A level transmitter converts the changes in level into constant electrical signals and sends those signals to tank’s monitoring station.
Types of level transmitter;
The most common method of determining tank’s level, especially in under-pressure tanks, is using differential pressure transmitter. In this method, the differential pressure between top and bottom of the tank is measured and using density, tank’s level can be measured with precision.
Any liquid enforces some amount of pressure on the tank’s bottom. Using this we can convert level changes into pressure changes, then convert those changes –using differential pressure transmitter- into electrical or digital signal or observe it with pressure gauge.
For measuring this differential pressure in open tanks, transmitter’s high pressure side (H) is connected to the tank’s bottom and low pressure side (L) to the Atmosphere, Thus the instrument works according to the static changes in level.
In closed tanks, high pressure side is connected to the bottom and low pressure side to a point above the liquid’s level (a point which only consists of gas or steam). This type of connection is called “dry leg”. Any pressure changes above the liquid level is compensated by the transmitter. The inside pressure is enforced on both sides of differential pressure transmitter and any changes in liquid level is sent to the transmitter’s output.
Pressure can be measured according to manometers’ performance and calculating liquid’s column pressure. In level gauges which work according to pressure calculation, by measuring pressure (and since we have the fluid’s density: ρ) we can use this equation to calculate the liquid column’s height in the tank; Pabs=P+ (ρ*g*H)
In the above equation “P” is the pressure for gas and vapor accumulated at the top of the tank; in open tanks this pressure is equal to the atmospheric pressure. For calculating fluid column’s height with this method, we need to measure both column’s pressure and the pressure for top of the tank. For this purpose, there are differential pressure transmitters which measure differential pressure in the tank and transmit a signal correspondent to the fluid’s height. The pressure which measures with this method, is static pressure for the static liquid in the tank. Thus this is called the method of measuring hydrostatic level. Since these sensors are inexpensive and the method is a simple one, using these sensors have become more and more common.
This company offers differential pressure transmitter level with American brand “Kleev” and Brazilian brand “Smar”.
Guided Wave Radar
This type of level transmitter operates by sending a microwave or an electromagnetic pulse via a sensor. When this signal reaches liquid level, it gets reflected and returns to the transmitter. The amount of time it takes for the signal to reach sensor bar’s bottom and go back up is measured and shown on transmitter’s monitor.
This transmitter is suitable for liquids. It is used in all sectors of process technology. Measuring level without contact and using radar level transmitter is independent of parameters such as temperature, pressure, density, vapor, dust and electrical conduction. Because of this, using radar level transmitter is an effective and precise method for measuring level in the following cases;
1. Tanks of powder material and turbulent liquid
2. Tanks of foamy or dusty materials
3. Solids such as tanks of lime or cement
In these cases, the measurement occurs with millimeter precision.
radar level transmitter come in various models and mechanisms. They can be used in heights up to 70 meters and in conditions with high pressure and high temperature. Radar level transmitters are used for a variety of solids and liquids such as acids, bases, organic solvents, alcohol, petroleum and petroleum products, liquefied petroleum gas, polymer granules, lime, cement, sugar, cereal grains, minerals, metal powder, carbon and coal.
Radar level transmitter come in two types of horn-shaped antenna and bar-shaped antenna.
This company offers radar level transmitter with American brand “Kleev” and Brazilian brand “Smar”.
Displacer level transmitters use Archimedes’ principle as the basis for their operation. Archimedes’ principle states that “the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.”Thus displacer is always heavier than buoyant force and this force increases with the increase in the liquid in which displacer is immersed in.
The displacer is cylindrical in shape with a constant cross-sectional area for enduring highest amount of pressure. As such, buoyant force is correspondent to changes in liquid’s height and ultimately, the relation between level’s change and signal is linear and proportional. Displacers consist of a microprocessor and a device for measuring level.
When displacer (or any part of it) is not immersed in liquid, enters torque tube with its whole weight. When it is fully immersed in liquid, its weight reduces according to liquid’s height and changes occur in the signal.
Displacer’s tube is connected to torque tube. Torque tube uses a rotating motion in order to display or control liquid’s height. Any changes in buoyant force lead to a corresponding change in rotation and it is not sensitive towards changes of the tank’s pressure. With this rotation, the required signal for measuring level is achieved.
Displaying level with liquid’s density is possible with a current change ranging from 4 to 20 mA. This happens through constant signals.
This company offers displacer level transmitter with Indian brand “General Instrument Consortium” and American brand “Kleev”.
Ultrasonic level sensors work by the "time of flight" principle using the speed of sound and the fact that speed of sound is constant in gases.
The sensor part has a piezoelectric element. By stimulating this element, an ultrasound wave emits and after passing gas and reaching liquid, it gets reflected and converts to voltage in the piezoelectric receiver. The liquid’s level is measured by the duration of time between sending and receiving the wave and the speed of sound in the gas.
Ultrasonic frequencies are upward 20 KHz. Ultrasonic pulses are used for measuring level of liquids and solids in industry.
This method is non-contact and is more suitable for measuring the level of hot, corrosive or boiling liquids. Normal frequency range for ultrasonic transmitters is between 40 to 200 KHz.
Ultrasonic transmitter does not have any moving part and can measure level without actual physical contact. This feature helps measuring level in tanks with boiling, corrosive and dangerous chemicals.
Even when chemical compound or dielectric constant changes in fluids of the process, the measured amount remains precise and intact.
This company offers ultrasonic level transmitter with American brand “Kleev”.