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Wescor

  basic principles

Similar to determining relative humidity in air-a thermocouple is cooled below the dew point, causing condensation to collect on the thermocouple junction. As the condensate evaporates, the junction is cooled below ambient temperature and a "wet bulb" vs. "dry bulb" temperature difference is obtained from which the water potential is calculated.

  advantages

Psychrometers measure water availability rather than water content. Water potential is an indicator of how much force the plant must expend to remove water from the soil. Calibration of sensors and sensor readings are independent of soil type or soil particle size.

  disadvantages

Psychrometers are highly susceptible to thermal gradient effects (so they do not perform well at shallow soil depths), specialized equipment is required for the sensor's excitation and reading, and the volume of soil that is "sensed" is rather small.

  general

  manufacturers

 Wescor Environmental Products
 P.O. Box 361
 Logan, UT 84323-0361
 www.wescor.com

 J.R.D. Merrill Specialty Equipment
 1105 W. 2200 S.
 Logan, UT 84321

Paper on the use of Psychrometry (by MJ Savage)

  details

  other

Text contributed by

John Manwaring
Applications Scientist
Wescor Environmental Division
Logan, UT 84321
(435)753-8311

johnm@mail.wescor.com

 

Relevant Links

• HCalc Humidity Conversion Utility - psychrometric conversion utility courtesy of Avatel.

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PRISM - CMP by Irrigation Scheduling Methods, Inc.

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