Self-calibrating; ideal for energy-balance in eddy-covariance and Bowen-ratio systems
Overview
The HFP01SC, manufactured by Hukseflux, measures soil heat flux—typically for energy-balance or Bowen-ratio flux systems. It is intended for applications requiring the highest possible degree of measurement accuracy. The HFP01SC outputs a voltage signal that is proportional to the heat flux of the surrounding medium. At least two sensors are required for each site to provide spatial averaging. Sites with heterogeneous media may require additional sensors.
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Benefits and Features
- Corrects for errors due to differences in thermal conductivity between the sensor and surrounding medium, temperature variations, and slight sensor instabilities
- Compatible with most Campbell Scientific data loggers
- Uses Van den Bos-Hoeksma self-calibration method to provide a high degree of measurement accuracy
The "-L" on a product model indicates that the cable length is specified at the time of order.
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Detailed Description
The HFP01SC consists of a thermopile and a film heater. The thermopile measures temperature gradients across the plate. During the in-situ field calibration, the film heater is used to generate a heat flux through the plate. The amount of power used to generate the calibration heat flux is measured by the datalogger. Each plate is individually calibrated, at the factory, to output flux.
Self-calibration corrects for errors due to differences in thermal conductivity between the sensor and surrounding medium, temperature variations, and slight sensor instabilities.
Note: In an energy-balance installation, all sensors must be completely inserted into the soil face before the hole is backfilled.
Specifications
| Sensor Type | Thermopile with film heater |
| Sensitivity | 50 μV W-1 m-2 (nominal) |
| Nominal Resistance | 2 Ω |
| Temperature Range | -30° to +70°C |
| Expected Accuracy | ±3% of reading |
| Heater Resistance | 100 Ω (nominal) |
| Heater Voltage Input | 9 to 15 Vdc |
| Heater Voltage Output | 0 to 2 Vdc |
| Duration of Calibration | ±3 minutes @ 1.5 W (typically performed every 3 to 6 hours) |
| Average Power Consumption | 0.02 to 0.04 W |
| Plate Diameter | 80 mm (3.15 in.) |
| Plate Thickness | 5 mm (0.20 in.) |
| Weight | 200 g (7.05 oz) without cable |
Documents
Downloads
CR1000X HFP01SC Example program v.2 (3 kb) 18-02-2020
CR1000X program that measures the HFP01SC, performs the self-calibration, and checks for calibration validity. Refer to the manual for the equations used for the self-calibration and calibration-validity checks. A table in the manual provides a cross reference of the terms in the equations in the manual with the constants and variables in the example data logger program.
Frequently Asked Questions
Number of FAQs related to HFP01SC-L: 13
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The in-situ calibration is helpful for quality assurance/quality control. The multiplier determined from the in-situ calibration should be within ±10% of the factory-determined calibration. If it is not, the plate may be damaged, not wired correctly to the data logger, or not making full contact with the soil.
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The example CRBasic program runs in either SequentialMode or PipeLineMode. To force the CRBasic program to run in PipeLineMode, add the instruction PipeLineMode to the beginning of the program.
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Can the HFP01SC-L be embedded in railroad ballast, which comprises a blend of coarse rock particles?
No. The HFP01SC-L must be in full contact with the media. Railroad ballast is too coarse.
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Rather than using a running average to find the millivolt output during a calibration, use a single sample with 50 or 60 Hz integration. See Example 1 in the 2014 or later version of the HFP01SC-L manual.
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A calibration shift occurs if the HFP01SC-L is not making full contact with the soil during the calibration cycle. The following could cause the plate to lose contact with the soil: a soil freeze/thaw cycle, soil swelling/contracting because of extreme drying/wetting cycles, or rodents burrowing past the plate.
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Add the appropriate code to the data logger program using a program example from the HFP01SC-L manual. Alternatively, contact Campbell Scientific for assistance.
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This depends on the information contained in the calibration sheet:
- If the calibration sheet contains coefficient information, Campbell Scientific keeps a copy, and a replacement copy can be requested.
- If the calibration sheet does not contain coefficients, Campbell Scientific does not keep a copy. It may be possible to contact the original manufacturer for a replacement copy.
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The information included on a calibration sheet differs with each sensor. For some sensors, the sheet contains coefficients necessary to program a data logger. For other sensors, the calibration sheet is a pass/fail report.
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Because of the loss of IR radiation, nearly all thermopile instruments typically have a negative offset. This offset is most easily visible at night-time, when a small negative value is read instead of zero. This same offset is present during the daytime, but it is not as visible because of the large solar signal.
Another common issue involves leveling an instrument. Leveling a thermopile instrument can cause errors in the direct beam component because the cosine response is not correct. These errors are more notable when the sun is close to the horizon because the angle is so shallow.
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