Hukseflux Thermal Sensors

Warning statements

Cautionary statements are subdivided into four categories: danger, warning, caution and notice according to the severity of the risk.

Introduction

HTR02 heaters are so-called foil heaters. They can be used for functionality checks of foil heat flux sensors like those of the FHF05 series. The heaters have a 4-wire connection with a known surface area and electrical resistance. Users can now easily and objectively check their sensor performance before and after use. HTR02 series is available in two different models: 50 x 50 and 85 x 85 mm. See also FHF05SC series heat flux sensor with integrated heater.

Measuring heat flux, users may wish to regularly check their sensor performance. A quick check is now possible with HTR02 heaters plus some accessories that most laboratories will have in-house. The HTR02 heaters have a well-characterised traceable surface area and electrical resistance.

In a typical test setup, the heat losses through the insulation are typically smaller than 3 % and may be ignored. Measuring the heater power (voltage U_heater² divided by resistance R_heater), and dividing by the surface area A_heater, gives the applied heat flux. The heat flux sensor sensitivity S is the voltage output U_sensor divided by the applied heat flux.

S = (Usensor * Rheater * Aheater) / Uheater2 (Formula 0.1)

The reproducibility of this test is much improved when using contact material between the heater, sensor and heat sink.

HTR02 series has unique features and benefits:

• Makes it possible to perform a simple test
• Guarantees sensor stability
• Matches all models of the FHF05 series heat flux sensors

Figure 0.1

Two Hukseflux heaters are shown. On the left is model HTR02-85X85, and on the right is model HF05-85X85, which can be used with the HTR02-85X85.

HTR02 comes in two models. These can be used for test purposes in combination with foil heat flux sensors, such as those of the FHF05 series, or as general-purpose heaters.

Options:

• Available with standard cable length -02 metre, or change -02 to -05 or -10 metres for the respective cable length.
• Cables can also be ordered separately in 2, 5 or 10 metres length.

See also:

• FHF05SC heat flux sensor with integrated heater
• FHF05 series general purpose heat flux sensor
• View our complete range of heat flux sensors

1 Ordering and checking at delivery

1.1 Ordering HTR02

The standard configuration of HTR02 series is model 50X50 with 2 metres of cable, order code: HTR02-50X50-02.

Common options are:

• Model HTR02-85X85
• -05 or -10 metres cable length
• With a separate cable in 2, 5 or 10 metres cable length

1.2 Included items

Arriving at the customer, the delivery should include:

• HTR02 heater with cable of the length as ordered
• Product certificate matching the instrument serial number

Figure 1.2.1

A Hukseflux HTR02-50X50 heater with its cable. A close-up shows the end of the cable with a label displaying the serial number 'HTRO2 3011' and resistance 'R = 44.8 Ω'.

1.3 Quick instrument check

A quick test of the heater can be done by connecting it to a multimeter:

1. Check the heater serial number on the label at the end of HTR02's cable against the product certificate provided with the heater.
2. Inspect the instrument for any damage.
3. Check the electrical resistance of the heater between any of the yellow wires and any of the grey wires. Use a multimeter at the 1 kΩ range. Typical resistance should be around 120 Ω for model -50X50 and around 40 Ω for model -85X85. Infinite resistance indicates a broken circuit; zero or a lower than 1 Ω resistance indicates a short circuit.
4. Check the electrical resistance between the 2 yellow wires. These resistances should be in the 0.1 Ω/m range, so 0.2 Ω in case of the standard 2 m wire length. Higher resistances indicate a broken circuit. Repeat this measurement for the 2 grey wires.

2 Instrument principle and theory

HTR02 series is a foil heater. It can either be used in combination with foil heat flux sensors such as FHF05 series for test and calibration purposes, or it can be used as a general-purpose heater.

2.1 Basic operation

If a voltage U_heater is applied to the heater such that an electrical current I_heater runs through the heater, the heater power P_heater may be calculated as:

Pheater = Uheater * Iheater = Uheater2 / Rheater = Iheater2 * Rheater

where R_heater is the heater electrical resistance. If the heater is placed in a uniform environment (i.e., same medium on both sides of the heater) the applied heat flux Φ in either direction may be calculated as:

Φ = Pheater / (2 * Aheater) (Formula 2.1.1)

where A_heater is the heater area. If, however, the heater is placed in between a thermal insulator and a good thermal conductor the heat flux Φ in the direction of the conductor is:

Φ = Pheater / Aheater (Formula 2.1.2)

Other cases exist as well. Users need to evaluate which case applies to their situation.

2.2 A self-test for heat flux sensors

In combination with a heat flux sensor such as FHF05 series, HTR02 can be used to test the response of the heat flux sensor. To this end, HTR02 should be positioned directly on top of the heat flux sensor so that HTR02 can be used to apply a heat flux through the heat flux sensor.

A self-test is started by switching on HTR02, while recording the heat flux sensor output signal and the HTR02 heater power and finalised by switching HTR02 off. During the heating interval, a current I_heater is fed through the foil heater which generates a known heat flux proportional to the heater power. To calculate this heat flux, the heater power P_heater must be measured accurately. This power can be measured in several different ways;

• Heater voltage and current, Pheater = Uheater * Iheater (Formula 2.2.1)
• Heater voltage and known heater resistance, Pheater = Uheater2 / Rheater (Formula 2.2.2)
• Heater current and known heater resistance, Pheater = Iheater2 * Rheater (Formula 2.2.3)

If performed in a four-wire configuration, the first method of formula 2.2.1 is preferred, because it is generally more accurate than the latter two methods. However, it requires both a voltmeter and an ammeter instead of just one of the two. This is why the method of formula 2.2.2 is more commonly applied.

Analysis of the heat flux sensor response to the heating (the self-test) serves several purposes:

• First, the amplitude and response time under comparable conditions are indicators of the sensor stability. See 2.5 for application examples.
• Second, the functionality of the complete measuring system is verified. For example: a broken cable is immediately detected.
• Third, under the right conditions, after taking the sensor out of its normal environment, the self-test may be used as calibration. See 2.4 for more details.

2.3 Using heaters with FHF heat flux sensors

HTR02 series is compatible with the models from the FHF05 series. Model FHF05-50X50 and -85X85 fit directly with HTR02-50X50 and -85X85 respectively.

Models FHF05-10X10 and FHF05-15X30 fit with HTR02-50X50. Model FHF05-15X85 can be used with HTR02-85X85. Because in these cases the heater is larger than the sensor, it is recommended to make a guard when using these models with HTR02. See Table 2.3.1 for an overview of the models and suggested heaters.

Figure 2.3.1

An HTR02-50X50 heater is shown positioned on an FHF05-15x30 heat flux sensor. A protective guard, made with the help of a 3D printer, surrounds the sensor.

To get a representative measurement, the heater should 'see' the same environment. Surrounded by a material like a metal heat sink, the sensor will locally increase the thermal resistance. The heat from the heater will follow the easiest path; in the case the heater is larger than the sensor, flow to the metal heat sink. Therefore, the sensor will measure an underestimation of the actual heat flux.

Figure 2.3.2

Diagrams illustrating the compatibility between HTR02 heaters and FHF05 sensors. It shows that FHF05 models -10X10 and -15X30 are compatible with HTR02-50X50, and FHF05-15X85 is compatible with HTR02-85X85. The diagrams highlight areas where a guard is recommended, especially when the heater is larger than the sensor.

To create an environment with constant thermal resistance, make a guard around the sensor with equal thermal resistance and thickness. FHF05 series have a thermal resistance of R_thermal = 11 × 10^-4 K/(W/m²) and a thickness of 0.4 × 10^-3 m.

We recommend a guard made from plastic. Most plastics have a thermal resistance in the same order of magnitude as the base material of the sensor. Use tape of comparable thickness or print a guard with a 3D printer. See Figure 2.3.1.

Table 2.3.1

2.4 Validation of heat flux sensors

It is recommended to recalibrate heat flux sensors at least once every two years. HTR02 series can be used to validate the performance of heat flux sensors such as the models in the FHF05 series.

In a typical calibration setup, as shown in Figure 2.4.1, a stack is made of a heatsink, the heat flux sensor to be validated, the heater and an insulating material. In such a setup, the heat losses through the insulation are for FHF-type sensors in the order of magnitude of 3%. In this case, heat generated by HTR02 flows through the heat flux sensor to the heat sink. Measuring the heater power P_heater, and dividing by the surface area A_heater, gives the applied heat flux:

Φ = Pheater / Aheater (Formula 2.4.1)

The heat flux sensor sensitivity S is the voltage output of the sensor U_sensor divided by the applied heat flux Φ:

S = Usensor / Φ (Formula 2.4.2)

The reproducibility of this test is much improved when using contact material (such as glycerol or a thermal paste) between heater, sensor and heat sink.

Figure 2.4.1

A diagram of a typical stack for heat flux sensor calibration. It includes an insulating foam (1), the HTR02 heater (2), the heat flux sensor (3), indicating heat flux flow (4) from hot to cold, and a metal block heatsink (5).

2.5 An in-situ test for heat flux sensors

The HTR02 series heater can be used to test the stable performance of the heat flux sensors such as FHF05 series.

HTR02 series should be installed on top of the heat flux sensor, preferably on the side of the heat flux sensors with the more insulating medium. In case the heater is used for repeated verifications at one location, consider using our FHF05SC series: sensor with a HTR02 heater integrated.

A typical stability check is performed based on the step response of the measured heat flux and sensor temperature to a heat flux applied by HTR02. Upon installing the heat flux sensor and HTR02, a reference measurement should be made. A time trace of the heater power, the measured heat flux and the measured sensor temperature should be stored as reference data. Stable operation of the heat flux sensor can then be confirmed at any time by comparing it to the reference measurement. The test protocol consists of the following steps:

1. Make sure that the absolute temperature is similar to that during the reference measurement.
2. Check the heater resistance stability. This can be done accurately by using the four heater wires to conduct a four-point resistance measurement.
3. Record a time trace of the heater power, the measured heat flux and the sensor temperature; the same parameters as in the reference data. Normalise the data by the heater power. Under normal circumstances (if the heater is stable) this process scales with U_heater².
4. Compare patterns of heat flux and temperature rise and fall. In both cases relative to the values just before heating. When the signal patterns match, amplitude differences, after correction for heater power, point towards sensor instability. In this case, recalibration of the sensor may be required (Figure 2.5.1). Non-matching patterns point towards changes in the sensor environment. This can, for example, be the result of a loss of thermal contact between sensor and object (Figure 2.5.2) or the presence of convective heat losses (Figure 2.5.3).

Figure 2.5.1

A graph showing in-situ sensor stability check results. It compares normalized heat flux (HF) and temperature (T) responses to stepwise heating against reference curves (black lines). Red lines indicate sensor non-stability, showing equal response times but lower heat flux and temperature rise.

Figure 2.5.2

A graph illustrating in-situ sensor stability check with a focus on thermal contact. It compares normalized heat flux (HF) and temperature (T) responses to stepwise heating against reference curves (black lines). Blue lines indicate a loss of thermal contact, resulting in slower response times, lower heat flux, and higher temperature rise.

Figure 2.5.3

A graph depicting in-situ sensor stability check under convective conditions. It compares normalized heat flux (HF) and temperature (T) responses to stepwise heating against reference curves (black lines). Grey lines represent exposure to convection, showing faster response times at lower heat flux and lower temperature rise.

3 Specifications of HTR02 heaters

3.1 Specifications of HTR02 heaters

Heaters of the HTR02 are thin, flexible foil heaters with 4-wire connection, a known surface area and a known electrical resistance. They are designed for validation and functionality checks of FHF-type heat flux sensors, but can also be used for general heating purposes. When used to test heat flux sensors, HTR02's must be combined with a switched or controlled power supply and a suitable measurement and control system.

Table 3.1 Specifications of HTR02 series

HTR02 SERIES SPECIFICATIONS

ELECTRICAL CHARACTERISTICS

3.2 Dimensions of HTR02 series

Figure 3.2.1

A diagram detailing the dimensions of HTR02 series heaters. It labels the heater area (1), passive guard (2), cable connection block for strain relief (3), and the standard cable length (4).

4 Installation of HTR02 series

4.1 Site selection and installation

Table 4.1.1

Figure 4.1.1

An illustration of the installation of an HTR02-50X50 heater using tape to secure the sensor and connection block. Cable tie mounts with double-sided tape are used for extra strain relief. Tapes are applied over the passive guard area, with an additional tape in the middle for support.

4.2 Electrical connection

4.2.1 Electrical diagram

Figure 4.2.1.1

An electrical diagram of the HTR02 heater foil, showing the four-wire connection with yellow wires for HEATER [+] and MEASURE [+], and grey wires for HEATER [-] and MEASURE [-].

4.2.2 Normal connection

To apply a heat flux, HTR02 should be connected to a power supply. If a variable heat flux is required, the heater is preferably connected to a programmable DC power supply (see Figure 4.2.1.1). The HTR02 electrical connections are explained in Table 4.2.1. When connecting HTR02, always observe the rated heater voltage. Users must make sure that the used power supply is able to source sufficient current.

Table 4.2.2.1

The electrical connections of HTR02. The heater has a cable consisting of four wires. The two yellow wires are equivalent, and the two grey wires are equivalent. Together they serve to make a 4-wire connection to the heater.

The heat generated by HTR02 can be accurately determined by measuring the heater voltage and current in a four-point measurement. To this end, HTR02 has a four-wire connection: two yellow wires and two grey wires. A voltmeter should be used to measure the voltage between one of the yellow and one of the grey leads (heater measure [+] and [-] in Figure 4.2.1.1). Working either with formula 2.2.1 or 2.2.2, either:

• An ammeter should be used to measure the current through the other yellow and grey leads (heater [+] and heater [-] in Figure 4.2.1.1), using I and V to estimate the heater power, or
• A voltage lower than 24 VDC should be applied to the other yellow and grey leads (heater [+] and heater [-] in Figure 4.2.1.1), using V and R to estimate the heater power.

Suggested HTR02 connections are shown in Figure 4.2.1. The heater serial number and electrical resistance R are shown on the HTR02 product certificate and on the label at the end of the cable.

When extending the HTR02 cable, please consider the thickness and electrical resistance of the wires: too thin wires may lead to excessive heating of the wires themselves.

Figure 4.2.2.1

A diagram illustrating suggested HTR02 heater wiring for a 4-wire connection to a programmable power supply. Two wires carry the heater current, while two others are used for voltage measurement.

4.3 Requirements for data acquisition and control

The selection and programming of the measurement and control system for heaters is the responsibility of the user.

Table 4.3.1

5 Maintenance and trouble shooting

5.1 Recommended maintenance and quality assurance

The HTR02 series performs reliably at a low maintenance level. Unreliable heater output can be detected by scientific judgement, for example, by looking for unreasonably large or small measured values. The preferred way to ensure a reliable heater output is a regular critical review of the measured data.

Table 5.1.1

Recommended maintenance of HTR02 series. If possible, the data analysis is done daily.

MINIMUM RECOMMENDED HEAT FLUX SENSOR MAINTENANCE

5.2 Trouble shooting

Table 5.2.1

Table 5.2.2

Indicative electrical resistances between wires for HTR02-50X50 with standard cable length.

Table 5.2.3

Indicative electrical resistances between wires for HTR02-85X85 with standard cable length.

6 Appendices

6.1 EU declaration of conformity

CE

We, Hukseflux Thermal Sensors B.V., Delftechpark 31, 2628 XJ, Delft, The Netherlands

hereby declare under our sole responsibility that:

• Product model: HTR02 series, all models
• Product type: heater for calibration and verification of performance of FHF-type heat flux sensors

is in conformity with the following directives:

2011/65/EU, EU 2015/863 The Restriction of Hazardous Substances Directive

This conformity is declared using the relevant sections and requirements of the following standards:

• Hazardous substances: RoHS 2 and EU 215/863 amendment known as RoHS 3

Eric HOEKSEMA
Director
Delft, 15 November 2022