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The HRM Test Block

The HRM Sap Flow Meter Test Block is a functional verification standard for use with the HRM Sap Flow Meter. It is designed using a thermal compound of known thermodynamic properties and encased in a housing of known dimensions.

The needle spacing’s are asymmetrically arranged (which is a divergence from the Heat Ratio principle) to induce an artificial Heat Pulse Velocity. Each block is designed to induce an artificial velocity of approximately 18 cm hr or a mid range value of expected sap velocities as measured by the HRM in many species.


Depending upon the temperature conditions under which the block is used the heat dissipation from the block will be higher or lower which will ultimately impact the measured heat ratio causing a small divergence from the expected 18 cm hr value.


The Test Block is not intended to be used as a calibration device rather as a known standard for operational verification of the Sap Flow Meter. This can be done in the lab as a pre deployment check before taking the equipment to the field and installation.


This will ensure correct functioning of the two measurement needles and the heater providing confidence in the operation of the equipment and subsequent results obtained, which will aid in data interpretation.

Error Codes

If the Sap Flow Meter or the needles have a problem one of the following error codes will typically be generated:
  • 19.19 – No heater current was detected, therefore no heat pulse could be fired.
  • 20.20 – Sap Velocity (Vs) was not calculated because the temperature rise of one of the thermistors was below the cut-off threshold.
  • 21.21 – Sap velocity (Vs) could not be calculated because the temperature rise of one of the thermistors was negative.
  • 22.22 Calculated Sap velocity (Vs) is less than the maximum practical physiological limit of reverse sap flow.
Values that are significantly different from the expected verification range of 18 cm hr and are not flagged with an error code will indicate that an error exists and you should consult your Sap Flow Meter manual and commence diagnostics check and trouble shooting.