HUKSEFLUX FHF03 (01) PDF Document

When using the HUKSEFLUX FHF03, please observe the following warning statements:

• Putting more than 12 Volts across the sensor wiring can lead to permanent damage to the sensor.

• Do not use “open circuit detection” when measuring the sensor output.

The following symbols represent quantities and subscripts related to the HUKSEFLUX FHF03 sensor.

Quantities

Subscripts

The economical HUKSEFLUX FHF03 foil heat flux sensor has the following unique features and benefits:

• flexible (bending radius ≥ 25 x 10⁻³ m)

• low thermal resistance

• wide temperature range

• fast response time

• integrated type T thermocouple

• robust: well-protected wire connections and a sturdy, shielded cable

• IP protection class: IP67 (essential for outdoor application)

• thermal spreader included, low thermal conductivity dependence

When you receive your HUKSEFLUX FHF03, the delivery should include:

• heat flux sensor FHF03 with cable of the length as ordered

• product certificate matching the instrument serial number

A quick test of the HUKSEFLUX FHF03 can be done by connecting it to a multimeter. Follow these steps:

1. Check the sensor serial number and sensitivity on the sticker against the product certificate provided with the sensor.

2. Inspect the instrument for any damage.

3. Check the electrical resistance of the sensor between the red [+] and black [-] wires. Use a multimeter at the 100 Ω range. Measure the sensor resistance first with one polarity, then reverse the polarity. Take the average value. Measured resistance should be the nominal sensor resistance of 25 Ω plus 0.2 Ω/m. Infinite resistance indicates a broken circuit; zero or a lower than 1 Ω resistance indicates a short circuit.

4. Check the electrical resistance of the thermocouple between the brown [+] and white [-] wires. Use a multimeter at the 100 Ω range. Measure the thermocouple resistance first with one polarity, then reverse the polarity. Take the average value. Measured resistance should be the nominal thermocouple resistance of 2.5 Ω plus 2.5 Ω/m. Infinite resistance indicates a broken circuit; zero or a lower than 1 Ω resistance indicates a short circuit.

5. Check if the sensor reacts to heat: put the multimeter at its most sensitive range of DC voltage measurement, typically the 100 x 10⁻³ VDC range or lower. Expose the sensor to heat. Exposing the back side to heat should generate a positive signal between the red [+] and black [-] wires. Doing the same at the front side reverses the sign of the output.

The HUKSEFLUX FHF03 measures heat flux density (in W/m²) through the sensor itself. The core of the sensor is a thermopile, which measures the temperature difference across its flexible polyimide body. The thermopile is a passive component that generates a small voltage linearly proportional to this temperature difference. The heat flux is proportional to this same temperature difference divided by the effective thermal conductivity of the sensor body. To calculate the heat flux (Φ), you divide the measured voltage output (U) by the sensitivity (S), a constant provided with each individual sensor.

The HUKSEFLUX FHF03 is designed so that heat flux from the back side to the front side (the side with the readable sticker and logo) generates a positive voltage output signal.

The HUKSEFLUX FHF03 is calibrated under the following reference conditions:

• conductive heat flux (as opposed to radiative or convective heat flux)

• homogeneous heat flux across the sensor and guard surface

• room temperature

• heat flux in the order of 600 W/m²

• mounted on an aluminum heat sink

If you use the sensor under conditions that differ from these, such as at extreme temperatures or with radiative flux, the sensitivity may differ from what is stated on the certificate.

The dimensions of the HUKSEFLUX FHF03 heat flux sensor are given in millimeters (mm), which corresponds to x 10⁻³ m.

• Sensor body length: 31 mm

• Sensor sensing area length: 25 mm

• Total length including strain relief transition: 60 mm

• Sensor body width: 14.5 mm

• Sensor sensing area width: 10 mm

• Sensor thickness: 0.8 mm

• Strain relief diameter: Ø 5.5 mm

• Cable diameter: Ø 3.6 mm

The main components are:

(1) Sensing area with thermal spreader

(2) Passive guard

(3) Type T thermocouple

(4) Sticker showing serial number and sensitivity

(5) Strain relief

(6) Cable, standard length 2 m

The flexibility of the HUKSEFLUX FHF03 allows it to be installed on singly curved surfaces like pipes. The sensor should be bent along its length. For installation on curved surfaces, it is usually not possible to tape only over the passive guard area. Use sufficient tape to ensure the sensor remains fixed and in good thermal contact with the curved surface, avoiding air gaps. Tape can be used over the sensing area if necessary. The HUKSEFLUX FHF03 is not suited for dynamic bending.

Please see the following recommendations:

The HUKSEFLUX FHF03 is a passive sensor that does not require power. It should be connected to a measurement system like a datalogger. The shield is not connected to the sensor itself; connect the shield to a local ground to protect from capacitive noise. The wiring is as follows:

You can connect multiple HUKSEFLUX FHF03 sensors electrically in series to increase the overall sensitivity. The resulting sensitivity is the sum of the individual sensor sensitivities. For two sensors, the heat flux (Φ) is calculated as Φ = U / (S₁ + S₂), where U is the total voltage (U₁ + U₂).

The electrical connection for two HUKSEFLUX FHF03 sensors (Sensor 1 and Sensor 2) in series is as follows:

The HUKSEFLUX FHF03 can be connected to read out the heat flux through only the left 50% or the right 50% of its sensing area. This can be used for quality assurance purposes. If the sensor is correctly installed, you should expect a constant percentage of the signal to be generated by the left and right sides.

Electrical connection for 100% signal:

Electrical connection for left 50% signal:

Electrical connection for right 50% signal:

The recommended calibration interval for the HUKSEFLUX FHF03 is 2 years. While recalibration is ideally done by the manufacturer, on-site field calibration is possible by comparison to a calibration reference sensor.

Hukseflux main recommendations for field calibrations are:

1) to compare to a calibration reference of the same brand and type as the field sensor

2) to connect both to the same electronics, so that electronics errors (also offsets) are eliminated

3) to mount all sensors on the same platform, so that they have the same body temperature

4) typical duration of test: > 24 h

5) typical heat fluxes used for comparison: > 200 W/m²

6) to correct deviations of more than ± 20%. Lower deviations should be interpreted as acceptable and should not lead to a revised sensitivity

Users may also design their own calibration experiment, for example using a well characterised foil heater.

In an electrically “quiet” environment, the cable of the HUKSEFLUX FHF03 may be extended. Proper extension will not significantly degrade the small sensor signal due to the sensor’s very low resistance and the absence of current flow. Preferred specifications for cable extension are summarized below:

The sensitivity of the HUKSEFLUX FHF03 has a temperature dependence specified as < 0.3 %/°C, with typical values around +0.2 %/°C. The calibration reference temperature is 20 °C. If you are measuring at temperatures that deviate significantly from 20 °C or over a wide temperature range, you may wish to correct for this dependence.

To correct for the temperature dependence of the sensitivity, use the following measurement function:

Φ = U/(S⋅(1 + 0.002⋅(T – 20)))

Where:

• Φ is the heat flux in W/m²

• U is the HUKSEFLUX FHF03 voltage output in V

• S is the sensitivity in V/(W/m²) at 20 °C (found on the product certificate and sticker)

• T is the HUKSEFLUX FHF03 temperature in °C

The specified measurement range of (-10 to +10) x 10³ W/m² for the HUKSEFLUX FHF03 is at a 20 °C heat sink temperature. The actual limiting specification is the maximum sensor temperature of +150 °C. The measurement range is therefore lower for higher heat sink temperatures.

The sensor temperature (T) can be estimated using the formula:

T = Theatsink + Φ⋅Rthermal,A

The maximum heat flux (Φmaximum) can be calculated as:

Φmaximum = (150 – Theatsink)/Rthermal,A

The table below shows measurement ranges for different heat sink temperatures. For applications without a heatsink, use the ambient temperature.