openQCM Q-1 · Benchtop QCM with dissipationAsk the crystal, point by point.

openQCM Q-1 does not wait for the crystal to oscillate: it questions it. A synthesiser sweeps a band of frequencies across the resonance and, at every step, the instrument reads how much signal passes and with what phase. From the shape of that curve come frequency and dissipation, one overtone at a time.

openQCM Q-1 in the lab, connected to a peristaltic pump, with the resonance curve and the frequency trace on the screen

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Fig. 1 · One sweep, one curveMeasured data

Overtone

Medium

View

f —bandwidth —D —fₚ − f —

Real sweeps, 1 Hz steps (plotted every 40th point), measured with the openQCM electronics (AD9851 synthesiser and AD8302 gain-phase detector, the same in Q-1), 5 MHz AT-cut crystal, cell at 25 °C, 11 September 2026. Frequency, bandwidth and D computed with the instrument software's own chain: the detector readings are converted to the crystal's resonance curve (the effect of its parallel capacitance largely removed: the residual asymmetry, larger in liquid and on high overtones, shows where the correction is incomplete), whose maximum gives f and whose width at a calibrated level below the maximum gives the bandwidth (drawn here at half height, its equivalent); D = bandwidth / f. Reading f at the centre of the half-height points instead of the maximum would differ by up to ~50 Hz on n = 9 in water: a fit of the full curve is the more robust estimator. Absolute frequencies carry the reference oscillator's uncalibrated offset; differences between sweeps are reliable.

odd overtones, 5 MHz crystals: n ≤ 9
per overtone: ~1.0s
frequency SD in liquid, acceptance test: ≤ 2.0Hz
flow cell: ~50µl

The principle

A sweep, then a curve.

An AD9851 direct digital synthesiser excites the crystal at one frequency after another around the chosen harmonic, and an AD8302 detector measures gain and phase across it at every step. When the curve is complete, a peak-detection algorithm finds the resonance frequency; the width of the peak at a calibrated level below its maximum gives the dissipation. The crystal is interrogated passively, so its parameters are read without an oscillator circuit pulling on them.

01AD9851 synthesisersteps the excitation frequency across the peak
02Quartz crystalpasses more signal near resonance
03AD8302 detectorgain and phase across the crystal, at every step
04Microcontrollerstores the curve in a buffer
05Python softwarepeak → frequency; width → dissipation

Overtones

Choose the depth you look at.

Q-1 follows one harmonic per run: the fundamental or an odd overtone, up to the 9th with 5 MHz crystals and the 5th with 10 MHz crystals. Higher overtones probe a thinner slice of a soft film. The software lists the peaks found at calibration and you pick the one to follow; to sweep all of them in every cycle, there is NEXT.

How far the shear wave reaches into water, 5 MHz crystal

n = 1238 nm
n = 3138 nm
n = 5107 nm
n = 790 nm
n = 979 nm

Penetration depth δ = √(η / πfρ) for water at 25 °C (η 0.89 mPa·s, ρ 997 kg/m³), as in the rest of the site. Each overtone sees a thinner slice of the liquid, or of a soft film.

Calibration

Once per crystal.

Every time you change the sensor, a calibration sweep of about a minute and a half maps the crystal's peaks and the baseline of the electronics, corrected with a least-squares polynomial fit. Press start, wait about 30 seconds for the first data, and frequency, dissipation and temperature are saved to CSV, with optional raw sweep files: frequency, amplitude, phase.

~1.5 minCalibration sweep: peaks and baseline
StartChoose the fundamental or an overtone
~30 sFirst data, then “Measuring!”
CSVFrequency, dissipation, temperature

Fluidics

Pump in, or pump out.

The cell holds about 50 µl. A syringe pump usually pushes the liquid through it. With a peristaltic pump, the manual suggests placing the pump downstream so that it pulls the liquid from the reservoir through the cell: less pressure on the seal, less risk of leaks.

Materials

Pick the right window.

The wetted parts are the window and an FKM (Viton) O-ring. PMMA windows are clear but swell and dissolve in many organic solvents, ethanol included: with organic solvents, use a PTFE window.

Your liquid	PMMA window	PTFE window
Water, buffers	Yes, and you can see through it	Yes
Ethanol and other organic solvents	No: it swells and dissolves	Yes

Validation

What every unit has to pass.

Each assembled unit is tested with a 10 MHz AT-cut crystal, at least 30 minutes in air and 35 minutes in static deionized water: frequency standard deviation ≤ 2.0 Hz in liquid, dissipation standard deviation ≤ 0.2 × 10⁻⁶ in air and ≤ 0.5 × 10⁻⁶ in liquid, equilibrium in about 10 and 20 minutes.

Acceptance test, 10 MHz crystal, fundamental	Air	Static liquid
Frequency standard deviation	—	≤ 2.0 Hz
Dissipation standard deviation	≤ 0.2 × 10⁻⁶	≤ 0.5 × 10⁻⁶
Typical equilibrium time	~10 min	~20 min

Specifications

The data sheet.

From the official manual. Where two documents disagree, the figure is left out until it is confirmed.

Sensor

Crystals: 5 and 10 MHz, 14 mm, wrapped electrode (single-side contact)
Flow cell: ~50 µl
Wetted parts: PMMA or PTFE window, FKM (Viton) O-ring

Measurement

Quantities: Frequency and dissipation
Method: Scalar network analysis: AD9851 DDS sweep, AD8302 gain and phase detector
Overtones: One at a time; up to the 9th (5 MHz) or 5th (10 MHz)
Sampling: ~1.0 s per overtone
Temperature: MCP9808 in the sensor module (monitoring)