HRM Heat Ratio Method

The Heat Ratio Method (HRM) is a scientific principle for the measurement of sap flow, or water use, in plants. HRM was developed by scientists at the University of Western Australia in the late 1990’s in response to the limitations of existing sap flow measurement techniques. The principal scientist in the development of HRM was Dr Stephen Burgess who was the lead author in the seminal paper published in Tree Physiology in 2001.

ICT International is the only manufacturer in the world of HRM sap flow sensors and data loggers. The SFM1 Sap Flow Meter is the instrument which contains everything needed to measure sap flow via HRM: sensors, data logger, software interface, and internal battery which is recharged via an external solar panel. The SFM1 Sap Flow Meter can store data as raw temperature measurements or heat velocity measurements according to HRM. These data can be downloaded into Sap Flow Tool software for conversion to sap velocity, sap flow and total plant water use.


Output Options Raw Temperatures: °C
Heat Pulse Velocity cm hr-1
Sap Velocity: cm hr-1
Sap Flow: cm3 hr-1(Litres hr-1)
Range -100 to +100 cm hr-1
Resolution 0.01 cm hr-1
Accuracy 0.5 cm hr-1
Measurement Duration 120 seconds


Computer Interface USB, Wireless RF 2.4 GHz
Data Storage MicroSD Card
Memory Capacity Up to 16GB, 4GB microSD card included.


Heat Pulse User Adjustable: 20 Joules (default) approx. Equivalent to a 2.5 second heat pulse duration, auto scaling.
User Adjustable: Minimum interval, 3 minutes, recommended minimum 10 minutes.


Needle Diameter 1.3 mm
Needle Length 35 mm
Measurement Positions 2 per measurement needle
Measurement Spacings 7.5 mm and 22.5 mm from the needle tip
Dimensions L x W X D 170 x 80 x 35 mm
Weight 400 g


Internal Battery Specifications
960mAh Lithium Polymer, 4.20 Volts fully charged
External Power Requirements
Bus Power 8-30 Volts DC, non-polarised, current draw is 190mA maximum at 17 volts per logger
USB Power 5 Volts DC
Internal Charge Rate
Bus Power 60mA – 200mA Variable internal charge rate, maximum charge rate of 200mA active when the external voltage rises above 16 Volts DC
USB Power 100mA fixed charge rate
Internal Power Management
Fully Charged Battery 4.20 Volts
Low Power Mode 3.60 Volts – Instrument ceases to take measurements
Discharged Battery 2.90 Volts – Instrument automatically switches off at and below this voltage when no external power connected.
Battery Life varies
  • With a recommended power source connected, operation can be continuous.
  • 3 days at hourly logging interval without chamber heating
  • 1 day with chamber heating