What is precision?
How do different environments influence precision?
How does the wet / dry transition threshold influence precision?
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What is precision?
Precision can be defined as the closeness of an agreement between independent measurements of a single quantity obtained by applying a stated measurement procedure several times under prescribed conditions (Anonymous 1983).

The precision can be measured as the 95% uncertainty, which is approximately twice the standard deviation. Potraz et al. (1994) estimated the 95% uncertainty of painted Campbell sensors to be between 1.5 ands 2 hours either over turf or at the top of a tomato canopy. Magarey et al. (unpublished # 2) found the precision over turf was between 1 and 2 hours under most circumstances.

How do different environments influence precision?

· Magarey et al. (unpublished #2) found the most important influence upon sensor precision was the environment in which the measurement is made. The uncertainty due to precision was lowest under controlled conditions and highest in the grape canopy. Under controlled conditions sensor uncertainty was 0.5 to 1.2 hours, but in a grape canopy it was as high as 5 hours when unpainted sensors were used.

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Figure The influence of environment upon sensor precision The 95% uncertainty for measurement of surface wetness with unpainted Campbell (UPC) and painted Campbell sensors (PC) at 150 and 6999 kOhm thresholds under controlled conditions, over turf and in a grape canopy.

The uncertainty inside the canopy is likely to be higher because of variation in the shielding of the sensor from the sky. There are also a number of problems with the use of sensors in a vineyard. These include contaminants on the sensor surface such as pesticide spray, bird droppings, and leaves touching the sensor surface.
 

How does the wet / dry transition threshold influence precision?
 
For a painted Campbell sensor, a high wet/dry transition threshold will minimize the uncertainty. The results of Potraz et al. (1994) showed that increasing the wet/dry transition from 900 to 6999 kOhm with painted sensors reduced the 95% uncertainty from 2.2 to 1.6 hours. The results of Magarey et al. (unpublished #2 also support this finding.

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