Hukseflux Thermal Sensors Heat Flux Sensor Installation Guide

: June 9, 2024
: Hukseflux

Table of Contents

Hukseflux Thermal Sensors Heat Flux Sensor
Product Information
How to Install a Heat Flux Sensor
Tips and Tricks for Heat Flux Measurement
Introduction
General considerations for heat flux measurement
Considerations for installation
Mounting
Why to avoid air gaps
What to do about air gaps
Why optical properties are important
References
Read User Manual Online (PDF format)
Download This Manual (PDF format)

Hukseflux Thermal Sensors Heat Flux Sensor

Hukseflux-Thermal-Sensors-Heat-Flux-Sensor-
product

Product Information

Hukseflux Thermal Sensors

Hukseflux Thermal Sensors are heat flux sensors that can be used for a variety of applications, such as thermal insulation analysis, pipeline fouling monitoring, and health monitoring of pigs. Measuring the heat flux can provide valuable insights into processes and system performance. It is important to choose the right sensor and install it correctly to get accurate data.

General Considerations for Heat Flux Measurement

When measuring heat flux, it is important to consider the radiative and convective heat transfer mechanisms that affect the measurement. The sensor should match the surface of the object it is mounted on to ensure accurate measurement. Avoiding air gaps and choosing the right mounting solution are also critical factors.

How to Install a Heat Flux Sensor

Installing a heat flux sensor involves choosing the right mounting solution and ensuring proper strain relief on the cable to avoid stressing the sensor. Table 1 provides options for mounting heat flux sensors, including materials that can be used to fix the sensor position and fill air gaps. Always ensure that the sensor surface matches that of the object it is mounted on.

Figure 1 shows an example of a heat flux sensor mounted on a wall using silicone glue. Note that the sensor’s optical properties do not match those of the metal wall it will later be covered with a metal cover so that optical properties match.

Tips and Tricks for Heat Flux Measurement

To get the most out of your heat flux measurement, consider using a sensor with a low thermal resistance, such as the FHF05 or HFP01. Avoiding air gaps is also important, as they can significantly affect the measurement accuracy. Glycerine and toothpaste can be used to fill air gaps, but they are only suitable for temporary or quick experiments. Double-sided tape and silicone glue are more permanent solutions.

Always refer to the product manual and follow the manufacturer’s instructions for best results.

How to install a heat flux sensor
Tips and tricks to get the most out of your heat flux measurement
Measuring heat flux is a powerful tool to gain insights in processes. You may measure for example how much heat flows through a wall, or to a specimen that must be cooled. Assuming the right sensor is used, installing this sensor correctly, so that it performs a stable measurement and measures the right heat flux (radiative and convective), is a critical step to get the right data. This paper dives into the do’s and don’ts when installing a heat flux sensor.

Introduction

Heat flux sensors have a wide variety of applications, from thermal performance analysis of thermal insulation, to monitoring of fouling of pipelines and the health monitoring of pigs. Measuring the heat flux can lead to useful insights in processes and system performance. Assuming the right sensor is used, mounting this sensor correctly, so that it performs a stable measurement and measures the right heat flux (radiative and convective), is a critical step to get the right data.
This paper focuses on sensor installation. What are the do’s and don’ts when installing a heat flux sensor; how can you get the best data from your sensor . Hukseflux-Thermal-Sensors-Heat-Flux-Sensor-fig 1

Figure 1 FHF05SC-85X85 Heat flux sensor mounted on a wall, using silicone glue (NR 5 in table 1). NOTE: sensor optical properties do not match those of the metal wall it will later be covered with a metal cover so that optical properties match (NR 4 in table 3).

General considerations for heat flux measurement

use the right sensor for the application. There are many different models each with its own temperature- and heat flux range. View our complete product range of heat flux sensors.
see also our video on YouTube: how to measure heat flux.
perform a representative measurement. This starts with choosing the right location, representative for the system to be monitored. Use multiple sensors. The representativeness may be reviewed using infrared cameras.

Considerations for installation

Regardless of the heat flux sensor type, it is important that it is mounted securely in order to avoid variations of contact resistance between the sensor and the object on which it is mounted.

air gaps between sensor and object may be significant thermal resistances and increase response time. This should be avoided.
sensors gradually getting loose essentially produce unreliable (apparently unstable) measurements. Use a stable glue or filler. Use high quality cabling and strain relief.
Also, optical properties must match.
pay attention to the optical properties of the sensor surface. These must match those of the object the sensor is mounted on.

Mounting

There are various ways to mount a heat flux sensor, depending on the application. Two important parameters are

temperature range
the duration of the measurement

These two parameters will help choosing the right mounting solution for the heat flux sensor.

Table 1 and the examples at the end of this note will help you review your options.
Always ensure strain relief on the cable to avoid unnecessary stress on the sensor.

Why to avoid air gaps

The thermal conductivity of air is in the order of 0.02 W/(m·K). Therefore, even small air gaps are significant thermal resistances.
The thermal conductivity of a plastic or thermal paste is in the order of 0.2 W/(m·K), so for the same thickness, thermal resistance is a factor 10 lower.
Take for example a 0.05 x 10-3 m, air gap. This has a thermal resistance of 20 x 10-4 K/(W/m2). This may be compared to 11 x 10-4 K/(W/m2) for FHF05 series or 70 x 10-4 K/(W/m2) for HFP01, so a small air gap produces an increase of thermal resistance of respectively 200 % for FHF and about 30 % for HFP01. Using a filler of 0.05 x 10- 3 m, with a thermal conductivity around 10 times higher than that of air, the thermal resistance is reduced to 2.5 x 10-4 K/(W/m2). The contribution the thermal resistance reduces to about 20 % for FHF05 and 3 % for HFP01. From this example you can also see that it is not necessary to use high-thermal conductivity tapes. Using a thin normal tape is enough. An air gap may not only lead to a higher thermal resistance for conductive heat, but also to an entirely different radiation balance. An air gap is a “resistance” (a radiation screen) for radiative transfer. If it is filled-up, it is no resistance any longer. Watch out in case radiative (far infra-red) heat flux is significant. In that case the presence of an air gap may be the dominant source of errors, because a sensor with an air gap acts as a radiation shield, reducing local radiative transfer by a theoretical maximum of 50 %.

Table 1 Options for mounting heat flux sensors. Materials may act to fix the sensor position, but also to fill up airgaps.

NR| product| duration| rated

temperature range

| functionality| comments
---|---|---|---|---|---
[#]| [description]| [description]| [˚C]| [description]| [description]
1| powerstrip| temporary,

easily removable

easily dissolves in water.

water-based

most commercially available toothpastes are suitable

be tested individually before usage)

silicone oil-based

DOW CORNING heat sink compound 340 OMEGATHERM conductive paste

6| silicone glue| permanent| -45 to 200| fixation and

gap filling

| most commercially available silicone glues are suitable

DOWSIL 3145 silicone sealant

TESA 51408 orange masking tape

most commercially available Kapton tapes are suitable

NR| product| duration| rated temperature

range

| functionality| comments
---|---|---|---|---|---
[#]| [description]| [description]| [˚C]| [description]| [description]
8| magnets| temporary or permanent| to 500| fixation only| on magnetic surfaces only

for sensors with optional “frame with magnet” only in case using welded treads or bolting is not

fixation only, use with other fillers such as silicone, graphite sheet material or cements

usually combined with springs

fixation only, use with other fillers such as silicone, graphite sheet material or cements

usually combined with springs

ERIKS silicone sheet 0.5 mm

users can cut sheets to size

ERIKS Ergaflex or similar sheet material

users can cut sheets to size.

13| high temperature

cement

epoxies

| various| various| various| OMEGA cements and epoxies

What to do about air gaps

Tapes, sheet (gasket) material, glues and cement al may be used to fill-up air gaps.
These gaps may occur:

because of the nature of the surface. It may not be smooth. Smoothen before installation
because of a curvature in the surface. For all practical purposes a surface with a radius of smaller than 5 m is considered “flat”. At smaller radii, use of flexible sensors may be considered. For industrial sensors like IHF01 and IHF02, we may also provide coupling pieces (flat on one side, curved on the other).
Table 1 summarises the different mounting options.

Why optical properties are important

When heat flux sensors are mounted at a surface, heat will often be transferred by a combination of radiation and convection. For the convective part, the thermal resistance of the sensor should be as low as possible. For the radiative part, the optical surface properties of the sensor should be representative of the surrounding area.
Some points to keep in mind:

radiation is not only transmitted in the spectral range that humans can see (visible radiation) but also as non-visible far infra-red
blank metal is reflective in the visible as well as in the far infra-red
paints and plastic coatings wood and stone absorb in different ranges, depending on their colour in the visible range. These materials typically all behave as “black” in the far infra-red. See figure 2.
To get some feeling how radiative heat transfer works: see Table 2.
To see recommendations how to adapt the surface optical properties of your sensor: see Table 3.
The representativeness may be reviewed using a combination of normal (visible range) and infrared (far infra-red range) cameras.

Figure 2 Measuring with BLK – GLD stickers; application of a BLK black sticker and a GLD gold sticker on FHF models for measuring radiative and convective heat flux on an espresso machine. The machine has a polished metal surface of about 45 ˚C. The IR image on the right shows that the black sticker on the left, as well as the sensor wires and connector blocks, emit radiation. They appear in red on the image. The gold sticker and the metal surface have lower emission and appear as “bluish” on the image. Mounted on the same surface, the BLK and GLD stickers have the same temperature. The measurement with the sensor with the GLD sticker is most representative of the heat flux at the polished metal surface, while the sensor with the BLK sticker overestimates the heat flux.

Table 2 Properties of some common heat flux sources and receiving surfaces. Optical properties of heat flux sensors must match those of the surface they are mounted upon, so that they react in the same way to radiative heat flux.

material / source| visible

0.3 to 0.7 micron

| near infrared

0.7 to 3 micron

| far infrared

3 to 50 micron

| examples
---|---|---|---|---
sun

(blackbody of 7000 ˚C)

| emission| emission| no significant

emission

| the sun emits radiation in the 0.3 to 3

micron range

blackbody

(-30 to 70 ˚C)

| absorption| absorption| absorption and emission| objects at normal ambient temperatures emit energy in the far infra-red (3 to 50

micron) to their environment

white object

(-30 to 70 ˚C)

| reflection| reflection| absorption and emission| objects that appear white to the human eye reflect solar radiation, but are “black” (behave like blackbodies) in the

invisible “far infra-red”.

coloured object (-30 to 70 ˚C)| partial absorption / partial reflection| partial absorption| absorption and emission| objects that are coloured to the human eye absorb selectively in the visible

range

blank metal object (all temperatures)| reflection (low emission, low absorption)| reflection (low emission, low absorption)| reflection (low emission, low absorption)| blank metal reflects and has a low emission. Low emission of far infra-red is why thermal insulation systems may

have a blank metal cover.

Table 3 : What you can do to adapt the surface properties of your heat flux sensor.

NR| material / source|
---|---|---
1| blackbody

(-30 to 70 ˚C)

| To create absorbing surfaces you may use tapes like 3M temflex PVC insulation tape

(rated temperature to 90 ˚C)

BLK black sticker is available for all sensor models of FHF05 series and HFP01 (rated temperature to 150 °C)

You may also paint sensor surface paints as used in industry like RUSTOLEUM spray paint

(rated temperature to 600 ˚C)

For high temperature black paint RUSTOLEUM Hard hat black or TEMPIL Pyromark 2500 (rated temperature to 650 and 1093 ˚C respectively)

2| white object

(-30 to 70 ˚C)

3| coloured object (-30 to 70 ˚C)
4| blank metal object (all temperatures)| To create a reflective surface you may use aluminium tapes like 3M 425 tape

Metal sensors like our model IHF01 and IHF02 are already reflective, and may be polished on request

GLD gold sticker is available for all sensor models of FHF05 series and HFP01

(rated temperature to 150 °C)

Example 1: HFP01
Heat flux sensor model HFP01 is used a lot on walls to analyse their thermal resistance.

For short term installation, thermal paste (typically silicone-oil based) or a water-based paste (toothpaste) can be used to fill up the space between the sensor and the surface.
In case of long-term use at one location, the sensor can be installed using double sided tape. Thermal paste is not recommended in this situation because it tends to dry out over time. Choosing double sided tape, make sure the pieces of tape fit neatly next to one another. Apply tape over the entire area of the sensor.
Figure 3 Installation of HFP01 on a wall using 2-sided “removable” carpet laying tape such as TESA 4939 (table 1, NR4) and a strain relief of the cable using a cable tie mount equipped with the same carpet laying tape as adhesive. Note that the optical properties in the visible range do not match those of the surrounding wall. This is acceptable if the heat transfer by visible radiation is negligible, in other words if there is no solar radiation or illumination by strong lamps.

Example 2: IHF02
At high temperatures, above 150 ˚C, sensors like HFP01 do not survive. For temperature above 250 ⁰C also the models of the FHF series are not suitable. Industrial heat flux sensors such as model IHF02 are then used. They can withstand up to 100 kW/m2 and temperatures up to 900 ˚C. IHF02 features mounting flanges so that bolts or threads can be used for mounting . Hukseflux-Thermal-Sensors-Heat-Flux-Sensor-fig 4

Figure 4 IHF02 industrial heat flux sensor bolted to the surface. Note that the optical properties of sensor and surrounding metal are matching (both blank metal). Springs are not visible.

Figure 5 Industrial heat flux (6) sensor (5) mounted using flanges (8) with tack welded (4) threads (1) and spring (3) loaded nuts (2). The space (7) between sensor and object is filled up to avoid air gaps.

Figure 6 Industrial heat flux (4) sensor (5) mounted using flanges (6) with bolts (1) in tapped holes (7) and spring (2) loading. The space (5) between sensor and object is filled up to avoid air gaps.

Bolts are often combined with springs and air gaps are typically filled up with gasket material or cements. Use of springs ensures constant pressure over a large temperature range and ensures that the pressure on the sensor is not too high.

For short-term installation on magnetic surfaces, consider using a magnet frame. The magnet frame may also be used for long term installation, if drilling or welding are not possible.

Figure 7 Mounting of IHF02 using an optional frame with magnets: note that optical properties of the sensor (black) do not match those of the metal object on which the sensor is mounted. This will lead to bad measurement data. (see also figure 3). Hukseflux-Thermal-Sensors-Heat-Flux-Sensor-fig
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