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Measuring Sap Flow: TDP compared to the Heat Ratio Method


The Heat Ratio Method (HRM) provides many benefits over the Thermal Disipation Probe (TDP) method. HRM, as used by the SFM1x Sap Flow Meter, allows for the measurement of very low/reverse sap flow, which TDP is unable to measure. The variety of sapwood thickness that a SFM1x is able to measure is greater than a TDP system can measure.


SFM1 Heat Ratio Method Sap Flow Meter SFM1x Sap Flow Meter with IoT connectivity


The importance of quality Sap Flow Measurement

In a world of ever decreasing water availability, the need to accurately quantify plant water use and screen for plants with the ability to redistribute water within their growing environment through mechanisms such as hydraulic lift and reverse flow, is crucial. The requirement to adopt reliable techniques and methodologies that can measure these mechanisms is of paramount importance.

The Heat Ratio Method (and the SFM1x Sap Flow Meter) has become established as a benchmark tool for the measurement of plant water use. With the ability to measure very low flow (approx 1 cm hr-1), zero flow, and reverse flow in a range of stem sizes, making it very robust and flexible while the TDP is unable to capture these important measurements. Missing those records will account, nearly 40% loss of Sap flow data from the system.

Comparison of operations between the TDP and HRM instruments

TDP is a constantly heated sap flow principle that assumes there is no background temperature gradient between the probes and the plant (thermal insulation is required around the sensors and the stem). However, data shows that Natural Thermal Gradients (NTG) from the soil (which cannot be incorporated into the TDP measurement) are significant (1°C or greater) hence, the temperature gradient measured is not a true indication of the actual sap flow. The result is a substantial overestimation due to the background temperature gradient in the mornings and again underestimation in the evenings, none of which are of a constant magnitude to even begin to attempt compensating for.

Another cause of underestimation of sap velocity using TDP is the thickness of the sapwood. A standard TDP-30 sensor has a length of 30 mm consisting of a full length line heater and only a single thermocouple located at the halfway point (15 mm). The sensor design is adequate for use in boreal Pinus species with thick sapwood of at least 30 to 40 mm, but is inadequate for most other species. TDP sensors cannot be used to accurately measure sap flow of Eucalyptus species for example, because on average the sapwood thickness is approximately 20 to 25 mm thick. Therefore, the TDP sensor will almost always have the heater extend significant depths into heartwood or non-conducting xylem. This results in highly variable and underestimated sap velocity because the non-conducting xylem (heartwood) artificially increases the dTmax as it does not dissipate the heat.

With these two conflicting variables causing both positive and negative errors of unknown quantities, it is clear that the TDP principle has significant limitations in application.

The Heat Ratio Method (HRM) sensor is a modified heat pulse technique that provides a better accuracy of measurements, compared to the TDP. HRM consists of three needles: two measurement needles located equidistant above and below a central line heater. Because it is a pulsed technique using a short two to six second pulse of heat and a 100 second measurement window, the effect of NTGs are avoided as ambient temperature changes within the measurement interval are insignificant or non-existent. This has additional benefits in practical deployment as the sensors also require no thermal shielding to thermally isolate the measurement site against NTGs as with TDP.

The HRM needles are 35 mm in length and have two temperature measurement locations, 7.5 mm from the tip and 22.5 mm from the tip. This provides a measurement of radial velocity across the sapwood and quantifiably measures the velocity gradient and/or identifies when the needle extends into non-conducting xylem (so the inner measurement point can be discounted).

The HRM can measure very low flow (approx 1 cm hr-1), zero flow, and reverse flow in a range of stem sizes, making it very robust and flexible while the TDP is unable to capture these important measurements. Missing those records will account, nearly 40% loss of sap flow data from the system.

Specification HRM TDP
Measurement Range
Measurement Units Heat Pulse Velocity (cm hr-1)
Sap Velocity (cm hr-1)
Sap Flow (g hr-1)
Sap Flux Density (cm3 hr-1 cm-2)
Measurement Range -20 to 60 cm hr-1 0 to 80 cm3 cm-2 hr-1 (with calibration)
Measurement Accuracy 0.5 cm hr-1 Not specified*
Measurement Resolution 0.01 cm hr-1 1 µV
Measures Reverse Flow Yes No
Measures Low Flow Yes Standard minimum measurable velocity is approx. 4 cm hr-1, can measure to 0 cm hr-1 with empirical calibration
Measures High Flow Yes Yes
Measures Multiple Radial Points Yes – 2 independent measurement points in the same radial profile, spaced 15 mm apart. Can be used to characterise flow in the inner and outer xylem. No – Standard sensor is one measurement point
Used on large diameter stems Yes – For trees of any diameter, but only for the outer 4 0mm of xylem. Using 2 measurement points, flow can be characterised in the inner and outer xylem, enabling correction for radial variation across sapwood. Yes – Only for the outer 20 mm of xylem
Used on Small Diameter Stems Yes – Stems larger than 10mm No – Sensor needle must be fully inserted and insulated to prevent errors due to Natural Thermal Gradients
Used on Roots Yes – Has been used to study hydraulic lift and hydraulic redistribution in root systems No – Does not measure reverse flow, unsuitable for installation in soil
Measurement Principles
Is wound response accounted for? Yes No
Heat Source Heat Pulse Continuous
Requires radiation shielding? No – Measurement time is so short that no significant change in air temperature or effect of direct incident radiation will occur that can effect the measurement. Yes – Requires thermal and radiation shielding around the installation site, and 50 cm to 1 m both above and below the installation, preferably to ground level.
Effected by Natural Thermal Gradients (NTG)? No Yes – Causes error in measurement accuracy and sensitivity
Does the sensor need to be inserted only in sapwood? No – multiple measurement points enable measurement of radial gradient and determination of sapwood/heartwood border should the needles extend beyond sapwood into heartwood. Yes – crucial that the entire length of the needle be in sapwood. If the needle extends into heartwood or non-conducting xylem heat will not be dissipated and errors of up to 50% can occur.
Do heat lags effect the measurement? No – A short heat pulse is generated for each measurement, eliminating the effect of ambient thermal conditions. Yes – the sensor is continuously heated
Data Processing & Analysis
Calibration method Specific wood properties and wound coefficients to convert heat pulse velocity to corrected sap velocity and sap flow. Empirical calibration requiring data corrections for dTmax; using extensive data modelling.
Raw Data Processing Required No – Units measured are cm hr-1 Yes – Conversion from analogue uV to temperature, then conversion to sap velocity
Software Yes – Sap Flow Tool and ICT Combined Instrument Software. Data files are in CSV format, and can be used in Excel No – Excel only, requires processing.
Data Logging
Data Output Raw Heat Pulse Velocity (cm hr-1)
Corrected Sap Velocity (cm hr-1)
Corrected Sap Flow (g hr-1)
Temperature Difference (uV/mV or °C)
Temporal Logging Resolution Minimum 3 minutes to ensure heat dissipation, standard 15 minutes. Minimum 1 minute, typically 15 to 60 minutes to prevent measurement noise
Number of sensors per logging system N/A (standalone) 32 (with multiplexer)
Communications Bluetooth, IoT RS-232 Serial
Memory Capacity MicroSD card 4MB (1 year hourly data; sap flow calculations: 8 months for 8 gages, 2 months for 32.), expandable up to 8MB