A key strength of the HRM is that it can distinguish both small sap velocities and negative velocities (when sap flows in reverse, i.e., in a proximal direction, sap transfers heat towards Probe #1, causingδ 1 > δ 2 and hence V 12 < 0).
2.1.2 Problems with high sap velocities in the HRM
In practice, Eqn. 4 is applied using experimental estimates ofδ 1 and δ 2(\({\hat{\delta}}_{1}\) and \({\hat{\delta}}_{2}\), respectively) that include measurement noise (error terms ε 1 andε 2, respectively). Adding temperature noise terms to Eqn. 4 gives