WHAT IS ZERO SUPPRESSION & ZERO ELEVATION ? 
 
Answer -- Zero Suppression: suppose a DPT is mounted at the bottom of 
the tank through an impulse pipeline,then even if the tank 
is empty the impulse pipeline will have some water in it.
This water will exert pressure on the HP side of the DPT 
and we will get some output even if the tank is empty.so 
the output need to be supressed to the zero value

Zero Elevation:suppose a DPT is mounted at the bottom of 
the closed tank and to the LP the impulse tube which is 
connected is filled with a refrence liquid.Then the 
pressure exerted on LP>HP.Hence the output will be 
negative .So it has to be elevated to reach the zero level.