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PSY1 Stem Psychrometer



Range-0.1 MPa to -10 Mpa
(1 to 100 Bars)
Resolution0.01 MPa (0.1 Bar)
Accuracy±0.1 MPa (1 Bar)
Response TimeMeasurement mode: 51 seconds
Live mode: 1 second
Sampling Rate10 Hz


Computer InterfaceUSB, Wireless RF 2.4 GHz
Data StorageMicroSD Card
Memory CapacityUp to 16GB, 4GB MicroSD card included.

Operating Conditions

Temperature Range-10 to 50°C
R/H Range0-99%


LoggerLength: 170 mm
Width: 80 mm
Depth: 35 mm
Psychrometer ChamberDiameter: 25.5mm
Depth: 20mm
Depth with calibration chamber: 30mm



Instrument Logging

Analogue Channels1x Input, (1x Sensor) High precision 24bit ADC circuit
1x Output – PSY Chamber Heater Circuit
Minimum Logging Interval1 second
Delayed StartSuspend Logging, Customised Intervals
Sampling Frequency10Hz


CommunicationsUSB, Wireless Radio Frequency 2.4 GHz
Data StorageMicroSD Card, SD, SDHC & SDXC Compatible (FAT32 format)
Software CompatibilityWindows 7, 8, 8.1, 10 and Mac
Data CompatibilityFAT32 compatible for direct exchange of SD card with any Windows PC and Mac
Data File FormatComma Separated Values (CSV) for compatibility with all software programs
Memory CapacityUp to 16GB, 4GB MicroSD card included.

Operating Conditions

Temperature Range-40°C to +80°C
R/H Range0-100%
UpgradableUser Upgradeable firmware using USB boot strap loader function

Internal Battery Specifications

960mAh Lithium Polymer, 4.20 Volts fully charged
External Power Requirements
Bus Power8-30 Volts DC, non-polarised, current draw is 190mA maximum at 17 volts per logger
USB Power5 Volts DC
Internal Charge Rate
Bus Power60mA – 200mA Variable internal charge rate, maximum charge rate of 200mA active when the external voltage rises above 16 Volts DC
USB Power100mA fixed charge rate
Internal Power Management
Fully Charged Battery4.20 Volts
Low Power Mode3.60 Volts – Instrument ceases to take measurements
Discharged Battery2.90 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 & Environment7(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 DendrometryInstrumentation 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 Psychrometer

When 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.

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