© 1998 Heron Publishing—Victoria, Canada
Limitations of a compensation heat pulse velocity system at low sap flow: implications for measurements at night and in shaded
trees
Peter Becker (1)
1. Biology Department, Universiti Brunei Darussalam, Bandar Seri Begawan 2028, Brunei, SE Asia (pbecker@ubd.edu.bn) / Received February 13, 1997
Summary
Unlike an ideal system, the return time to thermal balance (tb) between upstream and downstream thermistors, as measured by the (compensation) heat pulse velocity method, effectively depends
on the heat input and the water content of the wood at zero and low sap flow. Even when these factors were held constant and
ambient temperature was stabilized, a twofold variation in tb at zero flow was observed within and among Greenspan Technology sensors implanted in wooden posts, making it impossible to
distinguish zero flow from low sap velocities (< 0.01–0.02 mm s–1). This limitation has serious consequences because the contribution of low flow rates to water movement is important during
both daytime and nighttime in tropical understory and overstory trees. Measurements in an artificial flow system showed that
this technical limitation is exacerbated by erratic variation in sensor response at both zero and low flow rates. The limited
sensitivity of the tested sap flow sensors may be caused by their poor thermal contact with wood. Interim procedures are suggested
for estimating minimum detectable sap flow and delimiting the hydroactive zone until the sensitivity and interchangeability
of sap flow probes are improved.
Keywords:
Greenspan Technology sensors, measurement variation, nocturnal, sap velocity, thermal balance, thermistor.
