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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.

The Heat Ratio Method

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Ratio Method (HRM) is an improvement of the Compensation Heat Pulse Method (CHPM) and is now widely regarded as having superseded that technique.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.

How does it work?

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
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
  • SFM1 Sap Flow Meter
    The ICT International SFM1 Sap Flow Meter is a self contained, stand-alone instrument for measurement of sap flow in plants & plant water use
  • CH24 - 24 Volt Power Supply
    The CH24 is a 100 - 240Volts AC Mains to 24Volts DC power supply adapter; capable of outputting up to 2.5Amps. For most ICT Instruments.
  • SFT1 Sap Flow Tool
    Sap Flow Tool software for HFD and HRM. Single License. Unlimited access to any number HRM or HFD datasets. Configured to analyse HRMx, CHPM, Tmax data from the SFM Sap Flow Meter. Visualise PSY1, soil moisture, and meteorological data.

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

Buckley, T. N., Turnbull, T. L., & Adams, M. A. (2012). Simple models for stomatal conductance derived from a process model: Cross-validation against sap flux data. Plant, Cell & Environment, 35(9), 1647–1662. https://doi.org/10.1111/j.1365-3040.2012.02515.x

Buckley, T. N., Turnbull, T. L., Pfautsch, S., & Adams, M. A. (2011). Nocturnal water loss in mature subalpine Eucalyptus delegatensis tall open forests and adjacent E. pauciflora woodlands. Ecology and Evolution, 1(3), 435–450. https://doi.org/10.1002/ece3.44

Buckley, T. N., Turnbull, T. L., Pfautsch, S., Gharun, M., & Adams, M. A. (2012). Differences in water use between mature and post-fire regrowth stands of subalpine Eucalyptus delegatensis R. Baker. Forest Ecology and Management, 270, 1–10. https://doi.org/10.1016/j.foreco.2012.01.008

Burgess, S. S. O., Adams, M. A., Turner, N. C., Beverly, C. R., Ong, C. K., Khan, A. A. H., & Bleby, T. M. (2001). An improved heat pulse method to measure low and reverse rates of sap flow in woody plants. Tree Physiology, 21(9), 589–598. https://doi.org/10.1093/treephys/21.9.589

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

Burgess, S. S. O., & Dawson, T. E. (2004). The contribution of fog to the water relations of Sequoia sempervirens (D. Don): Foliar uptake and prevention of dehydration. Plant, Cell & Environment, 27(8), 1023–1034. https://doi.org/10.1111/j.1365-3040.2004.01207.x

Carbone, M. S., Williams, A. P., Ambrose, A. R., Boot, C. M., Bradley, E. S., Dawson, T. E., … Still, C. J. (2013). Cloud shading and fog drip influence the metabolism of a coastal pine ecosystem. Global Change Biology, 19(2), 484–497. https://doi.org/10.1111/gcb.12054

Dawson, T. E., Burgess, S. S. O., Tu, K. P., Oliveira, R. S., Santiago, L. S., Fisher, J. B., … Ambrose, A. R. (2007). Nighttime transpiration in woody plants from contrasting ecosystems. Tree Physiology, 27(4), 561–575. https://doi.org/10.1093/treephys/27.4.561

de Dios, V. R., Díaz‐Sierra, R., Goulden, M. L., Barton, C. V. M., Boer, M. M., Gessler, A., … Tissue, D. T. (2013). Woody clockworks: Circadian regulation of night-time water use in Eucalyptus globulus. New Phytologist, 200(3), 743–752. https://doi.org/10.1111/nph.12382

Doronila, A. I., & Forster, M. A. (2015). Performance Measurement Via Sap Flow Monitoring of Three Eucalyptus Species for Mine Site and Dryland Salinity Phytoremediation. International Journal of Phytoremediation, 17(2), 101–108. https://doi.org/10.1080/15226514.2013.850466

Drake, P. L., Coleman, B. F., & Vogwill, R. (2013). The response of semi-arid ephemeral wetland plants to flooding: Linking water use to hydrological processes. Ecohydrology, 6(5), 852–862. https://doi.org/10.1002/eco.1309

Eller, C. B., Lima, A. L., & Oliveira, R. S. (2013). Foliar uptake of fog water and transport belowground alleviates drought effects in the cloud forest tree species, Drimys brasiliensis (Winteraceae). New Phytologist, 199(1), 151–162. https://doi.org/10.1111/nph.12248

Falge, E., & Meixner, F. X. (2008). Validation of a 3D gas exchange model for a Picea abies canopy in the Fichtelgebirge, Germany. In Geophys. Res. Abstr (Vol. 10). Download PDF.

Gharun, M., Turnbull, T. L., & Adams, M. A. (2013). Stand water use status in relation to fire in a mixed species eucalypt forest. Forest Ecology and Management, 304, 162–170. https://doi.org/10.1016/j.foreco.2013.05.002

Gharun, M., Turnbull, T. L., Pfautsch, S., & Adams, M. A. (2015). Stomatal structure and physiology do not explain differences in water use among montane eucalypts. Oecologia, 177(4), 1171–1181. https://doi.org/10.1007/s00442-015-3252-3

Mitchell, P. J., Veneklaas, E., Lambers, H., & Burgess, S. S. O. (2009). Partitioning of evapotranspiration in a semi-arid eucalypt woodland in south-western Australia. Agricultural and Forest Meteorology, 149(1), 25–37. https://doi.org/10.1016/j.agrformet.2008.07.008

Palmer, A. R., Fuentes, S., Taylor, D., Macinnis‐Ng, C., Zeppel, M., Yunusa, I., & Eamus, D. (2010). Towards a spatial understanding of water use of several land-cover classes: An examination of relationships amongst pre-dawn leaf water potential, vegetation water use, aridity and MODIS LAI. Ecohydrology, 3(1), 1–10. https://doi.org/10.1002/eco.63

Patankar, R., Quinton, W. L., Hayashi, M., & Baltzer, J. L. (2015). Sap flow responses to seasonal thaw and permafrost degradation in a subarctic boreal peatland. Trees, 29(1), 129–142. https://doi.org/10.1007/s00468-014-1097-8

Pfautsch, S., Dodson, W., Madden, S., & Adams, M. A. (2015). Assessing the impact of large-scale water table modifications on riparian trees: A case study from Australia. Ecohydrology, 8(4), 642–651. https://doi.org/10.1002/eco.1531

Pfautsch, S., Keitel, C., Turnbull, T. L., Braimbridge, M. J., Wright, T. E., Simpson, R. R., … Adams, M. A. (2011). Diurnal patterns of water use in Eucalyptus victrix indicate pronounced desiccation–rehydration cycles despite unlimited water supply. Tree Physiology, 31(10), 1041–1051. https://doi.org/10.1093/treephys/tpr082

Pfautsch, S., Peri, P. L., Macfarlane, C., van Ogtrop, F., & Adams, M. A. (2014). Relating water use to morphology and environment of Nothofagus from the world’s most southern forests. Trees, 28(1), 125–136. https://doi.org/10.1007/s00468-013-0935-4

Rosado, B. H. P., Oliveira, R. S., Joly, C. A., Aidar, M. P. M., & Burgess, S. S. O. (2012). Diversity in nighttime transpiration behavior of woody species of the Atlantic Rain Forest, Brazil. Agricultural and Forest Meteorology, 158–159, 13–20. https://doi.org/10.1016/j.agrformet.2012.02.002

Staudt, K., Serafimovich, A., Siebicke, L., Pyles, R. D., & Falge, E. (2011). Vertical structure of evapotranspiration at a forest site (a case study). Agricultural and Forest Meteorology, 151(6), 709–729. https://doi.org/10.1016/j.agrformet.2010.10.009

Van de Wal, B. A. E., Guyot, A., Lovelock, C. E., Lockington, D. A., & Steppe, K. (2015). Influence of temporospatial variation in sap flux density on estimates of whole-tree water use in Avicennia marina. Trees, 29(1), 215–222. https://doi.org/10.1007/s00468-014-1105-z

Zeppel, M. J. B., Lewis, J. D., Medlyn, B., Barton, C. V. M., Duursma, R. A., Eamus, D., … Tissue, D. T. (2011). Interactive effects of elevated CO2 and drought on nocturnal water fluxes in Eucalyptus saligna. Tree Physiology, 31(9), 932–944. https://doi.org/10.1093/treephys/tpr024