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Enabling better global research outcomes in soil, plant & environmental monitoring.

PSY1 Stem Psychrometer

The PSY1 Stem Psychrometer is a very powerful tool integrating all the ambient environmental parameters acting upon the plant such as solar radiation, temperature, humidity, wind speed and water availability into a single continuously measurable variable.

The PSY1 is a stand-alone instrument for the measurement of stem water potential. It can continuously log changes in plant water status/potential, which directly reflect the energy required to access water or the stress the plant is under.


PSY1 Psychrometer Features

  • Stem, leaf and root water potential
  • Ambient environmental parameters integration
  • Hydraulic conductivity
  • Plant water relations
  • Growth potential
Optimal logging solution: Plant Water Potential


Units MPa
Range -0.1 MPa to -10 Mpa
(1 to 100 Bars)
Resolution 0.01 MPa (0.1 Bar)
Accuracy ±0.1 MPa (1 Bar)
Response Time Measurement mode: 51 seconds
Live mode: 1 second
Sampling Rate 10 Hz


Computer Interface USB, Wireless RF 2.4 GHz
Data Storage MicroSD Card
Memory Capacity Up to 16GB, 4GB MicroSD card included.

Operating Conditions

Temperature Range -10 to 50°C
R/H Range 0-99%


Logger Length: 170 mm
Width: 80 mm
Depth: 35 mm
Psychrometer Chamber Diameter: 25.5mm
Depth: 20mm
Depth with calibration chamber: 30mm



Instrument Logging

Analogue Channels 1x Input, (1x Sensor) High precision 24bit ADC circuit
1x Output – PSY Chamber Heater Circuit
Minimum Logging Interval 1 second
Delayed Start Suspend Logging, Customised Intervals
Sampling Frequency 10Hz


Communications USB, Wireless Radio Frequency 2.4 GHz
Data Storage MicroSD Card, SD, SDHC & SDXC Compatible (FAT32 format)
Software Compatibility Windows 7, 8, 8.1, 10 and Mac
Data Compatibility FAT32 compatible for direct exchange of SD card with any Windows PC and Mac
Data File Format Comma Separated Values (CSV) for compatibility with all software programs
Memory Capacity Up to 16GB, 4GB MicroSD card included.

Operating Conditions

Temperature Range -40°C to +80°C
R/H Range 0-100%
Upgradable User Upgradeable firmware using USB boot strap loader function
Internal Battery Specifications
960mAh Lithium Polymer, 4.20 Volts fully charged
External Power Requirements
Bus Power 8-30 Volts DC, non-polarised, current draw is 190mA maximum at 17 volts per logger
USB Power 5 Volts DC
Internal Charge Rate
Bus Power 60mA – 200mA Variable internal charge rate, maximum charge rate of 200mA active when the external voltage rises above 16 Volts DC
USB Power 100mA fixed charge rate
Internal Power Management
Fully Charged Battery 4.20 Volts
Low Power Mode 3.60 Volts – Instrument ceases to take measurements
Discharged Battery 2.90 Volts – Instrument automatically switches off at and below this voltage when no external power connected.
Battery Life varies
  • With a recommended power source connected, operation can be continuous.
  • 3 days at hourly logging interval without chamber heating
  • 1 day with chamber heating

About the PSY1 Stem Psychrometer

Thermocouple hygrometers, or psychrometers, of various designs have been used successfully in plant science research since the early 1950’s, most commonly used on a detached leaf sample. The PSY1 Stem Psychrometer developed by Professor Mike Dixon, University of Guelph, measures water potential in-situ and has been validated against Scholander-Hammel pressure bombs with excellent results and used in published research since 1984. Initially published as Dixon, M. A., & Tyree, M. T. (1984). A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb. Plant, Cell & Environment, 7(9), 693-697., the PSY1 Stem Psychrometer has since been used in many other studies.

PSY1-PWSC comparison

PSY1 Stem Psychrometer Comparison to Pressure Bomb – Courtesy George Koch, Northern Arizona University.

All temperature measurements (dT, Wet Bulb depression & chamber temperature) are then used in the determination of plant water potential. It can continuously log (at 10-minute temporal resolution) changes in the plant water status.

When simultaneously combined with the SFM1 Sap Flow Meter and Dendrometry Instrumentation the complete plant water relations and growth potential of the plant can be obtained, continuously monitoring ecophysiological change over time. The PSY1 Stem Psychrometer offers significant benefits over more common leaf psychrometers through the ease of attachment which minimises energy balance disruptions and improves measurement accuracy.

Instrument design

The PSY1 Stem Psychrometer head consists of two welded chromel-constantan thermocouples connected in series within a chromium plated brass chamber that forms a large insulating thermal mass.

PSY1 Chamber

Inside the chamber one thermocouple is in contact with the stem sample and the other simultaneously measures the chamber air temperature and subsequent to a Peltier cooling pulse, the set bulb depression. A third soldered copper-constantan thermocouple is located within the sample chamber body to measure the instrument temperature for the purpose of temperature compensation.

Within the chamber, the thermocouples are set so one is slightly above the surface of the sample chamber such that upon installation it is placed in contact with an exposed section of sapwood, while a second thermocouple remains within the sample chamber measuring the chamber air temperature. A Peltier cooling current is then applied to the junction, the differential output of the two junctions is a measure of the temperature gradient between the sample and the dew point measuring junction. By measuring the psychrometric (wet bulb) depression and applying automatic temperature correction of the error induced by temperature gradients within the chamber, precise and repeatable measurements of plant water potential are accurately obtained.

PSY Thermocouples


The PSY1 Stem Psychrometer is attached to the stem using a clamp to hold it in position using moderate pressure, and is then insulated against external temperature influences with additional insulation.

Small Stem PsychrometerWhen installing, insulation will effect the equilibration half-time and the quality of the results.

Equilibration Half-Time

The equilibration half-time for thermocouple psychrometers is varied. The range can extend from several minutes to several hours depending upon the design of the psychrometer. Variability stems from how accurately the differential temperatures are measured, whether the initial measuring junction and sample temperatures are measured or assumed and finally, how well the psychrometer is insulated from thermal gradients. The PSY1 Stem Psychrometer measures all temperatures and assumes nothing. With good insulation, equilibration half-times as short as 60 seconds can be achieved, making it a very rapid, repeatable and reliable instrument.

Instrument Configuration & Operation

All functions of the instrument’s operation and calculations are controlled by the microprocessor which automatically converts the analogue microvolt signals to a calibrated output. Programming variables such as; Peltier cooling pulse, duration & wait time; reverse Peltier warming, duration & wait time; Chamber heating, duration & period; measurement frequency and data logging options, are all held resident in non-volatile memory.

The PSY1 Stem Psychrometer displays information such as:

  • External Battery Status
  • Serial Number
  • Firmware Version
  • SD Card Status
  • Measurement Interval
  • Data File Logging Option
  • Calibration Factors

The utility software enables the instrument to be used in the manual mode. This provides the ability to perform lab based work with destructively sampled material, osmotic potential measurements or to evaluate the cleanliness or reliability of the chamber using the Plot Peltier Cooling Curve recording and plotting function.

Data Analysis

Data can be manually processed using a Spreadsheet such as Excel by opening the Comma Separated Values (CSV) file provided by the PSY1. The user can customise what values are logged in the data file choosing from all raw data parameters, processed stem Water Potential in MPa and the relevant calibration and correction factors used in the data processing. Regardless of the parameters chosen all are pre-processed into engineering units ready for interpretation and analysis.

Calibration Options

The PSY1 Stem Psychrometer requires calibration. Depending upon the level of accuracy required you can choose to utilise a generic batch calibration with coarse generalised accuracy or a specific chamber calibration. The specific chamber calibration can be requested from ICT International at the time of purchase and this calibration is at additional cost to the instrument. Alternatively, ICT International provides detailed calibration instructions and a calibration spreadsheet to enable the user to perform their own calibration at no extra cost.

It is recommended that a full 6-point calibration (0.1, 0.2, 0.3, 0.4, 0.5 and 1.0 Molal NaCl solutions) be performed every 6 to 12 months or immediately following a serious contamination and cleaning of the chamber. The need for a full calibration and/or change of calibration can then be determined using the integrated calibration function of the utility software. Calibrations should be performed both within the range of expected water potentials and at a range of expected temperatures. An extreme environment calibration range (1.2 to 2.0 Molal equivalent to -6 to -10 MPa) is included to facilitate this.

The calibration function automatically records raw wet bulb depressions, chamber temperatures and corrected wet bulb depression values for plotting against known solute potentials. The plot function includes each individual data point, r2 regression analysis and line, and the slope and intercept of the calibration curve. You can also choose to plot the current calibration against a historical calibration evaluating and comparing r2, slope and offsets of each. The calibration function is a very powerful and time saving feature. Multiple calibration files for a Psychrometer or calibration files for multiple Psychrometer chambers can be stored on the MicroSD card of the PSY1 and recalled for use with the specific chamber.

  • CH24 - 24 Volt Power Supply
    The CH24 is a 100 - 240Volts AC Mains to 24Volts DC power supply adapter; capable of outputting up to 2.5Amps. For most ICT Instruments.
  • MCC Mini
    The MCC Mini is a simple to use USB Serial to Radio Communications device providing a high level of integrity in data transfers. Its miniature design and minimalist approach make it an attractive solution for portable computers and less intrusive workstation setups where space and weight are of concern.
  • Data to the Web
    Wireless Communication Module - Includes; MCC Radio Frequency Logging Hub, Comms and ICT Data View Software, GSM/2G/3G modem, 3V 5Ah Lithium Polymer Battery, 11W solar panel, IP66 enclosure. 
  • Wireless Communication
    Wireless USB Radio communication device.
  • Wireless Data Collector
    Wireless data logger. 4GB SD Card storage. Communicates with any ICT International instrument. Includes SPBP07 power supply.
  • PSY Leaf Clamp
    Leaf clamp for use with the PSY1 Stem Psychrometer.
  • PSY-LC
    Large Clamp for the PSY1
  • PSY1 Installation Kit

Brodribb, T. J., Skelton, R. P., McAdam, S. A., Bienaimé, D., Lucani, C. J., & Marmottant, P. (2016). Visual quantification of embolism reveals leaf vulnerability to hydraulic failure. New Phytologist, 209(4), 1403-1409. doi:10.1111/nph.13846

De Groote, Stefanie. Impact of dew and rain on the water relations of the mangrove species Avicennia marina (Forssk.) Vierh. Diss. Master’s thesis, University Ghent, Faculty of Bioscience Engineering, 2013. View Paper

Dixon, M. A. (1987). In situ psychrometry for measuring cut flower water status. HortScience (USA). 22, 285-287.

Dixon, M., & Grace, J. (1982). Water uptake by some chamber materials. Plant, Cell & Environment, 5(4), 323-327.

Dixon, M. A., Grace, J., & Tyree, M. T. (1984). Concurrent measurements of stem density, leaf and stem water potential, stomatal conductance and cavitation on a sapling of Thuja occidentalis L. Plant, Cell & Environment, 7(8), 615-618.

Dixon, M., Graham, T., Bam, P., Kervin, J., Tran, N., Ping Zhang, , Reeves, B. and Downey, A., (2015). Reducing Drought Stress in Transplanted Trees Using Mycorrhizae©. Acta Hort. (ISHS) 1085:119-127 [PDF]

Dixon, M. A., & Tyree, M. T. (1984). A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb. Plant, Cell & Environment, 7(9), 693-697. http://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep11572454/abstract

Johnson, R. W., Dixon, M. A., & Lee, D. R. (1992). Water relations of the tomato during fruit growth. Plant, Cell & Environment, 15(8), 947-953.

Liu, N., Guan, H., Luo, Z., Zhang, C., Wang, H., & Zhang, X. (2016). Examination of a coupled supply-and demand-induced stress function for root water uptake modelling. Hydrology Research, nh2016173. DOI: 10.2166/nh.2016.173 http://hr.iwaponline.com/content/early/2016/02/18/nh.2016.173

Patankar, R., Quinton, W. L., & Baltzer, J. L. (2013). Permafrost‐driven differences in habitat quality determine plant response to gall‐inducing mite herbivory. Journal of Ecology, 101(4), 1042-1052.http://onlinelibrary.wiley.com/doi/10.1111/1365-2745.12101/abstract

Tran, N., Bam, P., Black, K., Graham, T., Zhang, P., Dixon, M., Reeves, B., Downey, A., (2014). Improving Irrigation Scheduling Protocols for Nursery Trees by Relating Cumulative Water Potential to Concurrent Vapour Pressure Deficit. Acta Hortic. 1085, 129-134. DOI: 10.17660/ActaHortic.2015.1085.22. [PDF]

Tran, N., Graham, T., Ping Zhang, Bam, P., Black, K., Reeves, R., Downey, A., and Dixon, M., (2014). Irrigation Management Strategies for Nursery Trees Based on Plant Water Status Measured with Automated Stem Psychrometers. Poster presented at the CSPB Conference, CSPB Conference – Irrigation Management Strategies for Nursery Trees

Vandegehuchte, M. W., Guyot, A., Hubeau, M., De Swaef, T., Lockington, D. A., & Steppe, K. (2014). Modelling reveals endogenous osmotic adaptation of storage tissue water potential as an important driver determining different stem diameter variation patterns in the mangrove species Avicennia marina and Rhizophora stylosa. Annals of botany, 114(4), 667-676. http://aob.oxfordjournals.org/content/early/2014/02/16/aob.mct311.full.pdf

Vandegehuchte, M. W., Guyot, A., Hubau, M., De Groote, S. R., De Baerdemaeker, N. J., Hayes, M., … & Steppe, K. (2014). Long-term versus daily stem diameter variation in co-occurring mangrove species: Environmental versus ecophysiological drivers. Agricultural and Forest Meteorology, 192, 51-58. http://www.sciencedirect.com/science/article/pii/S0168192314000628

Wang, H., Guan, H., & Simmons, C. T. (2016). Modeling the environmental controls on tree water use at different temporal scales. Agricultural and Forest Meteorology, 225, 24-35. http://www.sciencedirect.com/science/article/pii/S0168192316302477

Yang, Y., Guan, H., Hutson, J. L., Wang, H., Ewenz, C., Shang, S., & Simmons, C. T. (2013). Examination and parameterization of the root water uptake model from stem water potential and sap flow measurements. Hydrological Processes, 27(20), 2857-2863. http://onlinelibrary.wiley.com/doi/10.1002/hyp.9406/abstract