Scenario

You are the on-call engineer for a hydrogen compressor skid. The control system has five sensors arranged in a serial telemetry chain. Each sensor reports its own reading and passes all upstream readings downstream unchanged.

If exactly one sensor is faulty, it reports an incorrect value for its own measurement — but faithfully relays whatever the sensors upstream of it reported. Sensors downstream of a faulty one receive and relay the corrupted data without modification.

At 03:14 the master controller logs a warning. You pull the telemetry snapshot below. Exactly one sensor is malfunctioning. Your job: identify which one, what it actually reads, and determine the true system state.

Telemetry snapshot

Sensor 1 — Inlet pressure Reported: 12.4 bar Nominal range: 11–14 bar Note: S1 relays no upstream data. Physical constraint: inlet pressure must always be lower than stage-1 discharge pressure.

Sensor 2 — Stage-1 discharge pressure Reported: 9.1 bar Nominal range: 28–35 bar Note: S2 relays S1 = 12.4 bar. Compression ratio at this stage should be approximately 2.4–2.6×.

Sensor 3 — Inter-stage temperature Reported: 148 °C Nominal range: 130–160 °C Note: S3 relays S1 = 12.4 bar, S2 = 9.1 bar. Temperature is consistent with gas compressed to approximately 9 bar from 12 bar.

Sensor 4 — Stage-2 discharge pressure Reported: 87.3 bar Nominal range: 70–90 bar Note: S4 relays S1 = 12.4 bar, S2 = 9.1 bar, S3 = 148 °C. Compression ratio from S2 reading: 9.6×.

Sensor 5 — Final outlet flow rate Reported: 340 Nm³/h Nominal range: 310–370 Nm³/h Note: S5 relays all upstream values unchanged. Flow is within range for the reported outlet conditions.

Questions

1. Which single sensor is faulty?
2. What physical law or engineering constraint exposes the contradiction?
3. What is the sensor's true reading, given that all other sensors are accurate?
4. Bonus: Is the system in a safe state, or should you trigger a shutdown?

Admin note: This logic puzzle was created with the help of generative AI. Give us your feelings or criticisms of this in the comments below.

My watch is 1 second slow.

Your watch is 1.5 seconds fast.

We both agree to stand here until the show the same time again (12 hr clock)

Question 1: How long will we stand here, waiting for the watches to display the same time?

After that we will leave. But we will meet back up when we simultaneously have the correct time again.
Answer, updated 5.27.26
Rixter gets it, in answer No. 9 (down below)

Question 2: How long until we meet back up again?

A service worker is tasked with replacing some fasteners that are installed on the inner ceiling of a large underground pipe.

The pipe is 12 ft in diameter and she brings with her a 12 ft long ladder. The pipe is perfectly round and is also coated with a very low friction film.

The worker vertically positions the ladder underneath the fasteners she needs to replace.

1.Describe the behavior of the ladder as the worker begins to climb.

2. How can she replace those fasteners?

Disclosure: The accompanying image was created with generative AI.

Answer, updated 4.27.2026
As the worker climbs the ladder, she changes its center of gravity. With each step, the ladder rotates away from her. By the time she reaches the midpoint, the ladder is nearly horizontal in the pipe and she is underneath (and holding on for dear life). If she continues, the ladder will continue to rotate, until she either falls or is 'climbing' upside down. She cannot reach the fasteners by climbing the ladder in a conventional way.

Instead, she must rotate the ladder so it is perpendicular to her and then very carefully climb on from the side (vsar gets it!). Bonus points to HiTekRedNek as well, as that technique seems viable.

You and a friend are on a new reality TV game show. Your friend is placed in the middle of a dark maze and they cannot see at all.

You are given this map of the maze and are allowed to give your friend exactly one instruction to guide them out of the maze.

If they successfully get out of the maze before the timer ends, you and the friend win $100,000.

What instruction do you give your friend?

Answer, updated 3.26.26

Randall gets it right in answer #1. You can keep one hand on either wall, and it will eventually lead your friend out.

The vessel Ironweed leaves a dock from the western shore (A), destined for the dock on the east side of the lake.

At the same time, it's sister vessel Lake Dog leaves a dock from the eastern shore (B), destined for the dock on the west side of the lake.

The vessels cross paths for the first time 500 yards off-shore from (A). Each vessel completes its passage and prepares for the return journey.

The vessels then leave docks for return trips at the same time again and this time meet 300 yards off-shore from (B).

What is the distance from shore A and shore B? What is the differential in the vessel's respective speeds?

Answer, updated 2.24.26

Rixter gets it in the comments below.