Smart Inhaler Flow Sensor | mysimulator.uk

Patient Effort

Inspiratory effort ΔP 4.0 kPa

Ramp time to peak 0.6 s

Device Resistance

Idle. Press Inhale to trigger a breath.

Sensor Readouts

Peak flow (PIFR)

0 L/min

Lung deposition

0.0%

Oropharyngeal deposition

0.0%

How it works

A dry-powder inhaler (DPI) has no propellant — the patient's own inhalation pulls air through a fixed internal resistance R, and that turbulent airflow is what breaks the powder into breathable particles. The onboard flow sensor measures the pressure drop and derives flow rate from the classic DPI resistance equation:

ΔP = (R · Q)²   ⇒   Q = √ΔP / R

ΔP  = patient inspiratory effort (kPa)
R   = device resistance constant (√kPa per L·min⁻¹)
Q   = flow rate through the mouthpiece (L/min)

The sensor only lets the dose actuate once flow crosses a trigger threshold Q_trig ≈ 30 L/min — too weak a breath never releases the dose. Once released, the delivered particles split three ways, governed by two competing, flow-dependent effects:

η_deagg(Q) = Q² / (Q² + Qc²)          Qc ≈ 20 L/min  (turbulent deaggregation)
η_oro(Q)   = min(0.6, (Q / 120)²)                     (inertial throat impaction)

Lung (fine-particle) fraction   = η_deagg · (1 − η_oro)
Oropharyngeal deposition        = η_oro
Exhaled / not deposited         = remainder

Too little effort — flow stays low, powder never fully deaggregates, most of the dose is exhaled as large lumps.
Too much effort — flow is high enough to deaggregate the powder well, but inertial impaction throws a growing share of it against the throat instead of the lungs.
Device resistance R — a lower-resistance device (e.g. a Diskus-type) reaches a higher flow for the same effort; a higher-resistance device (e.g. a Handihaler-type) needs more effort to trigger but is gentler on throat deposition.

This deaggregation-vs-impaction trade-off is why real DPIs are engineered around a target PIFR window, and why inhaler coaching (and now onboard flow sensors) focuses on hitting a "firm, steady" breath rather than the hardest possible one.

Powder in device Throat deposition Lung deposition Exhaled / not deposited

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