Radiation Level Detection

The radiation type level measuring device operates on the principle that radiation can be blocked by any substance to a limited degree. This degree of blockage is measured and translated to indicate level.

The system itself consists of a radiation source, a detector, a conversion circuit, and an indicator. The source and detector are situated above and below the substance to be measured. As the level of the substance increases, the amount of radiation blocked increases. Therefore, the amount of radiation received by the detector decreases. This received radiation is translated into an electrical signal by the converter and sent to the indicator. A typical radiation type level measuring system is shown in Figure

radiation level detector

see also

level measurement using hydrostatic pressure ultra sonic level measurement indirect methods for level measurement level measurement: direct methods difference between sensor and transducer

Ultra Sonic Level Measurement

This type of instrument uses “time of flight” principle. An ultrasonic level measuring device uses reflected sound to determine the height of a substance in a vessel. The time required for a sound wave to be transmitted, reflected, and returned to the device may be translated directly into a level indication. A high frequency sound signal is used for this purpose, normally in the range of 20KHZ to 200KHZ.

The timing circuit of the device is started when the transmitter sends out the sound wave. It is stopped when the receiver detects the return of the reflected signal. The device is calibrated in an empty vessel; therefore, as the level in the vessel increases, the time required for the reflected signal to return decreases.

Although this principle is very accurate and used in many industries, it is not widely used as an indicator in power plants. The device is generally fragile and affected greatly by outside forces. Because of this, it requires frequent calibration to maintain its accuracy. It is, however, used to some extent as a level switch. A typical example is illustrated in Figure .

ultrasonic level measurement

see also

level measurement using hydrostatic pressure indirect methods for level measurement level measurement: direct methods difference between sensor and transducer

Water Level measurement using hydrostatic Pressure

Water Level measurement using static Pressure

The pressure at any point below the surface of a liquid and the level of water in height are related directly. Simply, the pressure at any point below the liquid surface is a measure of liquid level. Level measuring instruments using static pressure utilize this principle.

Static pressure type level instruments are normally only applied to open vessels, because they depend entirely on the static head. Any change in pressure on the surface of the liquid would affect the calibration, because it would cause a change of static head pressure. The pressure and Level relation can be summarized using the equation

h=P/ρg

Indirect Methods for Level Measurement

In case of indirect method or inferential method, levels are measured in terms of other parameters like pressure. Measuring instrument will be calibrated in terms of level. As discussed previously, direct reading level devices are normally for local indication only. When remote indication like in the control room or a control signal is required, an indirect measurement device must be used. There are different types of indirect level-measurement devices like differential pressure, static pressure, ultrasonic, and radiation level measuring instruments. Some are discussed below.

 Differential pressure level measurement

This type of instrument measures the differential pressure resulting from two columns of liquids with unequal height. The height of one column will be always constant and that of the other will be variable. A typical example of this type would be the steam drum water level measuring device, similar to the one shown in Figure.