Instrument design

When simultaneously combined with the SFM-1 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 PSY-1 Stem Psychrometer consists of two welded chromel-constantan thermocouples connected in series within a chromium plated brass chamber that forms a large insulating thermal mass. 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. All temperature measurements (dT, Wet Bulb depression & chamber temperature) are then used in the determination of plant water potential.

Principle of Measurement

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 PSY-1 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. Dixon, M.A., & Tyree, M.T., 1984 A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb Plant, Cell and Environment 7, 693- 697.

The PSY-1 Stem Psychrometer is attached to the stem using a clamp to hold it in position using moderate pressure. A thermocouple is lifted up from the sample chamber and 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.

The PSY-1 Stem Psychrometer offers significant benefits over more common leaf psychrometers through the ease of attachment which minimizes energy balance disruptions and improves measurement accuracy.

Equilibration Half-Time Equilibration half-time for thermocouple psychrometer 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 PSY-1 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.

Data Analysis

Data can be manually processed using a Spreadsheet such as Excel by opening the comma separated Values (CSV) file provided by the PSY-1. 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 preprocessed into engineering units ready for interpretation and analysis.

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 nonvolatile memory.

The PSY-1 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.

Calibration Options

PSY-1 Stem Psychrometer requires calibration. Depending upon the level of accuracy required you can choose to utilise a generic batch calibration with coarse generalized 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.

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, slope & 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 Micro SD card of the PSY-1 and recalled for use with the specific chamber.

PSY-1 Stem Psychrometer requires calibration. 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.