Spray droplet size testing measures the size distribution of liquid droplets produced by nozzles, atomizers, pumps, and aerosol devices, delivering critical data for product performance, quality control, and regulatory compliance. This technical guide explains the complete spray particle size testing workflow—from sample preparation and instrument calibration to laser diffraction analysis and report issuance—in accordance with ISO 13320 and related international standards.
Technical Overview
Spray droplet size testing, also referred to as spray particle size analysis or atomization characterization, is a laboratory procedure used to quantify the size distribution of liquid droplets generated by spray nozzles, hydraulic atomizers, air-assisted sprayers, metered-dose devices, and pressurized aerosol systems. Because droplet size governs critical performance attributes—such as drift potential in agricultural spraying, respiratory deposition in inhalation products, evaporation rate in combustion systems, and coating uniformity in industrial finishing—accurate and repeatable droplet size measurement is essential for product development, quality assurance, and regulatory submission.
The most widely accepted technique for spray droplet size measurement is laser diffraction, standardized internationally under ISO 13320, with supporting data treatment and reporting practices described in ASTM E799 and ISO 9276. Results are typically reported as volume-weighted percentiles (Dv10, Dv50, Dv90), the Sauter mean diameter (D32), and derived distribution parameters such as span and uniformity. Accredited third-party laboratories perform these measurements under controlled environmental conditions using calibrated instruments and traceable reference materials, ensuring that reported data are defensible, comparable, and reproducible across organizations.
Testing Samples
Agricultural Spray Nozzles and Pesticide Formulations
Flat-fan, hollow-cone, full-cone, and air-induction nozzles used in crop protection are tested to determine their droplet size spectrum and to assign an industry-standard spray classification category (fine, medium, coarse, very coarse, or extremely coarse). Both water and specific pesticide tank mixtures may be evaluated at defined pressures and flow rates, since formulation properties such as viscosity and surface tension significantly influence atomization behavior.
Pharmaceutical and Consumer Aerosol Products
Metered-dose inhalers, soft-mist inhalers, nasal sprays, throat sprays, and cosmetic mist products are characterized to verify consistent plume geometry and respirable or depositable droplet fractions. Testing is performed using dedicated spray inhalation cells and actuation devices that simulate real-use conditions, including controlled actuation force, stroke length, and air flow.
Industrial Nozzles and Fuel Injection Components
Pressure swirl injectors, twin-fluid atomizers, fuel injectors, and coating spray guns are tested to optimize atomization efficiency, spray angle stability, and combustion or deposition performance across the operating envelope of pressure, liquid viscosity, and gas-to-liquid ratio.
Fire Suppression, Humidification, and Cleaning Systems
Water mist nozzles, humidifier atomizers, and cleaning-in-place spray heads are evaluated to confirm that the generated droplet spectrum meets the design intent, such as achieving fine mist for heat absorption or coarse droplets for surface wetting.
Testing Items
Depending on the product type and the applicable standard, the following parameters are routinely determined during spray droplet size testing:
- Dv10, Dv50, and Dv90 — the droplet diameters below which 10%, 50%, and 90% of the total spray volume is contained
- Volume median diameter (VMD), equivalent to Dv50 for volume-based distributions
- Sauter mean diameter (D32), the diameter of a droplet having the same volume-to-surface-area ratio as the entire spray
- Number-weighted mean diameter and number-based size distribution
- Span, calculated as (Dv90 − Dv10) / Dv50, indicating distribution width
- Uniformity coefficient and relative span for spray homogeneity assessment
- Percentage of spray volume within specified size bands, such as droplets smaller than 100 µm
- Spray classification category for agricultural nozzles
- Shot-to-shot or actuation-to-actuation variability for pharmaceutical sprays
Testing Methods
Laser Diffraction Method
Laser diffraction is the reference technique for spray droplet size measurement and is specified in ISO 13320. A laser beam passes through the spray plume, and droplets scatter light at angles inversely related to their size. The scattered pattern is captured by an array of detectors and converted into a size distribution using Mie theory (for transparent liquids with known optical properties) or Fraunhofer approximation (when optical properties are unknown). The technique covers a broad measurement range, typically from approximately 0.1 µm to 3000 µm, and delivers rapid, statistically robust results suitable for transient sprays.
Phase Doppler Interferometry Method
Phase Doppler interferometry, also known as phase Doppler anemometry, provides localized, point-by-point measurements of droplet size and velocity simultaneously. Two intersecting laser beams create a measurement volume; the phase shift between signals received by multiple detectors yields droplet diameter, while the signal frequency yields velocity. This method is particularly valuable for characterizing spatial variations within a spray plume and for two-phase flow research.
High-Speed Imaging and Dynamic Image Analysis
High-speed photography combined with automated image analysis captures individual droplet images, enabling direct visualization of droplet morphology, ligament formation, and breakup processes. This method is often used as a complementary or verification technique, especially for coarse sprays and transient events.
Testing Process
A typical spray droplet size testing project in an accredited third-party laboratory follows a structured workflow designed to guarantee traceability, repeatability, and data integrity:
Step 1: Inquiry and Standard Confirmation
The laboratory reviews the client's requirements, including the sample type, operating pressure, flow rate, test liquid, and intended use of the data. The applicable standard—such as ISO 13320, ASTM E799, or the relevant GB/T method for the Chinese market—is confirmed, and a formal quotation and test plan are issued upon acceptance.
Step 2: Sample Receipt, Inspection, and Conditioning
Upon arrival, samples are inspected for integrity and registered into the laboratory information system. Nozzles, actuators, and devices are checked for dimensional conformity, and test liquids are conditioned to the specified temperature, since viscosity and surface tension vary with temperature and directly affect atomization.
Step 3: Instrument Calibration and Verification
The laser diffraction analyzer is qualified and calibrated in accordance with ISO 13320. Verification is performed using certified reference materials—typically polymer microsphere standards traceable to national metrology institutes—and background measurements are recorded to confirm a clean optical path before testing begins.
Step 4: Test Setup and Spray Delivery Configuration
The spray rig is configured with the correct nozzle mounting, pressure regulation, flow control, and timing electronics. Pharmaceutical aerosols are mounted on automated actuation stations with synchronized triggering, while agricultural and industrial nozzles are operated at the agreed pressure points using calibrated pressure transducers and flow meters.
Step 5: Data Acquisition Across the Spray Plume
Measurements are performed at multiple positions along the spray axis and, where required, across radial traverses of the plume to capture spatial homogeneity. Each condition is replicated to demonstrate repeatability, and measurement quality indicators such as obscuration, data fit residual, and detector signal quality are monitored in real time against the acceptance criteria of ISO 13320.
Step 6: Data Processing and Technical Review
Raw scattering data are converted into size distributions using validated optical models. Statistical parameters (Dv10, Dv50, Dv90, D32, span) are calculated per ISO 9276 representation rules, replicate variability is evaluated, and results undergo independent technical review by qualified engineers before release.
Step 7: Report Issuance and Archival
A comprehensive test report is issued containing sample identification, test conditions, instrument configuration, calibration status, full size distribution tables, key statistical parameters, and statements of conformity where applicable. All raw data and records are archived to ensure full traceability and support any subsequent audit or regulatory submission.
Applications
Spray droplet size testing data support a wide range of engineering, quality, and regulatory decisions across multiple industries:
- Crop protection: selecting nozzle and pressure combinations that balance biological efficacy against off-target drift risk
- Pharmaceutical development: demonstrating consistent delivered dose and droplet size for inhalation and nasal products during formulation and device development
- Automotive and aerospace: optimizing fuel injector atomization for combustion efficiency and emission reduction
- Surface coating and cleaning: verifying droplet spectra that determine transfer efficiency and coating quality
- Fire protection: qualifying water mist systems against design droplet size criteria
- Consumer products: ensuring uniform mist delivery and user experience for cosmetic and household sprays
- Research and development: characterizing new atomizer geometries and two-phase flow phenomena
FAQ
How long does spray droplet size testing take?
Standard testing of a single nozzle or device at several operating points is typically completed within five to ten working days after sample receipt, depending on the number of conditions, replicates, and the complexity of the test plan. Projects involving method development or pharmaceutical actuation simulation may require additional time for protocol alignment.
What sample and information should be provided for testing?
Clients should supply representative nozzles or finished devices, sufficient test liquid (or specify that standard substitute liquids are to be used), and key operating parameters such as pressure range, flow rate, actuation conditions, and any applicable standard or specification. Clear declaration of flammability and toxicity is required for safe handling.
Why can Dv50 results differ between laboratories?
Differences typically arise from variations in measurement distance from the nozzle, spray rig geometry, test liquid properties, temperature, optical model assumptions, and instrument configuration. Aligning these parameters with ISO 13320 recommendations and documenting them precisely in the test report minimizes inter-laboratory variation.
Can volatile, flammable, or viscous liquids be tested?
Yes. Specialized enclosed spray enclosures, inert gas purge systems, and heated delivery lines allow the safe testing of volatile and flammable liquids as well as heated viscous fluids, provided that adequate safety declarations and handling procedures are in place.
Is laser diffraction suitable for all spray types?
Laser diffraction covers the majority of industrial and pharmaceutical sprays with droplet sizes from submicron to several millimeters. For very coarse, dense, or spatially structured sprays, phase Doppler interferometry or high-speed imaging may be recommended as complementary methods, and an accredited third-party laboratory can advise on the most appropriate technique for a specific application.