Understanding and Correcting Probe Misalignment in the Implexx Sap Flow Sensor.
The Implexx Sap Flow Sensor comprises three probes: a central heater probe and two temperature probes—one upstream (bottom) and one downstream (top). Accurate measurements of sap flux density and sap flow depend on the precise alignment of these probes. For valid data, the probes must be exactly parallel, and the distance between the heater and each temperature probe must be consistent: 0.6 cm for Gen1 and 0.8 cm for Gen2 models (Figure 1). Any deviation from these distances, or lack of parallelism, is considered probe misalignment and necessitates post-measurement data correction.
Figure 1. The Gen1 (left) and Gen2 (right) models of the Implexx Sap Flow Sensor. Note that probe spacing differs in Gen1 (0.6cm) versus Gen2 (0.8cm).
Causes of probe misalignment.
(1) Installation error.
The most common cause of misalignment is minor error during the drilling process. Even when using Implexx’s custom-built drill guide (as demonstrated in Figure 2), perfect alignment is difficult to achieve. Deviations as small as 0.01–0.02 cm—the approximate diameter of a human hair—can affect data accuracy. Fatigue or haste—particularly at the end of a long day in the field—can result in poor installation. Precision is critical; a rushed job will likely need to be redone, increasing time and resource costs.
Figure 2. The three probes of the Implexx Sap Flow Sensor appear aligned in this installation. However, even imperceptible deviations affect the accuracy of sap flow measurements.
(2) Xylem anatomy and plant structure.Xylem vessels are often assumed to be straight and aligned with the direction of sensor insertion. However, natural anatomical variability, including angled or irregular xylem tissue, can contribute to misalignment. Figure 3 illustrates this with an example from grapevine, a species known for complex stem anatomy. Because destructive sampling is required to confirm internal alignment, installers must rely on best practices and post hoc data correction.
Figure 3. Installing sensors at unusual angles or tortuous xylem anatomy may cause probe misalignment.
An example dataset of probe misalignment.
Probe misalignment can be inferred when sap flow values deviate from zero during periods when flow is expected to cease, such as at night. For example, Figure 4 shows a dataset with apparent probe misalignment. The blue arrows indicate nighttime periods when sap flow should align with the red zero-flow line; however, values are negative (approximately –0.15 cm/hr heat velocity). In some cases, misalignment may also produce positive nighttime values.
Figure 4. When sap flow is expected to be zero but measured values are consistently positive or negative, this indicates probe misalignment.
Not all positive or negative nighttime sap flow values indicate probe misalignment. For instance, Figure 5 shows positive nighttime values that could be mistaken for misalignment. However, these measurements correspond to hot, dry, and windy conditions that likely induced nocturnal sap flow. In contrast, nights 5 and 6—following rainfall and under high humidity—show zero nighttime flow. In this case, the data reflect true physiological responses, not sensor error or probe misalignment, and no correction is required.
Figure 5. Example dataset showing positive nighttime sap flow that may be mistaken for probe misalignment. In this case, the probes are properly aligned, and the observed flow is due to environmental conditions.
Detecting probe misalignment.
Because the probes are embedded within the tree, direct observation of misalignment is impossible. Instead, several methods are available to detect probe misalignment.
- Cut-stem method (Figure 6): Cutting the stem below the sensor instantly halts sap flow, revealing any offset in the data. While accurate, this method is destructive and often impractical.
Figure 6. Cutting the stem just below the sensor immediately halts sap flow (heat velocity). The resulting flat line represents the zero-flow offset used for data correction.
- Dormant periods: In deciduous species, leafless periods signal the cessation of transpiration, and thus sap flow should be zero.
- Saturation technique: Based on biophysical principles, sap flow is driven by environmental energy (solar radiation, relative humidity, vapor pressure deficit, wind etc.). At night, particularly after rainfall when VPD is zero, relative humidity is 100%, and soil moisture is saturated, energy input is minimal and sap flow is presumed to be zero (for example, the nighttime data displayed in Figure 4). That is, the atmosphere and soil are saturated, and there is no energy for sap to flow in plants. Any non-zero sensor reading under these conditions indicates misalignment (Figure 4).
Correcting for probe misalignment.
There are two main approaches to correct for misalignment:
(1) Zero flow offset (data shift):
This simple method applies a vertical shift to the dataset so that periods of known zero flow register as zero. For example, using the saturation technique shown in Figure 4, the offset is estimated at approximately –0.15 cm/hr. Therefore, +0.15 cm/hr is added uniformly to all data points. This "zero-flow offset" correction aligns zero-flow periods with the red reference line, as shown in Figure 7. While quick to implement, this method lacks precision.
Figure 7. The dataset from Figure 4 after applying a zero-flow offset. A value of +0.15 cm/hr was added to all data points to align zero-flow periods with the red reference line.
(2) Thermal theory-based correction:
A more accurate correction is based on thermal conduction and convection theory (see Forster, 2020). The Implexx Sap Flow Workbook, an Excel-based tool that is free to users of the Implexx Sap Flow Sensor, applies this theory to estimate the actual probe offset. However, this method is only reliable if accurate measurements of sapwood dry density and water content are included in the Implexx Sap Flow Workbook.
Conclusion.
Probe misalignment is a subtle but significant source of error in sap flow measurements. It is often unavoidable due to biological variability and practical limitations during installation. However, by using careful installation techniques, identifying periods of zero flow, and applying appropriate correction methods—either through simple offset adjustment or rigorous thermal theory—researchers can ensure high-quality sap flow data.