UNIT – VI
Current to Pressure converter : It is mainly based on the nozzle / flapper
system .
Flopper Nozzle mechanism :
The current to pressure convereter works on flapper nozzle system .
The signal conversion is from pressure to mechanical motion and viceversa.
This conversion can be provided by a nozzle flapper system ( sometimes
called a nozzle baffle system ) . The diagram of this device is given below
A regulated power supply is usually over 20 psi provides a source of a
through the restriction . The nozzle is opened at the end where the gap is
adjusted between the nozzle and the flapper and air escapes in this region .
If the flapper moves down and closes of the nozzle opening so that no air
leakage . A signal pressure will rise the supply pressure as the flapper
moves away the signal pressure will drop because of leakage air . Finally
when the flapper is far away the pressure will stabilize at some value
determined by the max. leakage through the nozzle . The graph shows the
relation between the pressure and air gap distance . A nozzle flapper is
designed to operate in the certain region where the slope of the line is high
in this region the response will be such that very small motion of flapper
can change the pressure by an order of magnitude .
I – P CONVERTER : The current to pressure converter is simple I-P
converter is an important in process control when we wants to use the low
level electric current signal to do work it is easier to let the work be done by
a pneumatic signal . The I –P converter gives us a linear way of translating
the 4- 20 ma of current in 3-15 psi
signal . The following diagram
illustrates a simple way to construct such a converter .The current through a
coil produces a force that tend to move the flapper down and close the
Flapper .A high current produces high pressure so that the device is direct
acting adjustment of the spring and perhaps the motion related to pivot to
which their attachment allows the unit to be calibrated so that 4 mA
correspond to 3 psi and 20 mA corresponds to 15 psi .