Because these are the signals which can change with respect to signal.4-20 mA signal is use because if it can start from 0-20 then if there is an signal of instrument is not equal to or less then original signal and if there instrument is show 10% open valve at 4 mA then we can test it with giving it less then 4 mA.
I hope that your question is the difference between mmwc (millimeter of water column) and mmHg (millimeter of Mercury column)
The force exerted by a water column of say 100 mm (ie. the force exerted by a water column of height 100 mm at the bottom point of unit
area ) is 100 mmwc. The pressure is defined as Force per unit area and the unit of pressure is mmwc. When the liquid column is replaced by Mercury then the pressure is measured as mmHg. The density of Hg(Mercury) is approximately 13.6 times greater than water and hence the pressure exerted by Hg column is 13.6 times greater than water column of same height.
Pressure will be added up a bit more, see its detail:
1. Suppose,you want to install a pressure transmitter 10
meter below of pipe line.
2. Density of methane (Examlpe) 0.656 kg/m3 at 25C.
3. your pressure transmitter will show (10 x 1000 x0.656 =
6560mm) 6569 mm of water pressure more.
4. Suppose gas pressure is 20Kg/cm2, then due to this
elevation, transmitter will show 20.66Kg/cm2
Cryogenic means to work in lower temperature range (-50 and
below than that).
Following are the mode of control. Ratio Control, PID
Control, PI Control, ON/OFF Control, Cascade Control,
Override Control, Feedback Control, Feed Froward Control.
These control are can be set in to MAN and AUTO Mode, If
this in MANUAL the mode of control is disable, If this is in
AUTO the mode of control is enable.
This meter uses the Coriolis effect to measure the amount of mass moving through the element. The substance to be measured runs through a U-shaped tube that is caused to vibrate in a perpendicular direction to the flow. Fluid forces running through the tube interact with the vibration, causing it to twist. The greater the angle of the twist, the greater the flow
In closed systems, the transmitter location is
restricted by the maximum allowable distance
above the lower tap. In pressurized systems, this is
the same as the 1 atmosphere equivalent seen
previously. In sub-atmospheric systems (vacuum
systems), the transmitter should be mounted at or
below the lower tap. This ensures the transmitter
always sees a positive pressure on both the
measurement and the reference sides.
In two seal systems, the distance between the taps
becomes the reference offset from zero. The
calculations are the same regardless of where the
transmitter is mounted.
zero will be -400mm and span will be LRV will be -400mm and
URV will be 0 mm.
CNC (Computer Numerical Control) is a particular application
usually to control a multi axis machine tool such as a
milling machine or a lathe but also could be a tube bending
machine punch press or similar. The CNC controller is
usually a purpose built controller using a micro computer
but could be a general purpose computer or a PLC. The
forerunner was NC (Numerical Control) before computers were
readily available, it used solid state logic and relays,
timers etc to provide the application.
A PLC (Programable Logic Controller) is a general purpose
logic controller specifically designed to allow the user to
create their own application. They select appropriate I/O
(Inputs Outputs) and perhaps function modules for purposes
such as communications and multi axis servo control and
program the PLC to perform the required functions.