Phosphine (PH₃) is the most important fumigant for pest control in the food industry. From the active fumigation phase with concentrations up to 2,000 ppm to monitoring residual concentrations during degassing, phosphine measurement places high demands on selectivity, long-term stability, and low maintenance. Fresenius Umwelttechnik demonstrates in this article how the patented two-pressure reference method meets these requirements – without continuous calibration during operation.

Phosphine Fumigation: Why Precise Process Monitoring is Crucial

In the food industry, controlled fumigation with phosphine is an indispensable method for protecting stored goods from pest infestation. Grains, nuts, flour, spices, cocoa, and coffee are treated with PH₃ in silos, bunkers, or containers – typically at concentrations of 200 to 1,500 ppm over a period of several days.

The precise monitoring of PH₃ concentration during fumigation is indispensable for two reasons: Firstly, it must be ensured that the required effective concentration is maintained throughout the fumigation period – underdosing leads to ineffective control. Secondly, measurement after fumigation serves for release monitoring: Only when the residual concentration has dropped sufficiently may storage areas be re-entered.

The occupational exposure limit (OEL) for phosphine is 0.1 ppm (TRGS 900). Our GA210 PH₃ analyzer is designed as a process monitoring system for the fumigation phase and supports release measurement down to a detection limit of 2 ppm. For occupational safety-related release according to OEL (≤ 0.1 ppm), supplementary measurement methods are required (e.g., electrochemical personal protection sensors or detector tubes). We would be happy to advise you on the optimal combination. Chat with our AI agent Tory for more information.

Why NDIR? Selectivity and Longevity in Comparison

For phosphine process monitoring in the concentration range up to 2,000 ppm, electrochemical sensors (EC) and NDIR analyzers are generally suitable. However, the differences in practical suitability are considerable:

The decisive advantage of NDIR technology lies in the physical nature of the measurement principle: phosphine absorbs infrared radiation at a characteristic wavelength. The measurement signal is generated contactlessly – there is no chemical consumption, no electrolyte aging, and no sensor wear.

The Fresenius Measurement Principle: Two-Pressure Reference Method

For phosphine measurement, Fresenius Umwelttechnik uses the patented two-pressure reference method – a measurement principle that significantly increases long-term stability compared to conventional NDIR systems.

Operating Principle

The method uses an elegant physical approach: the sample gas itself serves as its own reference. In two successive measurement phases, the gas is measured in the same cuvette at two different pressures:

• Phase 1 – Normal Pressure: The sample gas is in the cuvette at a defined pressure, and IR absorption is recorded.
• Phase 2 – Reduced Pressure: The pressure in the cuvette is reduced to approx. 200 mbar absolute. Since the molecular concentration decreases with pressure, the absorption also changes.

The PH₃ concentration is precisely calculated from the ratio of the two absorption values.

Why is this method so drift-stable?

Since both measurements take place successively in the same cuvette with the same detector, all slow changes – source aging, detector drift, contamination of optical surfaces – affect both measurements equally. These influences are completely eliminated when forming the quotient. In addition, the cuvette is gold-plated on the inside to ensure maximum reflection and long-term stability of the optical surfaces.

The result: A long-term drift of ≤ 3% per year – without any zero point or span correction during operation. No reference gas, no second optical path, no filter change. The two-pressure method eliminates drift structurally, not by compensation.

Our Solution: GA210 PH₃ – Technical Data

The Gas Analyzer GA210 from Fresenius Umwelttechnik in PH₃ configuration is specifically designed for process monitoring of phosphine fumigation:

Note on measurement cycle: The 21 seconds correspond to the complete two-pressure measurement cycle (measurement at normal pressure + measurement at reduced pressure + pressure equalization). Each cycle provides a fully drift-compensated measured value.

Temperature-Stabilized Measuring Cell: Why 50 °C is Crucial

The measuring cell of the GA210 PH₃ is actively temperature-stabilized to 50 °C. In the practice of food storage – with fluctuating temperatures in unheated warehouses, silos, and bunkers – this is a decisive factor:

• Condensation-free: At 50 °C, the measuring cell is significantly above typical dew points. Moisture precipitation and associated measurement falsifications are excluded – especially important in humid grain storage facilities.
• Constant measurement conditions: Regardless of day-night fluctuations or seasonal temperature variations, identical conditions always prevail in the measuring cell.
• Reproducible gas kinetics: Absorption and desorption processes on the cell surfaces proceed reproducibly at constant temperature. Memory effects – particularly relevant for PH₃ due to the molecule’s reactivity – are minimized.
• Minimized cross-sensitivities: Temperature-dependent interferences from accompanying gases are eliminated.

Sampling and Gas Routing: Stainless Steel is Mandatory

Phosphine is a reactive molecule that interacts with many plastics – through adsorption on surfaces and chemical reactions. At concentrations up to 2,000 ppm and the typical alternation between fumigation phase (high concentration) and degassing (low residual concentration), plastic lines lead to carry-over effects and falsified measured values.

Our recommendation: Sample gas lines made of 316L stainless steel for the entire sampling path. Stainless steel is chemically inert to phosphine and minimizes memory effects – crucial for precise measurements during the transition between high and low concentrations.

Further Components of Sample Gas Conditioning

• Particle filter: Stainless steel sintered filter or PTFE membrane filter (2–5 µm) to protect optical components from dust – especially important with grain dust.
• Pressure stabilization: In case of fluctuating ambient pressures in large silos, we recommend installing a pressure regulator before the analyzer to keep the inlet pressure stable within the compensation range (0.7–1.1 bar abs.).
• Sample gas pump: The internal pump delivers approx. 4 l/min and ensures rapid gas exchange in the measuring cell.

Calibration: Why Almost No Effort is Required

One of the most compelling advantages of the two-pressure reference method over conventional NDIR systems and electrochemical sensors is the minimal calibration effort.

During operation, no zero point and no span correction is required. The system self-references with each 21-second measurement cycle – every single measured value is automatically drift-compensated.

Nevertheless, we recommend annual maintenance and calibration with certified test gas as a quality assurance measure. Since a drift of up to 3% per year is possible even with the two-pressure method, the annual test confirms the measurement accuracy and documents traceability to national standards.

Comparison: Conventional EC sensors require calibrations weekly to monthly and sensor replacement every 1–2 years. The difference in life cycle costs is considerable.

Integration into Process Control Systems

The GA210 PH₃ offers a 4–20 mA analog output as standard for direct connection to PLC systems. Optional fieldbus interfaces (Profinet, Profibus, Ethernet/IP, Modbus TCP) are available, enabling seamless integration into existing process control systems.

Typical integration scenarios:

• Fumigation monitoring: Continuous recording of PH₃ concentration during the fumigation phase – as proof of correct fumigation duration and concentration.
• Degassing monitoring: Tracking the concentration drop after fumigation down to the system’s detection limit (2 ppm). Supports the decision for release.
• Automation: Control of ventilation systems or shut-off valves based on real-time measured values.
• Documentation: Complete data recording for quality assurance (IFS, BRC) and regulatory evidence.

Practical Application Examples

Grain Storage and Large Silos

Monitoring of wheat, barley, corn, and other grain fumigation. Continuous measurement during the 5–7 day fumigation phase and subsequent degassing. Typical concentrations: 200–1,000 ppm during fumigation, drop to NDG during degassing.

Nut and Dried Fruit Storage

Fumigation monitoring of almonds, walnuts, hazelnuts, and dried fruits in containers and warehouses. Special challenge: High fat content can absorb phosphine – precise measurement prevents underdosing.

Flour and Spice Mills

Monitoring of phosphine concentrations in production areas during and after fumigation. Integration into ventilation control for automatic ventilation.

Cocoa and Coffee Storage

Fumigation monitoring of raw cocoa and raw coffee. Compliance with strict maximum residue levels according to Regulation (EC) No. 396/2005. Documentation for quality certification and export evidence.

Standards and Regulations

• TRGS 512: Fumigations – Activities with fumigants and pesticides in rooms and on goods.
• TRGS 900: Occupational Exposure Limits – PH₃ OEL: 0.1 ppm (shift average).
• Regulation (EC) No. 396/2005: Maximum residue levels for pesticides in food.
• GefStoffV § 6: Information gathering and risk assessment.

Note: The GA210 PH₃ is a process monitoring system for the fumigation phase (measuring range 0–2,000 ppm, detection limit 2 ppm). It is not designed for occupational safety-related OEL monitoring at 0.1 ppm. For personal protection monitoring, we would be happy to advise you on supplementary solutions.

Cost-effectiveness: Less Effort, More Availability

The investment in NDIR measurement technology with the two-pressure reference method pays for itself over the device’s lifespan through several factors:

• No sensor replacement: While electrochemical sensors need to be replaced every 1–2 years, the NDIR system operates wear-free for many years.
• Minimal calibration effort: Annual maintenance and calibration instead of regular test gas calibrations save material costs and personnel time.
• High availability: No downtime due to sensor replacement or frequent recalibration.
• Process optimization: Precise measurement enables optimized control of fumigation. Both overdosing and underdosing are avoided, which saves phosphine costs and increases process reliability.

Over a 10-year period, significantly lower total cost of ownership is achieved compared to electrochemical solutions – despite higher acquisition costs.

Drift-Free Phosphine Measurement for the Food Industry

Phosphine monitoring in the food industry requires a measurement system that delivers stable measured values over months and years – without continuous calibration and without sensor wear. The two-pressure reference method of the GA210 PH₃ fulfills this requirement: Every single measured value is automatically drift-compensated, the temperature-stabilized and gold-plated measuring cell ensures condensation-free operation, and the stainless steel gas routing minimizes carry-over effects of the reactive PH₃ molecule.

Are you planning phosphine monitoring for your fumigation system? Contact us – we will work with you to develop the optimal measurement concept for your application, including sampling, system integration, and documentation for IFS/BRC certification.

Frequently Asked Questions (FAQ)

What is the two-pressure reference method and why is it so drift-stable?

The two-pressure reference method measures the same gas successively at two different pressures in the same cuvette with the same detector. Since all drift influences (source aging, detector drift, contamination) affect both measurements equally, they are eliminated when forming the quotient. The resulting long-term drift is ≤ 3% per year – without any calibration during operation.

Can the GA210 PH₃ monitor the occupational exposure limit (0.1 ppm)?

No. The GA210 PH₃ is designed as a process monitoring system for the fumigation phase (measuring range 0–2,000 ppm, detection limit 2 ppm). The OEL for phosphine is 0.1 ppm (TRGS 900) and cannot be measured with this system. Supplementary methods are required for occupational safety-related release. We would be happy to advise you on the optimal combination.

How often does the GA210 PH₃ need to be calibrated?

No calibration is required during operation – the two-pressure reference method self-references with each measurement cycle. We recommend annual maintenance and calibration with certified test gas as a quality assurance measure.

Why stainless steel lines instead of PTFE?

At higher concentrations, phosphine is reactive with many plastics. Adsorption effects on plastic surfaces lead to carry-over and falsified measured values – especially when transitioning from high fumigation concentrations to low residual concentrations. 316L stainless steel is chemically inert to PH₃.

Why is the temperature-stabilized measuring cell at 50 °C important?

Active temperature control to 50 °C ensures condensation-free operation (no moisture falsification), constant measurement conditions regardless of ambient fluctuations, reproducible gas kinetics without memory effects, and minimized temperature-dependent cross-sensitivities.

For which foods is phosphine fumigation used?

Typical applications: Grains (wheat, barley, corn), nuts (almonds, walnuts, hazelnuts), dried fruits, flour, spices, raw cocoa, and raw coffee. Fumigation takes place in silos, bunkers, containers, or warehouses.