance level gauge; the float and its isolation pipe in the float level gauge; the probe and the wave guide of the gas-media ultrasonic level gauge Wait.

5. Converter

The function of the converter is to amplify or process the electricity or non-electricity signal sent by the sensor, and convert it into a standard signal that can be used for display or sent to the control machine, such as 0~10 mA DC; 4~20mA DC; 1~ 5V DC etc.

6. Transmitter

Liquid level instrument made specifically to provide a unified standard signal for industrial control machines is called a liquid level transmitter. It completes the functions of both the liquid level sensor and the converter. The difference between the transmitter and the secondary instrument is that it is not necessarily equipped with a display, mainly to provide standard signals to the display instrument, recording instrument or control machine.

7. Measurement range, non-measurement area, insensitive area

The range of liquid level changes that can be measured with the specified accuracy is called the measurement range. The algebraic difference between the upper limit and the lower limit of the measurement range is called the range of the level gauge.

The length section that is not required to be measured due to the installation conditions of the liquid container (equipment) being tested is called the non-measurement area. For the liquid level meter of the cubic structure, the distance from the high liquid level to the mounting flange of the sensor is usually called the non-measurement area.

The change of the liquid level cannot cause obvious changes in the output signal of the sensor-a distance is called the insensitive zone, also called the blind zone. The insensitive zone of the dead zone is caused by factors such as the performance of the liquid level instrument itself, the structure of the sensor, and the measurement method. . For example, due to the after-vibration of the transmitted wave of the ultrasonic level gauge, the received wave cannot be distinguished at a certain distance (within the blind zone) close to the probe; the capacitance level gauge is located at the bottom of the sensor due to the presence of the seal at the bottom of the insulating sleeve. Level measurement cannot be performed for a short distance, etc. The insensitive area is usually smaller than the non-measurement area. The insensitive area is also one of the important parameters to measure the performance of the level gauge.

8. Error and accuracy

The measurement error is a parameter that marks the reliability of the measurement result.

In order to assess the quality of the level gauge, even if it is carried out under the same conditions, the results of multiple measurements are not necessarily the same, but they are all in a very narrow range, forming an error band. The middle line of this zone is called the rated characteristic curve. The difference between each measurement result and the true value is called measurement error.

Measurement error is divided into basic error and additional error due to the continuous measurement conditions.The basic error is the measurement error determined by the characteristics of the level gauge in the standard state.The additional error is the error caused by the deviation of the field conditions from the standard state during the use of the level gauge.

9. Return difference

Hysteresis is also called variation, and it is an important parameter to measure the performance of the level gauge just like the accuracy of the instrument. The return difference at any point in the measurement range is equal to the difference between the upstream and downstream errors of that point. The size of the backlash value indicates the ability of the level gauge to approach the true value when the level gauge is reversed under the same accuracy. The hysteresis selection range in the level gauge is 0.5~1% of its basic error.

10. Sensitivity

The slight change in the liquid level when the indication value of the liquid level gauge changes significantly is called the sensitivity of the liquid level gauge.

11. The tested object and working conditions

The measured object refers to the liquid to be measured and its container (equipment).

Working conditions refer to the conditions experienced when the level gauge is working, such as: ambient temperature, ambient pressure, electromagnetic fields, gravity, tilt, power changes (voltage, frequency, harmonics), radiation, shock, vibration, etc.

The use conditions of the sensor and converter of the level gauge are different, so their working conditions can also be different. For the sensor, factors such as the corrosion, viscosity, conductivity, and explosion-proof requirements of the liquid to be measured must also be considered.

12. Display method

Display mode of the liquid level measuring instrument can be divided into the numerical value output display mode, the analog output display mode and the signal alarm display mode according to the different output modes.