As instruments for liquid level measurement and display, liquid level transmitters have been widely adopted in engineering applications due to their ease of installation, high measurement accuracy, and long service life. Commonly used types of liquid level transmitters in daily operations include submersible liquid level transmitters, single-flange liquid level transmitters, dual-flange liquid level transmitters, and RF capacitance liquid level transmitters.
Characteristics and Selection of Liquid Level Transmitters
Given the wide variety of liquid level transmitters available-and the fact that each type requires specific considerations depending on the application environment-our design and operations departments must understand the unique product characteristics of each type to make targeted and appropriate selections during their daily work.
1. Single-Flange Liquid Level Transmitters
Single-flange liquid level transmitters are available in two configurations: flat-flange and flange with an insertion tube. Both types measure liquid level height by utilizing the hydrostatic pressure (gravity) of the medium being measured. Single-flange liquid level transmitters are suitable for media with high viscosity or a tendency to crystallize; they are typically installed on open-top vessels or equipment.
2. Dual-Flange Liquid Level Transmitters
Dual-flange liquid level transmitters are a type of differential pressure transmitter that provides a reliable measurement solution by preventing the medium being measured from coming into direct contact with the transmitter's isolation diaphragm. Their primary utility lies in applications where: high-temperature media must be isolated from the transmitter; the medium is corrosive to the transmitter's sensing elements; the medium consists of suspended solids or is highly viscous; the medium may solidify or crystallize due to changes in ambient or process temperatures; or strict cleaning of the measurement head is required when switching between different media. Dual-flange liquid level transmitters are primarily used for measuring liquid levels within sealed pressure vessels.
3. Submersible Liquid Level Transmitters
Submersible liquid level transmitters are available in two configurations: rod-type and cable-type. Their operating principle is based on the fact that the hydrostatic pressure of a liquid is directly proportional to its height (depth). By utilizing the piezoresistive effect of diffused silicon or ceramic sensing elements, they convert this hydrostatic pressure into an electrical signal. Following temperature compensation and linearity correction, this signal is converted into a standard 4–20 mA DC current output. Due to its ease and simplicity of installation, as well as its strong adaptability, this transmitter is capable of performing high-precision measurements on a wide range of media-from water and oil to highly viscous pastes-without being affected by foaming or sedimentation within the measured medium. It is typically utilized for installation in open-top vessels or equipment.
4. RF Capacitive Level Transmitter
The operating principle of the RF capacitive level transmitter is based on the formation of a capacitor between the probe and a conductive liquid. In this configuration, the metal core of the probe rod serves as one electrode of the capacitor, while the conductive liquid acts as the other electrode; the space between them is occupied by highly stable polytetrafluoroethylene (PTFE)-specifically, the insulating outer sheath of the probe rod-which functions as the dielectric medium between the two electrodes. As the liquid level fluctuates, the surface area of the probe rod immersed in the liquid changes, thereby altering the effective surface area of the capacitor's electrodes. Consequently, when the liquid level rises, the capacitance increases; the RF circuit then converts this change in capacitance into an analog output signal. For the RF capacitive level transmitter, measuring levels within high-temperature, high-pressure vessels is just as straightforward as measuring levels under standard temperature and pressure conditions; furthermore, the measured values remain unaffected by the temperature, specific gravity, or composition of the liquid being measured, nor by the shape or internal pressure of the vessel. This device is primarily employed for level measurement applications involving acids and bases, chlorides, organic solvents, liquid CO2, aqueous ammonia, PVC powder, fly ash, oil-water interfaces, traditional Chinese medicinal liquids, and similar media.