Practice missions
Practise in
0 XP
Practice missions
Mission 38 · Pro Alarms exercise

Alarm limits that don’t chatter

High and low level alarms with a deadband.

Turn HighAlarm ON when Level reaches 85%. Keep it ON until Level falls below 80%. Turn LowAlarm ON when Level falls to 15%. Keep it ON until Level rises above 20%. While SignalFault is ON, both alarms are OFF: a failed signal has its own alarm.

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

High alarm with a 5% deadband

Test 1 of 4
Set by the testChecked by the testStep 8 of 8 · 480 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
23. Alarm limits that don’t chatter
Automatic tests

What the tests check

  1. HighAlarm comes ON at 85% and goes OFF only below 80%.
  2. LowAlarm comes ON at 15% and goes OFF only above 20%.
  3. Ripples around a limit don’t flash an alarm. SignalFault turns both alarms OFF.
  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 high-level alarm flashes on and off every few seconds as the surface ripples. The operator starts to ignore it.

Chattering alarms bury the ones that matter. A deadband keeps each alarm steady, and a failed signal must not raise false level alarms.

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

Next in the course