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Mission 36 · Pro PLC fault-finding exercise

Empty until a fifth full

The level reads 0% while the tank fills. Find out why.

Find and fix the . LevelRaw is 0 counts at 4 mA (empty) and 27648 at 20 mA (full). LevelPV must read 0–100% in proportion, with decimals, and stay within 0–100%.

This mission is part of ProTry the full course, every mission and the expert challenges free for 7 days. No card needed.
The 3D process tank this mission runs on Pro mission
Test replay

Operator report: a fifth full reads 0%

Test 1 of 5
Set by the testChecked by the testStep 5 of 5 · 300 ms
Level
Applied
Time
About 12 min
Reward
+200 XP
Rig
3D process tank
Siemens editors
SCL, LAD, FBD
Rockwell editors
Structured Text, Ladder, FBD
Course stage
21. Read a real transmitter
The operator’s report · process tank
We refilled TK-101 after a clean. The sight glass showed about a fifth full, but LT-101 on the screen still said 0%. Then it caught up, and it reads 100% when the tank is full. The transmitter was calibrated last week.
Automatic tests

What the tests check

  1. About a fifth full, 5530 counts, reads 20%.
  2. Half full, 13824 counts, reads 50%. Empty and full still read 0% and 100%.
  3. Readings a little outside the range stay at 0% or 100%.
  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

After a tank clean, the sight glass shows it a fifth full while the screen still says empty. Both ends of the scale look right.

A scaling range that doesn't match the input card hides the bottom of the tank. Low-level alarms and pump protection then act on the wrong 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.

Keep going

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