PSY1 Stem Psychrometer

About the PSY1 Stem Psychrometer

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.

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.

Installation

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.

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 Power 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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  Clamps for PSY1-Stem
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  PSY1-Stem Leaf Clamp

  Leaf clamp for use with the PSY1 Stem Psychrometer.
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  PSY1 Installation Kit
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  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.
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  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.

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