GE Signa 1.5T HDe: doubtful helium level reading
What happened
Helium level reads 4.15 (about 4%) as reported in the case, a value the engineers distrusted and suspected of being a false reading.
What was checked
- Re-measure the helium level through the adjacent cryostat connector (designated 403 in the case) to get a second, independent reading.
- Check the cryostat pressure: as reported in the case, about 5 psi together with a true level near 0% corresponds to roughly 400 L of helium still at the bottom.
- Substitute a ~230 Ω resistor (as used in the case) for the MagMon sensing chain — a healthy chain should read about 70%.
- Check the continuity of the sensor circuit inside the cryostat.
Cause
Not proven in the case: either the helium was genuinely down to a few percent, or the MagMon chain was giving an unreliable reading. Since the second connector and the cryostat pressure both agreed with the low value, the engineers judged a genuinely low level to be probable and the sensor to be likely fine.
The cause is probable and was not fully confirmed.
What was done
Diagnosis only, as reported: the checks pointed to a real low level, so the sensor was not condemned and the outcome was judged probable — the system was handed over for an authorised helium refill. Had the resistor test pointed to the MagMon instead, the repair would be replacement of the sensing chain per the manufacturer's service documentation, done by authorised personnel.
Safety
A near-empty cryostat has little thermal margin before a quench: keep ferromagnetic tools and test equipment out of the fringe field, do no cryogenic work without the manufacturer's precautions, and remember that vented helium can displace oxygen in the room.
Lesson: Cross-check a doubtful helium gauge with an independent measurement — the second connector and the cryostat pressure — before deciding whether to refill the magnet or replace the sensor.
Covered in the course: MRI Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.