Siemens / GE / IGC MRI magnets: switch heater handling during magnet ramping
What happened
Engineers compared switch heater (superconducting switch) procedures during magnet ramping, because the correct algorithm differs between magnet manufacturers.
What was checked
- Ramp voltage check across the switch — reported around 9–11 V on the Siemens magnets.
- Switch heater current monitoring — reported around 110 mA on the IGC magnet.
- Allowance for magnet current overshoot after reaching field — reported at roughly 1.8–3 A depending on model.
Cause
Differences in magnet inductance, as reported in the case: the Siemens magnets (about 100 H) are largely self-heating from the ramp voltage itself, while the low-inductance GE magnet (about 11 H) needs the switch heater right through the end of the ramp. Continuous switch heating was reported to increase helium loss (about 4% per ramp cycle).
What was done
No repair was needed — the case confirmed that only the manufacturer's documented ramping procedure applies, done by authorised personnel: on the Siemens magnets the MPS drives the switch heater automatically, energising it at the start and at the end of the ramp; on the GE magnet the heater is switched on before ramping and kept on for the whole run-up to field, followed by the documented waiting period; on the IGC magnet the switch heater is operated manually for the entire ramp. Each magnet follows its own algorithm, not a universal one.
Safety
A ramped superconducting magnet stores huge energy, and a wrong switch-heater step can quench it with rapid helium venting — respect cryogen zones, vent lines and oxygen-depletion risk, and leave ramping to authorised personnel.
Lesson: The switch heater routine is part of the magnet design itself — never copy a ramping procedure from one manufacturer's magnet to another, always follow the algorithm in that magnet's own service documentation.
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.