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Compensation Heat Pulse Method

The Compensation Heat Pulse Method (CHPM) was the forerunner to the Heat Ratio Method (HRM) as a sap flow measurement principle. The CHPM was popular in the 1980's however it is now largely superseded by the more accurate HRM principle.

For those scientists still wanting to measure sap flow via CHPM, the SFM1 Sap Flow Meter can be configured to measure this principle. All that is needed is the SFM-CHPM Drill Guide so that needles can be accurately installed according to the requirements of CHPM.

The SFM1 Sap Flow Meter configuration is Needle Temperature Mode whereby all temperature measurements are recorded by the SFM1 self-contained logger. The raw data file is then downloaded into Sap Flow Tool where the temperature data is quickly and simply converted to heat velocity according to the CHPM equation.

MEASUREMENT

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

DATA

Computer Interface

USB, Wireless RF 2.4 GHz

Data Storage

MicroSD Card

Memory Capacity

Up to 16GB, 4GB microSD card included.

OPERATING CONDITIONS

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 DESIGN

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

POWER

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.
  • Approximately 1.5 days with a heat pulse of 50 Joules and a measurement interval of 30 minutes – no external power present to recharge battery.

CHPM (Compensation Heat Pulse Method)

For the CHPM, two probes containing temperature sensors are aligned with the axis of a plant stem or root and inserted radially to equal depths in the xylem. A heater element is similarly inserted at a fixed distance upstream from the midpoint between the temperature probes. A common configuration locates the heater 0.5 cm from the upstream temperature probe and 1.0 cm from the downstream probe (here denoted as a –0.5, 0, 1.0 cm configuration). During measurement, wood and sap are heated in pulses and convection through the flowing sap stream carries the heat toward the midpoint between the temperature probes. When both temperature probes have warmed to the same degree, the heat pulse has moved the 0.25 cm from the heater, i.e., to the midpoint between the probes. The time taken for the heat pulse to move this distance is used to calculate heat pulse velocity (vh):
vh = (x1 + x2 / 2t0) * 3600

where: t0 is time to thermal equilibration of the downstream and upstream probes after release of the heat pulse, and x1 and x2 denote distances (cm) between the heater and the downstream and upstream temperature probes, respectively (we use these terms as they apply in the case when water flows from soil to leaves). A negative value is assigned to x2 because it is located on the opposite side of the heater to x1.

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  • 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.
Bleby, T. M., Burgess, S. S., & 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. http://www.publish.csiro.au/paper/FP04013.htm