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.
Developed by the University of Western Australia and partner organisations, ICRAF and CSIRO, the HRM principle has been validated against gravimetric measurements of transpiration and used in published sap flow research since 1998. Burgess et al. (2001) developed the theory of HRM and Bleby et al. (2004) validated the technique:
Burgess, S.S.O., Adams, M.A., Turner, N.C., Beverly, C.R., Ong, C.K., Khan, A.A.H. and Bleby, T.M. (2001) An improved heat pulse method to measure low and reverse rates of sap flow in woody plants. Tree Physiology, 21: 589-598.
Bleby, T.M., Burgess, S.S.O. and Adams, M. A. (2004) A validation, comparison and error analysis of two heat-pulse methods for measuring sap flow in Eucalyptus marginata saplings. Functional Plant Biology, 36: 645-658.
Heat Ratio Method (HRM) is an improvement of the Compensation Heat Pulse Method (CHPM) and is now widely regarded as having superseded that technique. The major point of difference between HRM and CHPM is the former is based on a ratio principle whereas the latter is based on a time principle. Therefore, HRM has the ability to measure high, low, zero and reverse rates of sap flow. In contrast, CHPM can only measure high rates of flow. This limitation means the CHPM is highly inaccurate in determining total sap flow.
Burgess et al. (2001) thoroughly explain how HRM works from first principles. The SFM1 Sap Flow Meter Manual also details HRM in Chapter 6 and a summary of the theory is found in the HRM Explained presentation.
Briefly, temperature sensors spaced equidistant above (downstream) and below (upstream) a line heater measure initial temperature conditions for about 30 seconds. A pulse of heat is fired along the heater needle for 2.68 seconds. The system is left to equilibriate for 60 seconds and then temperature downstream and upstream the heater needle is measured again for 40 seconds. The rise in temperature from initial conditions to post heat pulse conditions in the downstream and upstream temperature sensors are noted.
The ratio of the downstream to upstream temperature rise is then calculated and entered into a formula to further calculate heat velocity (vh):
vh = heat velocity
k = thermal diffusivity
v1 = average increase temperature downstream
v2 = average increase temperature upstream
x = distance of temperature needles from heater needle
3600 = converting from seconds to hours
The Sap Flow Tool software is capable of converting vh into sap velocity and volumetric sap flow values once additional parameters are known. These parameters include thermal diffusivity, wood density and moisture content, bark depth, sapwood depth, and stem diameter.
|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|
|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|
Ambrose, A. R., Sillett, S. C., Koch, G. W., Van Pelt, R., Antoine, M. E., & Dawson, T. E. (2010). Effects of height on treetop transpiration and stomatal conductance in coast redwood (Sequoia sempervirens). Tree Physiology, 30(10), 1260–1272. https://doi.org/10.1093/treephys/tpq064
Bleby, T. M., Burgess, S. S. O., & Adams, M. A. (2004). A validation, comparison and error analysis of two heat-pulse methods for measuring sap flow in Eucalyptus marginata saplings. Functional Plant Biology, 31(6), 645–658. https://doi.org/10.1071/FP04013
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Burgess, S. S. O., M. A. Adams, N. C. Turner, C. K. Ong, A. A. H. Khan, C. R. Beverly and T. M. Bleby (2001) Corrections: An improved heat pulse method to measure low and reverse rates of sap flow in woody plants. Tree Physiology, 21(16), 1157. doi:10.1093/treephys/21.16.1157 http://treephys.oxfordjournals.org/content/21/16/1157.full.pdf
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