Delta-T Devices Ltd.

- Model AP4 - Leaf Porometer


AP4 Porometer, Direct readout of stomatal conductance or resistance, Simple absolute calibration in the field, Minimises leaf stress during measurement, Award-winning user interface. Stomatal aperture is the dominant factor in the diffusion conductance of leaf surfaces, which controls both the water loss from plant leaves and the uptake of CO2 for photosynthesis. Measurements of diffusion conductance are therefore important indicators of plant water status and provide a valuable insight into plant growth and plant adaptation to environmental variables.


The AP4 Leaf Porometer measures diffusion conductance by comparing the precise rate of humidification within a small cuvette (chamber) to readings obtained with a calibration plate. The plate has 6 diffusion conductance settings whose values have been accurately determined by finite element analysis.

Quoted accuracy figures for other porometers and gas analysis systems are based on time-consuming laboratory set-up and calibrations which bear little comparison to field conditions. In contrast, the AP4 Leaf Porometer features simple direct calibration in the field against a tested physical standard.

The AP4 Leaf Porometer has many other features designed to ensure that accurate, reproducible readings can be taken as easily as possible:

  • Sophisticated temperature compensation
  • Unstirred leaf chamber minimises unwanted stomatal closure
  • Lightweight ergonomic sensor head
  • Large clear LCD display (8 lines by 40 characters)
  • Full QWERTY keypad for annotating up to 1500 readings
  • A rugged and reliable tool for phenotyping projects
  • Over 2,000 Delta-T porometers in use worldwide

Porometer design collaboration with Prof John Monteith
Delta-T Devices worked closely with Professor John L. Monteith (1929 - 2012) over a period of four decades - from manufacture of the world's first automatic porometer in the 1970s, through several subsequent versions - leading to the current AP4 Porometer.

Delta-T Devices Founder, Ed Potter, shared a platform with a number of speakers to pay tribute to Professor John L. Monteith at the 2013 American Society of Agronomy Annual Meeting in Tampa, Florida. The talk included a history of the development of the automatic porometer.


  • Resolution varies with the magnitude of the value obtained. The range shown corresponds to the reading range. In relative terms, the resolution is better than 2%, but at least the smallest amount shown.
  • The stated accuracy applies over the range of the calibration plate and for optimum cup conditions, i.e. from +10 to -5 °C difference between actual cup temp. and that existing at calibration, and for +2.5 to -2.5 °C difference between leaf and cup temperatures.
  • Spectral and cosine responses are approximate only.
  • Ambient pressure may be read from a Wristwatch Barometer type PBR1.
  • Cycling at extreme combinations of temperature, conductance and RH level may not always be possible.

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