A level transmitter is a device that translates the sensed level of a vessel or tank into an electronic signal. Direct, visual, or float level detection methods are usually not possible, nor practical, for safety system measurements, so most safety system fluid level detection is accomplished with a differential pressure device. This is a device that compares the constant pressure of the reference leg to the variable pressure of the hydrostatic volume in the vessel that represents the level of the fluid. This comparison, called differential pressure (DP), is correlated and calibrated to actual fluid level in the tank or vessel.

The safety system level measurements of pressurizer level, steam generator level, safety accumulator level, and refueling water storage tank level are performed with the differential pressure method in most PWRs today. This method is desired for its ability to accommodate level measurements where the vapor/water interface between steam and liquid is not distinct.

One of the key factors affecting the use of DP methods in level detection is availability of room or space for the sensing lines. Another issue is the practical challenge of maintaining a constant reference leg pressure. In a traditional containment, there is room for the sensing lines necessary for DP-type level transmitters and the reference leg level is maintained due to milder environmental conditions. The ‘room and environment’ may not be available in some of the iPWR designs. For these cases, new technology may provide the much needed solutions.

Acoustic or ultrasonic signal measurement devices have been commercially developed that shoot a sound wave frequency signal from the top of the tank directly to a fluid level below. These devices use the time of the signal reflection to determine level distance. These devices could be used in tanks where there is no blocking hardware between the signal source and the fluid level and in an environment where there is a clear distinction between the liquid and gas interface. It is mechanically feasible to use this type of methodology for a refueling water storage tank for example, where the gas to liquid level is distinct, but steam generator and pressurizer level might provide a challenge for this type of technology with a non-distinct water to vapor surface and a high degree of water vapor in the air/vapor space. The acoustic or ultrasonic devices for most safety purposes would need to be temperature compensated as the ultrasonic wave speed is affected by the temperature of the medium it penetrates.23

Another new technology which may be developed for level measurements is fiber optic sensing. A fiber optic sensor uses light (optics) to carry information about a process. In the distributed fiber sensing methodology, the optical signal transforms in an area of changing stress, strain, or temperature. These transformations are detected by the optical signal and calibrated to detect temperature differences or strain differences. Developing fiber for level measurement is in the early stages of development.1,4-6

Much the same as the distributed fiber measurement discussed above, level detection can also be accomplished with strands of traditional temperature devices, such as resistance temperature devices (RTDs) or thermocouples. For this method, one solution is to distribute two strands of closely located discrete heated and unheated junction thermocouples along the length of the vessel. Level is sensed by comparing the temperature difference between the heated and unheated thermocouples, on the principle that heat transfer properties between air and water will result in varying temperature differences between the heated and unheated thermocouples, thus establishing the air (steam) to water interface. This method is only as accurate as the vertical distance between the temperature sensors.