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in this video I will discuss about dead
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problem the previous videos we discussed
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about 4 to 20 milliamp right the four
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represents the zero scale reading or 0%
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of the process variable
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so it is 0 percentage and the 20
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the full scale value and it represents
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the 100% of the process
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variable so this concept is very
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clear now what I will do I will assume 0
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to 20 milliamp as the range instead of 4
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I assumed 0 to 20 milliamp as a range
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instead of 4 to 20 in this
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video the zero represents the 0% of the
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process variable or zero scale and 20
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represents the full scale value which is
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variable now I will go to the same
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example we have have a tank and there is
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a level transmitter which measures the
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level this transmitter range is 0 to 20
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milliamp when the tank level is
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0% transmitter sends 0 milliamp to the
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PLC and the PLC calculates the
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equivalent tank level and displays the
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tank level on the computer
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if the tank is full then transmitter
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sends 20 mamp to the PLC PLC updates the
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tank level status on the
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graphics if the tank is 50 percentage
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then transmitter sends 10 mamp to the
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PLC again PLC calculates the equivalent
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tank level and displays on the computer
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okay in this video you have to remember
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one point main point which is
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transmitter range it is 0 to 20
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okay let's say the present tank level is
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percentage then how much current the
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transmitter will send it is 0 milliamp
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current signal it will send it to the
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PLC PLC calculates the level and updates
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Graphics now I will take one
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table the first one is tank level the
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level right now how much percentage is
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there tank level 0% so I will enter
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zero next one is level transmitter right
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right now what is the level transmitter
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milliamp so I will enter 0
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milliamp right now how much current the
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PLC is receiving it is zero so I will
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milliamp so what is the computer status
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how much it is there level reading on
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the computer it is 0 percentage so I
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will enter 0 percentage
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okay let's say due to some
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reason the cable the wire between the
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transmitter and the PLC is damaged or
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broken between transmitter and PLC due
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there may be a short circuit there may
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be a open circuit there may be a sensor
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failure this level sensor may be a
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failure some damage is
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happened okay in this case what happens
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check now I will update the table again
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see the tank level is 0
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percentage right now it is 0 percentage
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see I am using yellow color for the
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readings this case is for the broken
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failure that level transmitter is
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sending 0 milliamp so 0
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milliamp but the wire is
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broken the PLC is not receiving any
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transmitter okay so in this case the PLC
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much default value is zero so it will
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milliamp and the PLC calculates the 0
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milliamp into the equivalent level value
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and updates the computer which is 0
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percentage if you see if you compare
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these two cases the first one and the
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one you will not find any
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fault okay this is the
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problem the PLC don't know whether the
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reading is true reading or false
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reading in the second case it is dead
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zero the line is dead the wire is dead
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still the PLC is displaying
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0% this is the problem with the zero if
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the zero scale value or lower range
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value if it is zero then it will be
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difficult for PLC to identify the
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fault now you can some people can say
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right now both are same values even
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though the second one is faulty reading
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but it is matching with the first
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one the values are matching but we don't
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know whether the zero is a true reading
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true measurement or due to some faulty
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line or due to some sensor
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damage now I will take one more case You
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observe so this is the dead zero problem
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which is highlighted here now I will
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take one more example see I will
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level now the tank level is
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okay the level transmitter sending 10
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milliamp that is why the level
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transmitter output is 10
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milliamp but the wire is still broken
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so how much the PLC will
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PLC receives 0 milliampere that means it
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will not receive anything the default
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state is zero the starting state is zero
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that is why it will receive zero you see
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the PLC unlock input right now is 0
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milliamp but the level transmitter is
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milliampere and again the PLC calculate
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the level signal based on the 0 milliamp
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and it will update the computer
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percentage so this is also a faulty
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reading right the present level is
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50% but the PLC is receiving 0
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milliamp that is why it is showing zero
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level this is very serious concern if
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this it may lead to the
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accidents damage to the equipment and
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right also one more important point you
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have to remember when understanding this
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problem you have to consider the plcs
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from the 1970s 1980 is in that era
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okay with the modern technology we have
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softwares uh diagnostic Hardware
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available we can easily detect the
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faulty readings faulty circuits faulty
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lines short circuit open circuit sensor
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but these 4 to 20 milliamp are
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19 uh 60s 1970s at that time so at the
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time the plcs don't have any inbuilt
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Diagnostics or inbuilt advanced Safety
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Futures so if you are already working in
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Industry do not compare present
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technology present PLC
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Futures to solve this
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problem we have to solve this problem
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modern technology I mean modern safety
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Futures okay so this is the dead zero
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problem which was faced during
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the 1970s 1980s at that
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time so I will discuss the solution in
