Airflow Calibration Test Bench

A high-precision, modular 125mm wind tunnel designed for the calibration and validation of airflow meters. This rig serves as the absolute reference for the Zero-Pressure Airflow Meter project.

1. Project Overview

The Calibration Test Bench establishes a known, stable volumetric flow rate ($m^3/h$) by drawing air through a laboratory-grade Venturi nozzle. It allows the user to set a fixed fan power and use the resulting flow as a "Golden Standard" to calibrate secondary measurement devices.

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2. Hardware & Electronics

The rig uses an ESP32-WROOM to process environmental data and differential pressure, calculating the reference flow in real-time. To eliminate extreme static underpressure when testing restricted valves, the rig utilizes a coordinated dual-fan setup.

Components

• Sensor 1: SDP810-500PA (Differential pressure across the Venturi).
• Sensor 2: SDP810-125PA (Differential pressure at the inlet of the Venturi).
• Sensor 3: BME280 (Ambient temperature, humidity, and barometric pressure).
• Display: 1.3" OLED (SH1106) for real-time reference data.
• Encoder: Rotary encode with pushbutton for speedcontrol and menu navigation.
• Actuator: 2 x Arctic S12038-8K High-Static Pressure Fan (12V PWM).

ESP32 Pinout

Component	Function	ESP32 GPIO	Notes
I2C Bus 0	Data / Clock	21 / 22	BME280, OLED, SDP810 (Venturi)
I2C Bus 1	Data / Clock	25 / 26	SDP810 (Balance-Pressure Sensor)
Encoder	CLK / DT / SW	18, 19, 5	Speed control, Menu & Calibration
PWM Pull Fan	Speed Control	27	25kHz PWM signal
PWM Push Fan	Balance Control	14	25kHz PWM signal

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3. Mechanical Design

Modular 125mm Construction

The rig is built from 125mm diameter sections, optimized for standard ventilation parts:

1. Inlet Interface: A 500x500mm mounting panel for airextract valves.
2. Push Drive Section: A high-power server fan capable of overcoming the resistance of filters and valves.
3. Flow Conditioning: Dual honeycomb straighteners to eliminate swirl and ensure a laminar profile before the Venturi.
4. The Venturi Core: A 3-part 3D-printed assembly ($D=120\text{mm}$, $d=90\text{mm}$).
5. Pressure Sensing: Dual piezometric (averaging) rings at the inlet and throat for stable readings.
6. Pull Drive Section: A high-power server fan capable of overcoming the resistance of filters and valves.
7. Outlet Interface: A 500x500mm mounting panel for airsupply valves.

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4. Active Static Pressure Balancing

The Test Bench features an advanced "Active Balance" control loop to ensure measurement linearity regardless of inlet restriction.

• Primary Loop (Flow Control): The Exhaust (Pull) fan is set to a specific PWM or RPM to achieve the target flow rate.
• Secondary Loop (Pressure Balance): A dedicated SDP810 sensor monitors the static pressure difference between the Venturi inlet and the ambient room.
• Active Compensation: A PID controller adjusts the Inlet (Push) fan in real-time. If a restrictive valve causes a pressure drop, the Push fan increases power until the internal static pressure returns to 0.0 Pa (Atmospheric Neutral).

Result: The Venturi operates in a "Stagnation-Free" zone, eliminating the "Negative Delta-P" phenomenon and ensuring laboratory-grade accuracy even at >90% valve restriction.

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5. Calibration Physics

The reference flow $Q$ is calculated using the ISO 5167 standard principles, corrected for the actual air density at the time of measurement.

Air Density ($\rho$)

Density is calculated using the Magnus-Tetens formula to account for water vapor:

$$\rho = \frac{p_{dry}}{R_d \cdot T} + \frac{p_{vapor}}{R_v \cdot T}$$

Volumetric Flow ($Q$)

$$\text{Reference } Q = 3600 \cdot C_d \cdot A_{throat} \cdot \sqrt{\frac{2 \cdot \Delta P}{\rho \cdot (1 - \beta^4)}}$$

The discharge coefficient ($C_d$) is determined during initial rig validation (typically 0.975).

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6. Usage: Calibrating the Flow Meter

This rig is designed to be used in conjunction with the Zero-Pressure Airflow Meter interactive calibration modes.

Calibration Workflow:

1. Establish Reference: Run the Test Bench fan at a fixed power (e.g., 50%) and note the reference flow (e.g., $120.0 \text{ m}^3/h$).
2. Attach Device Under Test (DUT): Place the Zero-Pressure Flow Meter against the rig's inlet.
3. Zero-Compensation: Use the Flow Meter's Tune Zero-Comp menu to shift its setpoint until the Test Bench returns to exactly $120.0 \text{ m}^3/h$.
4. Flow Gain: Use the Tune Flow menu to match the Flow Meter's display to the Test Bench's reference value.

Traceability & Validation

The reference flow of this Test Bench is validated against a calibrated Testo 420.

1. A multi-point measurement was performed across the full PWM range of the fan.
2. The Discharge Coefficient ($C_d$) was empirically determined by comparing the Venturi $\Delta P$ to the calibrated reference flow.
3. This ensures that the Test Bench acts as a traceable "Golden Standard" for secondary devices.

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7. Manufacturing Notes

• Material: PETG (0.2mm layer height).
• Assembly: Parts are flanged for easy airtight coupling. The Venturi sections are bonded with epoxy to prevent leaks at the pressure rings.
• Precision: The internal surface of the Venturi should be sanded or printed with high precision to maintain a consistent $C_d$.

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License

MIT License - Created for the open-source ventilation community.