HFP01SC Self Calibrating Heat Flux Sensortm USER MANUAL HFP01SC Manual v0811 Edited & Copyright by: Hukseflux Thermal Sensors http://www.hukseflux.com e-mail: info@hukseflux.
Hukseflux Thermal Sensors Warning: Putting more than 20 volts across the heater of HFP01SC may result in permanent damage to the sensor HFP01SC Manual v0811 page 2/33
Hukseflux Thermal Sensors Contents List of symbols Introduction 1 Theory of Self-Calibration 1.1 Introduction 1.2 Theory on heat flux measurement errors 1.3 Self-calibration 1.4 HFP01SC calculation 1.
Hukseflux Thermal Sensors List of symbols Heat flux Thermal conductivity Voltage output HFP01SC sensitivity (factory value) HFP01SC sensitivity (newly found value) Thermal conductivity dependence of Esen Time Surface area Deflection error (+ when ϕ is overestimated) Electrical resistance Thermal resistance Measurement at t is 0, 180 seconds Difference before and during heating ϕ Wm-2 λ W/m.
Hukseflux Thermal Sensors Introduction The HFP01SC self-calibrating heat flux sensortm is a sensor intended for high accuracy measurement of soil heat flux. Also it offers improved quality assurance of the measurement.
Hukseflux Thermal Sensors The self calibration results in an improved sensitivity estimate for the heat flux measurement. The HFP01SC design dates from 1998. In its first years of existence the HFP01SC has rapidly been accepted as the state-of-the-art method for measurement of soil heat flux in meteorology. In meteorological applications there are two reasons for the popularity of HFP01SC.
Hukseflux Thermal Sensors As for the Quality Assurance: measuring with a conventional heat flux sensor, after the sensor is dug in it is assumed that its remains where it is, in good contact with the soil, all cabling in good condition. With the HFP01SC during the calibration implicitly the cabling is checked, thermal connection and also the soil thermal conductivity (as the correction that we make is largely dependent on soil thermal conductivity).
Hukseflux Thermal Sensors Figure 0.1 HFP01SC outlook 80 5m 5.0 Figure 0.2 HFP01SC dimensions in mm: film heater (1) heat flux sensor body (2), cable (3).
Hukseflux Thermal Sensors 1 Theory of Self-Calibration 1.1 Introduction This chapter is written for users that are familiar with the general theory of heat flux sensors and error sources in the heat flux measurement. In case more information on this subject is required, literature (most notably the HFP01 manual) is available free of charge via e-mail as a PDF file. Please request at info@hukseflux.com. The self calibrating principle can only be used if the sensor is surrounded by at least 40 mm of soil.
Hukseflux Thermal Sensors 1.2 Theory on heat flux measurement errors Using a normal heat flux sensor, there are several common error sources. In soil these are in particular the deflection error and the temperatre dependence. In case of use of HFP01SC, typically in the soil, the resistance error is neglected. As a first approximation, the heat flux is expressed as: ϕ = Vsen / Esen 1.2.
Hukseflux Thermal Sensors Figure 1.2.2 The resistance error: a heat flux sensor (2) increases or decreases the total thermal resistance of the object on which it is mounted or in which it is incorporated. An otherwise uniform flux (1) is locally disturbed (3). A first order correction of the measurement is: ϕ = (Rthobj+Rthsen ) V sen /E sen Rthobj 1.2.
Hukseflux Thermal Sensors Figure 1.2.3 The deflection error. The heat flux (1) is deflected in particular at the edges of the sensor. As a result the measurement will contain an error; the so-called deflection error. The magnitude of this error depends on the medium thermal conductivity, sensor thermal properties as well as sensor design. In addition, there is a temperature dependence TD reflects the fact that the sensitivity changes with temperature: Esen = E sen, cal (1+TD (Tcal - Tsen )) 1.2.
Hukseflux Thermal Sensors 1.3 Self-calibration Figure 1.3.1 Explanation of the self-calibrating principle: On the left the normal situation with a heat flux ϕ. Due to the fact that sensor and medium do not match, the actual flux through the sensor is reduced by a factor (1-X). This error is called deflection error. On the right, the film heater that is mounted on top (1) is activated to generate a well known heat flux ϕ. The response of the heat flux sensor is measured.
Hukseflux Thermal Sensors Figure 1.3.2 electrical connection HFP01SC. Sensor (2) wiring 1 and 2. Heater (1) heater voltage input wires 3 and 4. Heater current measurement, typically performed by putting a 10 ohm resistor (3) in series (not included with HFP01SC), and by measuring the voltage across the resistor, wires 5 and 6. Dashed line (4), sensor on the left and cable and datalogger on the right. Figure 1.3.
Hukseflux Thermal Sensors 1.4 HFP01SC calculation The heater generally is switched on every three or six hours. The totoal calibration takes about 6 minutes. During 3 of these 6 minutes, a current is lead through the film resistor for self test, in order to generate a well-known heat flux. The sensor output signal, Vsen, is measured.
Hukseflux Thermal Sensors The factory delivered calibration factor Esen is determined at the factory using an electrically generated heat flux that is forced through the sensor. In soils, corrections of up to +5 to -20% relative to the factory supplied calibration coefficient can be expected. (the +5% due to temperature dependence) During the calibration process it is suggested to discard the measured values of the heat flux.
Hukseflux Thermal Sensors 1.5 Additional quality assurance If possible, additional measures for quality assurance of the measurement can be programmed. It is suggested to generate an error message if Esen2 is larger than the factory delivered Esen by more than 5%, and smaller by more than 20%. A possible error source is that there has been too much fluctuation between of the heat flux in the soil during the calibration process.
Hukseflux Thermal Sensors 2 Application in meteorology In meteorological applications the primary purpose is to measure the part of the energy balance that goes into the soil. This soil heat flux in itself is in most cases of limited interest. However, knowing this quantity, it is possible to “close the balance". In other words, apply the law of conservation of energy to check the quality of the other (convective and evaporative) flux measurements.
Hukseflux Thermal Sensors Heat flux sensors in meteorological applications are typically buried at a depth of about 5 cm below the soil surface. Burial at a depth of less than 5 cm is generally not recommended. In most cases a 5 cm soil layer on top of the sensor offers just sufficient mechanical consistency to guarantee long-term stable installation conditions.
Hukseflux Thermal Sensors 3 Specifications of HFP01SC HFP01SC self-calibrating heat flux sensor is intended to be used for determining the heat flux in soil. It is normally used in combination with a suitable measurement and control system, including power supply and relay to activate the self calibration process. HFP01SC GENERAL SPECIFICATIONS Specified Heat flux in W/m2 perpendicular to the measurements sensor surface Installation See the product manual for recommendations.
Hukseflux Thermal Sensors Response time (nominal) Range Non stability ± 3 min (equals average soil) + 2000 to - 2000 W.m-2 < 1% change per year (normal meteorological use) compensated during self-calibration Required readout / 1.1 one differential voltage channel hardware or possibly (less ideal) 1.2 one single ended voltage channel. When using more than one sensor and having a lack of input channels, it can be considered to put several sensors in series, while working with the average sensitivity.
Hukseflux Thermal Sensors 4 Short user guide Preferably one should read the introduction and the section on theory. Really important items are put in boxes. The sensor should be installed following the directions of the next paragraphs. Essentially this requires a data logger and control system capable of switching, readout of voltages, and capability to perform calculations based on the measurement. The first step that is described in paragraph 5 is and indoor test.
Hukseflux Thermal Sensors 5 Putting HFP01SC into operation It is recommended to test the sensor functionality by checking the impedance of the sensor and heater, and by checking if the sensor works, according to the following table: (estimated time needed: 20 minutes) Check the connection of the heater. Use a multimeter at the 200 ohms range. Measure between two wires that are connected to the heater. The typical impedance of the wiring is 0.1 ohm/m. Typical impedance should be 1.
Hukseflux Thermal Sensors The HFP01SC should be connected to the measurement and control system as described in the chapter on the electrical connection. The programming of data loggers is the responsibility of the user. Please contact the supplier to see if directions for use with your system are available.
Hukseflux Thermal Sensors 6 Installation of HFP01SC HFP01SC is generally installed at the location where one wants to measure at least 4 cm depth below the surface. A typical depth of installation is 5 cm. Typically 2 sensors are used per measurement location in order to promote spatial averaging, and to have some redundancy for improved quality assurance. Sensors are typically several meters apart. The more even the surface on which HFP01SC is placed the better.
Hukseflux Thermal Sensors 7 Maintenance of HFP01SC Once installed, HFP01SC is essentially maintenance free. Usually errors in functionality will appear as unreasonably large or small measured values. As a general rule, this means that a critical review of the measured data is the best form of maintenance. In case 2 sensors are mounted on one location the ratio of measurement resuls could be monitored over time; this will give a clue if there is any unstability.
Hukseflux Thermal Sensors 8 Requirements for data acquisition and control Capability to measure microvolt signals (sensor signal) Capability of measuring currents Capability of switching Requirements for power supply of the heater Capability for the data logger or the software 5 microvolt resolution or better Around 0.1A, with 1% accuracy, typicaly perfromed by using a 10 Ohm resistor and measurement of the voltage across this resistor. 9-15 volt at 0.
Hukseflux Thermal Sensors 9 Electrical connection of HFP01SC In order to operate, HFP01SC should be connected to a measurement and control system as described above. A typical connection is shown in figure 9.1. For the purpose of making a correct measurement of the heater power there is a current sensing resistor in the wire that leads to the heater. The voltage over a 10-ohm current sensing resistor should be of the order of magnitude of 1 to 2 Volts. The required resistance must be 0.1%, 50ppm or better.
Hukseflux Thermal Sensors Wire colour code Heat flux sensor + White Heat flux sensor Green Heater Brown Heater Green Table 9.1 typical HFP01SC colour code Cable Number 1 1 2 2 NOTE: Sensors suplied by Campbell Scientific USA sually have a diferent wiring diagram; see the Campbell manual for ore information. In most cases the 10 ohm is included into th sensor wiring.
Hukseflux Thermal Sensors 10 Programming for HFP01SC The central formula is: Esen 2 = 2Vamp (R2cur.Aself)/(V2cur.Rself) 1.4.4 Rself and Aself are given on the sensor calibration certificate. Rcur is user supplied. The soil heat flux is calculated as: ϕ =V sen /E sen2 1.4.6 Possible quality assurance tests are: 1: Esen2 < 0.8 Esen 2: Esen2 > 1.05 Esen 3: Vsen (0) - Vsen (360) < 0.
Hukseflux Thermal Sensors Sensor specific part, entering Rself, Rcur and Aself, possibly also Esen. Repetitive loop typically 3 or 6 hour repetition. This is typically done in the data logger, but could also be done in a later stage, during processing. The heater resistance, Aself and Esen can be found on the calibration certificate. Measure Vsen(0) Store Vsen(0) There usually will be a significant signal representing the present heat flux at that moment.
Hukseflux Thermal Sensors 11 Appendices 11.1 Appendix on cable extension for HFP01SC HFP01SC has 2 cables, one for the sensor, and one for the heater. It is a general recommendation to keep the distance between data logger and sensor as short as possible. Cables generally act as a source of distortion, by picking up capacitive noise. HFP01SC cable can however be extended without any problem to 100 meters.
Hukseflux Thermal Sensors 11.
