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This article in AJ

  1. Vol. 73 No. 5, p. 808-817
     
    Received: Nov 5, 1980


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doi:10.2134/agronj1981.00021962007300050015x

Techniques for Thermocouple Placement and the Automation of Temperature Measurements in the Microenvironment of Kentucky Bluegrass1

  1. M. S. Welterlen and
  2. T. L. Watschke2

Abstract

Abstract

A Kentucky bluegrass (Poa pratensis L.) sward is comprised of many closely spaced, minute plants forming a dense, low canopy. Measurements of soil and basal tiller temperatures require careful placement of many small sensors. The objectives of this research were to develop expedient methods for obtaining precise point measurements of tiller, thatch, and soil temperatures with thermocouples in the field. Measurements were made in ‘Touchdown’ Kentucky bluegrass mowed to 5 an.

Throughout the study, Chromel-Alumel thermocouples were used. A 40-channel, microprocessor-based data acquisition system and a computer compatible nine-track magnetic tape were used for uninterrupted data collection. Irradiance, measured approximately 1 km from the experimental site, and air temperature, sensed by an aspirated thermocouple located 1.5 m above the turf, were recorded simultaneous with sod temperature measurements.

Thermocouples 120 AWG (0.81 mm)] placed horizontally and vertically, thermocouples [40 AWG (0.076 mm)] laced vertically, and thermocouples [40 AWG] fasteneg to 0.6 an diameter wooden dowels were placed 2 an below the thatch-soil interface and compared for precision and placement expedience. Vertically placed inegauged thermocouples (the most expedient means of installation) were as precise as the other methods. Although gauge did not influence precision in this test, in some situations (particularly vertical placement) the potential for error may exist due to heat flow along the wue.

Two methods of thermocouple (40 AWG) placement into basal tillers were compared for expedience of placement and relative precision. In one method, thermocouples were threaded through the tiller base leaving the thermocouple bead inside the tiller with lead wire connection on opposite sides. The other method involved direct insertion into a hypodermic puncture in the tiller base. The inserted thermocouple method was found to be the most expedient, and no significant temperature differences were found between the methods. Tiller base temperatures measured in the field on a cloudy afternoon on 11 June I979 ranged from 16.8 to 21.1 C, with a maximum difference of 0.6 C between the two methods (insignificant at the 0.05 level). The coefficients of variation for each method were not greater than 10.6. Temperatures measured during a clear afternoon on 12 June 1979 ranged from 23.1 to 31.5 C. At 1500 hours, the difference between the methods of placement was greatest (3.3 C) but was not significant at 0.05. The maximum coefficient of variation was 15.3 and 13.7 for the inserted and threaded thermocouples, respectively. Throughout the day, the threaded method had generally lower coefficients of variation.

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