working on a fourth class power engineering ticket here, so how can u use DP to tell water level
Assuming you are referring to DP of the steam drum of a boiler, one of the legs will be connected to the bottom while the other will be connected to the top of the drum. Since the top connection will fill with condensate, it will be at a higher pressure than the bottom connection, which will reflect the level changes. Therefore, there will be an increasing DP signal when the level drops. This is counter intuitive, since in most applications it works the other way. In addition, the density of the water-steam mixture in the drum is not the same as at atmospheric pressure, and varies with load changes. Therefore, the DP signal must be compensated for density to get the real level. .
The level of water can be measured by measuring its hydrostatic pressure. A differential pressure transducer has two pressure inputs, one to the pressure at the top of the boiler and the other to the pressure at the bottom of the boiler. The differential pressure is the weight of the column of liquid above the low point plus the weight of the column of gas above the liquid. Since the liquid is much more dense than the gas it is the predominant cause of the force to the differential pressure transducer. See the link below.
The links below will give you further information on the use of differential pressure to tell a level.
