Why does the replacement fan report a different speed (RPM)?

Three different problems look identical on the host display: a tach scaling mismatch (reporting error, real speed unchanged), a duty-map difference (real speed changed at the same command), or a genuinely different operating speed (real speed changed at the same rail). Diagnose in that order — each has a different fix.

Step 1 — rule out tach scaling

Compute expected RPM independently: count tach pulses with a meter for 30 seconds and convert using RPM = 60 × f / PPR. If the meter and the host disagree, the pulses-per-revolution of the candidate differs from the incumbent — a pure reporting mismatch. Fix by reconfiguring the host's conversion constant, or by selecting a candidate with the same PPR. The fan itself may be running perfectly.

Symptom	Meter vs host reading	Cause	Fix
RPM looks 2× or ½	Meter correct, host wrong	PPR differs	Host conversion constant
Speed shifts at same duty	Meter matches host	Duty→RPM map differs	Recalibrate control table
Speed shifts on rail only	Meter matches host	Load/rail difference	Check system curve ＆ voltage

Always carry a tach meter through qualification — the host's own display is the least trustworthy instrument in the loop.

Step 2 — compare duty maps

If real speed differs at the same PWM duty, the candidate's duty-to-RPM table differs. Record RPM at 20/50/100% duty on both fans. Options: accept and re-map the host's control table (common and cheap), or request a speed-matched variant from the candidate maker. Do not assume 50% duty means 50% speed on either fan — most maps are monotonic but not linear, and flatten near maximum duty.

Log duty→RPM at five points on both fans.
Re-map the host table if the shape is close; re-select if the floor differs.
Verify minimum-speed behaviour: below some duties many fans stall.

Step 3 — verify the operating point

If the candidate genuinely runs faster on the same rail at full duty, its P-Q curve is higher — which can change noise, and can even mean the incumbent was system-limited rather than speed-limited. Re-check installed flow or thermal result, not just RPM: a faster-spinning fan at the same operating point is not automatically better cooling. The thermal result on the unit is the acceptance criterion.

Need a fan matched to your operating point?

Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.

WhatsApp +86 135 7056 7086 · Email inquiry@cnmegatech.com · Custom fan development