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Mission 35 · Pro Analogue instruments exercise

Read a 4–20 mA transmitter

Turn raw input counts into a level the operator can trust.

LT-101 is a level transmitter. Its signal reaches the as LevelRaw: 0 counts when the tank is empty (4 mA) and 27648 when full (20 mA). Scale LevelRaw into LevelPV, from 0 to 100%, with decimals. If the reading goes a little outside that range, show 0% or 100%.

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The 3D process tank this mission runs on Pro mission
Test replay

Empty, half and full

Test 1 of 4
Set by the testChecked by the testStep 3 of 3 · 180 ms
Level
Applied
Time
About 15 min
Reward
+250 XP
Rig
3D process tank
Siemens editors
SCL, LAD, FBD
Rockwell editors
Structured Text, Ladder, FBD
Course stage
21. Read a real transmitter
Automatic tests

What the tests check

  1. 0, 13824 and 27648 counts show 0%, 50% and 100%. Readings in between keep their decimals.
  2. Counts a little below 0 show 0%. Counts a little above 27648 show 100%.
  3. LevelPV comes from LevelRaw, so it follows the real tank.
  1. Try it Use the buttons below the model and watch the rig respond.
  2. Test solution Every check runs on your code in one press.
  3. Replay If one fails, step through it one scan at a time to see why.
Why it matters

Where you’d see this on site

A tank's level transmitter sends 4–20 mA to the PLC. The operator's screen must show how full the tank really is.

A wrong scale can show half full when the tank is nearly full. Operators fill, empty and dose from that number.

Where you'd find thisMaking one batch of drink

At a drinks factory, valves let measured amounts of water and syrup into a mixing tank. An electric mixer blends them into one batch. An outlet valve then sends the drink to a holding tank. Filling machines bottle it from there.

  1. Ingredient tanksHold the water and syrup
  2. Mixing tankFill, mix and measure one batchYou are here
  3. Holding tankKeeps the finished drink ready
  4. Bottle fillerPuts the drink into bottles
On a real site: places, parts and safety
  • Drinks factory — blending water and syrup
  • Paint factory — mixing ingredients into a consistent colour
  • Water-treatment works — preparing a measured chemical mixture
  • Brewery — heating water for washing equipment

A full tank does not close its own inlet. Water keeps arriving until the valve closes, so an open inlet can flood the room.

A heater that is not covered by liquid can overheat. Real tanks have separate safety devices as well as the PLC programme.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

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