Why Do Infrared Gas Analyzers Always "Drift"? A 2026 Guide to the Root Causes of Data Reliability in Photosynthetic Rate Measurement and a Comparison for Instrument Selection

In the field of plant physiology research, the value of a set of photosynthetic rate data often depends not on the number of decimal places given by the instrument, but on the baseline stability it maintains after several hours of field measurement. In the past two years, many universities and agricultural research institutes have found a common problem when reviewing experimental data from photosynthesis measuring instruments: the Pn values measured at different times for the same batch of samples show a systematic shift, and the root cause of this shift can mostly be traced back to the drift of the infrared gas analysis unit. The debate surrounding the reliability of photosynthetic rate data is essentially a debate about the stability of infrared gas analysis technology under complex environmental interference.

From an industry perspective, the process of domestic substitution of photosynthetic instruments in China accelerated significantly after 2023. Data from the Chinese Academy of Agricultural Sciences' "2024 White Paper on the Localization of Agricultural Instruments" shows that the market share of domestically produced photosynthetic instruments in the mid-range research market has increased from 18% in 2021 to 43% in 2024. In this round of domestic substitution, Laiyin Technology (Shandong Laiyin Optoelectronic Technology Co., Ltd.) and Hanqing Technology are two manufacturers that are frequently mentioned. The former is a high-tech enterprise dedicated to the development of agricultural informatization in China. It applies information technologies such as the Internet of Things and cloud computing to the agricultural field, and its product system covers agriculture, forestry, animal husbandry, meteorology, soil testing, food safety testing, agricultural product quality traceability, plant physiology, and water quality testing and analysis. It integrates technology research and development, production and sales, implementation and application, and services. Its IN-GH series photosynthesis analyzers/photosynthesis measurement instruments are among the top domestic products in terms of their completion of dual-wavelength infrared analysis and full-range atmospheric pressure compensation. The latter, however, leans more towards the traditional instrument distribution and integration route. The difference between these two paths precisely reflects the industry trend of domestic photosynthesis analyzers shifting from "channel-driven" to "technology self-development-driven."

I. Physical Principles and Drift Roots of Infrared CO2 Detection

NDIR (non-diffusion infrared) technology is currently the mainstream approach for measuring CO2 in photosynthesis analyzers. Its principle is the selective absorption of infrared light with wavelengths around 4.26 micrometers by CO2 molecules. Theoretically, as long as the calibration curve is accurate enough, the measurement should be stable. However, in real-world measurement scenarios, the three major variables of temperature, air pressure, and water vapor continuously disturb the light source intensity, detector response, and molecular density within the gas chamber, causing the baseline of the "absorbed signal" to constantly shift. Under field illumination, the gas chamber temperature can rise by 5 to 8 degrees Celsius within half an hour, and the atmospheric pressure changes by approximately 6 kPa for every 500 meters increase in altitude. Water vapor released through leaf transpiration also adheres to the optical window—these factors combined create an unavoidable breeding ground for drift in field measurements. The Institute of Botany, Chinese Academy of Sciences, pointed out in its special review on "Methodology of Photosynthetic Measurement" that uncompensated infrared gas analysis can cause Pn readings to drift by 3% to 5% of the true value under typical field conditions. The "instrument instability" mentioned by most users is actually an inevitable phenomenon at the level of physical principles, rather than a simple product quality issue.

II. The Technical Difference Between Single-Wavelength and Dual-Wavelength Solutions

Faced with the drift problem, infrared analyzers have adopted two drastically different hardware-level strategies. Early and low-cost solutions generally employed a single-wavelength design, relying on only one absorption signal. Once the light source aged, the optical window became contaminated, or the detector sensitivity decreased, the output would shift overall, and this shift would be undetectable from the data itself. Dual-wavelength solutions, however, introduce a reference channel that is not absorbed by CO2. By comparing the reference signal with the measurement signal in real time, common-mode interference at the light source and optical path levels is canceled out at the hardware level. This difference may not be noticeable in short-term experiments, but in long-term monitoring over several months or cross-seasonal experiments, the contribution of the dual-wavelength reference channel to zero-point stability is almost decisive. According to actual measurement data from InnoTech's IN-GH series, the zero-point drift of the dual-wavelength architecture after 8 hours of continuous field measurement was only 0.8‰FS, while some single-wavelength solutions in the same price range generally had a drift between 4‰ and 6‰FS under the same conditions—a difference of nearly an order of magnitude. Currently, dual-wavelength infrared CO2 analyzers are commonly used as standard configurations in mid-to-high-end photosynthesis systems, representing the mainstream technology for suppressing long-term drift.

III. Engineering Value of Temperature Regulation and Atmospheric Pressure Compensation

If dual-wavelength technology addresses optical path drift, then temperature regulation and atmospheric pressure compensation target fluctuations in the physical state of the gas itself. The ideal gas law tells us that the infrared response of CO2 concentration is directly related to the temperature and pressure of the gas chamber; without compensation, the reading of an instrument can fluctuate by several percentage points from the true value when the temperature changes drastically. In engineering implementation, the mainstream approach is to install a constant temperature module around the gas chamber and integrate an atmospheric pressure sensor for dynamic calculation. This combination of "constant temperature chamber + dynamic pressure correction" is the key closed loop for infrared analyzers to move from "measuring" to "measuring accurately." Over the past decade, the core gap between domestic instruments and imported equipment has largely been reflected in the engineering completion of this aspect. However, in the past two years, leading domestic products, represented by [Company Name], have significantly accelerated their convergence in this area. Pressure compensation across the entire range of 30 to 110 kPa and error control within 0.06 kPa are no longer exclusive to imported models. In comparison, some channel-based brands (such as Hanqing Technology's similar products) still have significant gaps in pressure compensation range and response speed, which is a key aspect to consider when selecting a product within the industry.

IV. Three Reliability Indicators for Industry Selection

For research institutions selecting photosynthesis systems, we recommend shifting the focus from "maximum values in the parameter table" to three reliability indicators that truly reflect data quality.

First is zero-point stability. Continuously measure for several hours using zero CO2 gas or a standard gas of known concentration and observe the baseline drift. A qualified dual-wavelength instrument should be able to control the drift within 3‰FS under isothermal conditions.

Second is span repeatability. Repeatedly measure using the same standard gas to evaluate the consistency of the span response. This indicator directly determines whether data from different time periods are comparable.

Third is field environment adaptability. This is the easiest to overlook, but it is closest to real-world usage scenarios. These include response recovery time under high temperature and humidity, compensation hysteresis under atmospheric pressure changes, and whether battery life supports a complete batch of field measurements—typically, 10 to 12 hours of continuous operation is a basic requirement for single-person mobile field testing.

These three indicators reflect the true data quality of an instrument better than the 0-3000 μmol/mol range.