Heat Flux Sensing

What is heat flux?

Heat flux is the rate of heat that passes through a surface. It is proportional to the temperature difference. Heat flux occurs wherever a temperature disequilibrium occurs between two objects. Heat flux or thermal flux is the rate of heat energy transfer through a given surface, per unit surface. The SI derived unit of heat rate is joule per second, or watt. Heat flux density is the heat rate per unit area. In SI units, heat flux density is measured in [W/m2]. There are three types of heat fluxes: conduction, convection, and thermal radiation.

What is the advantage of heat flux sensor compared to temperature sensors?

Heat flux sensors offer a higher resolution of temperature gradients than temperature sensors. Heat flux sensors enable dynamic measurements for an in-depth understanding of thermal systems.

How do heat flux sensors function?

Tiny, serially connected semiconductor piles inside the sensor generate a voltage, which is proportional to the heat passing through the surface . The voltage is read out and depending on the sensor ‘s sensitivity the results are converted into the heat flux.

What is the temperature range where gSKIN® Heat Flux Sensors function?

gSKIN® Heat Flux Sensors function in a temperature range of -50° C to +150° C. The silicone encased gSKIN® (model XO) functions from -40°C to +100 °C. The silicone encasing ensures a good thermal coupling when measuring on rugged surfaces.

Does greenTEG offer customized heat flux sensors?

Yes, greenTEG can customize heat flux sensors to meet specific needs. Customization depends on the volume you intend to order. Contact us for questions regarding customization.

How do greenTEG’s heat flux sensors compare to heat flux sensors of other manufacturers?

greenTEG’s sensors are more sensitive and thinner than most commercially available heat flux sensors.

How do I choose the right heat flux sensor model?

This depends on your application. The larger the sensors surface, the higher the measurement resolution. Smaller sensors are easier to integrate. If you are unsure which sensor you should choose for your application, contact us. We are happy to answer your questions.

Where are heat flux sensors typically applied?

The range of heat flux sensor applications is almost endless. The sensors can be used to characterize and improve materials, to monitor and control processes in industry, home appliances, and wearable technologies, or to monitor core body temperature to name a few examples. Contact us to discuss your application idea.

Can I measure thermal radiation with a gSKIN® sensor?

Yes, but you will need to coat the sensor’s surface with a light absorbing coating. Contact us for further details.

U-Value Measurement

Norms and requirements

What is the ISO 9869 norm?

The ISO norm is established to secure the reliability of a heat flux measurement for investigating the thermal transmission properties of plain building components . If a measurement is conducted in line with ISO 9869, the measurement results can be considered as accurate.

What are the ISO 9869 requirements?

About the measurement itself ISO 9869 states that a measurement of a wall should last at least  72 hours and a maximum difference of 5% is tolerated between the last value and the value 24 hours before. For further guidance please refer to our instruction manual.

How is the U-Value calculated by greenTEG’s U-Value KIT software?

The U-value is calculated according to the moving average method as described in ISO 9869. Therefore, the measured heat flow rate is divided by the temperature difference for all data points. The average of these calculations (taken over a long time period) represents the actual U-value of a building envelope.

Why do I need to measure a wall at least 72h according to ISO 9869?

A wall has a thermal mass which enables it to store heat. Due to temperature fluctuations inside and outside the building, the amount of heat stored in the wall is fluctuating too. The influence of the thermal mass of the building envelope on the outcome of the U-value calculations is getting smaller over time as the average of more data points is taken. Under the right circumstances the calculated average is reaching the actual U-value of the building envelope.

Is it possible to get reliable measurement results for a wall in less than 72 hours?

Yes. However, the inside temperature should be kept as constant as possible during the measurement to diminish the effect of the thermal capacity of the wall. Moreover,  the measurement should preferably last a multiple of 24 hours.

Can I measure glass (windows) as well?

Yes, however, since glass has different thermal characteristics, the measurement procedure is a bit different. According to ISO 9869 three subsequent night measurements (e.g. of 7 hours) should be conducted. An example of such an assessment can be found in our application note.

Can I measure the whole year through?

When the difference between the inside temperature and the outside temperature is too small, the measurement results become unreliable. Therefore, we advise a minimum temperature difference of roughly 5° C degrees over the whole measurement time. This means usually that an assessment of a wall cannot be made during the summer months. As glass measurements are conducted over night, these measurement can usually be conducted during the whole year.

Can I measure during sunny days?

Yes, if certain aspects are taken into account, solar radiation will have no influence. You should try to avoid measuring the south wall of a building since the thermal storage capability of walls should not be underestimated.
Furthermore, always cover the temperature sensor to block any direct sunlight if necessary. In order to protect the sensor from the influences of the solar radiation you can cover it with aluminium foil.

Does the measured, dynamic U-value include the common heat transfer coefficients?

Yes, it does. Therefore the λ-values of the single materials can’t be assessed.

 U-value measurement routine & analytics

Does a measurement has any impact on the daily routines of building occupants?

The measurement set-up is rather small. If the sensor is located properly, it won’t bother any building occupant. To increase the accuracy of the obtained data building occupants might try to keep the inside temperature as constant as possible (e.g. by not switching the radiator off during the nights or limit the opening of the window).

I measured more than 72 hours but the measurement is not in line with ISO 9869. Why is this?

This means that the average U-value still fluctuates significantly and a stable value has not been reached yet. Unstable external conditions or a very small temperature difference (< 5°C) could be reasons why it takes longer to reach the actual U-value of the building envelope. You could decide to extend the measurement or start a new measurement when conditions are better.

My measurement set-up is not fully in line with ISO 9869, is the measured U-value unreliable?

Such a measurement could still be rather reliable. However, it requires some more in-depth interpretations of the measurement results. The length of the measurement and the volatility of the data should be considered. If the overall standard deviation and the deviation during the last 24 hours are not too high, the measurement results are likely to give a good representation of the measured results.

The measurement outcome seems to be unrealistic but in line with ISO 9869. Why?

First, you should make sure that the measurement set-up meets all requirements as described in our instruction manual. A reason for a unrealistic good U-value might be that the sensor does not have a good contact with the wall surface. If all criteria were taken into account, the U-value can be considered as correct. Values can be different than expected due to the heterogeneity of the walls or thermal bridges. Moreover, walls in historic buildings tend to have a better U-value than is expected by most models.

Is the software also available for Mac OS X?

Currently the software is only available for computer working with an operating system from Windows. However it is possible to run the Software on a mac over a virtual machine with windows. We tested this with VMware Fusion with Windows 10.

How do I import the .csv-file into excel without having formatting issues (some numbers are shown as dates)?

Firstly, you have to open a new excel sheet and get the csv as an import from text. Now you set the delimiter semicolon (“;”).
Press “Next” and select the format “Standard”. Additionally, you need to change the decimal seperator to dot (“.”), the thousands seperator to nothing (” “) and then press “Finish”.

Contact us if you need further information about U-value measurements.

Further ressources (application notes, white papers) are available for free download at www.shop.greenTEG.com

Textile Thermal Properties Measurement

What is the best product for measuring thermal storage and the thermal conductivity of a material?

The gSKIN® U-Value KIT with XP Heat Flux Sensor has 2 temperature sensors, a data logger and a software, which can easily read out: heat flux, 2 temperatures and U-Value at the same time. Since the data logger is small, robust and can record data even at a sampling rate of 1sec for several days, it is well suited for textile field measurements.

Why are the gSKIN® Heat Flux Sensors better, than other available methods for measuring textile thermal properties on the market i.e. hot guarded plate method?

The sensors are very sensitive and have a small size allowing measuring thermal properties of also small samples. They are affordable in comparison to for example a standard guarded hot plate method, and can be used under real conditions, where you can directly mount them on the material or skin.

Which sensor is the most suitable for textile thermal properties characterization?

Depending on the size of your sample and the required accuracy we either recommend 4.4x4.4 mm XM Sensor, 10x10 mm XP Sensor, or 18x18 mm XI Sensor.

What kind of data acquisition tool do I need?

If you do not wish to you use a data logger provided by greenTEG, you need a data logger which can resolve at least 1uV.

How do I integrate or attach the sensor to the textile material?

Best is to mount the sensor on/underneath the fabric by using either a double sided thermal tape from greenTEG (only valuable for smooth surfaces) or by a normal tape, which can be placed over the surface of the fabric. Make sure to use alcohol, isopropanol or acetone to remove the tape from the surface of the sensor. Never tear at the cable, because that can damage the contact of the sensor, which will break it.

How are the measurements performed?

Field measurements:
Mount the sensor at the desired spot on the fabric (better inside than outside) and mount the temperature sensors on the outside and the inside of the textile close to the heat flux sensor (for better understanding visit our website on u-value measurements). The human body act as a heat source.
Experiments in the lab:
Tape the temperature sensor to the surface of a hotplate and then place the material onto it. Attach the heat flux sensor and the second temperature sensor on the upper surface fabric. Then heat up the hot plate with a constant power resulting in a temperature 5°C to 10° above room temperature. Make sure, that the measurements are not disturbed by convection or radiation.

Thermal Storage & PCM

Which sensor is most suitable?

This depends on the application:

  • If you would like to measure the heat flux from the air into the material or vice versa, we suggest using our XO sensor. The silicon package of the sensor has the advantage of adapting the surface profile very well. Furthermore, the thermal mass of the silicon helps to reduce noise from thermal convection

  • When you would like to integrate the sensor into a measurement setup or a heat exchanger, then, depending on the available space and the required resolution, we suggest you to use the Al packaged low thermal resistance XM (4mm x 4mm), XP (10mm x 10mm) or XI (18mm x 18mm) sensors.

What kind of data acquisition tool do I need?

This depends as well on your application:

  • If you would like to measure the characteristics of the PCM vs. air (for example a wall of a building), we suggest you are using our U-Value KIT. It includes a small powerful data logger with low energy usage which can store more than 2 million data points. The logger can also be combined with the XM, XP and the XI sensor.

  • When using heat flux sensors for application where the heat flux can be larger than 500W/m2 as for example in a heat exchanger, we suggest to use any kind of data logger which is able to resolve at least 1uV.

How do I integrate or attach the sensor?

For the measurements vs. air, the sensor can simply be attached to the surface by normal “scotch” tape.

For the integration into a measurement setup or a heat exchanger, we suggest you make a notch of the appropriate size into the heating/cooling plate which gets in contact with the sample. Clean the places where you would like to attach the sensor with Isopropanol. You can either glue the sensor into that notch by a thermal epoxy glue or mechanically sandwich it between the sample and the notch in the plate using thermal paste between the sample and the sensor and the sensor and the cooling/heating plate.

Example: Possible integration of a heat flux sensor into a measurement setup

Another possibility is to fix the sensor on the surface of the heater/cooler in a manner that all heat is forced through the sensor. This can be done by a thermal double side sticky tape, thermal glue or thermal paste.

Removing of the sensor from a surface: You need to work very carefully. Use Isopropanol or Acetone to weaken the thermal tape. Then you can remove the sensor with a sharp blade. Avoid any force to the cable.

Are the sensors resistant against water and chemicals?

The sensors survived the standard highly accelerated stress test (HAST) at 85% humidity and 125°C for 95h without any problems. We have been also using the sensors for several experiments in deionized water. Since the surface of the sensor consist of Aluminum and Polyimide, it might also be chemical resistant. However, we have not done any tests yet into this regards. You would need to cover the contacts and the sidewall of the sensors by a protective coating (for example epoxy glue). All usage of the sensors in environments different than air are on your own responsibility.

How are the sensors calibrated?

The sensors are calibrated in a special measurement setup using NIST traceable thermal reference materials. For further information see here.

Thermal characterization of batteries using heat flux sensors

What kind of data logger do I need for my measurements?

For high resolution measurements, you need a data logger which can resolve at least 1uV. For synchronized measurements (for example channel 1 cell voltage, channel 2 heat flux voltage measurement) it is often useful to plug the sensor to the A channel of the multichannel potentiostat.

How do I mount the heat flux sensor?

Important when attaching the sensor to the battery surface is optimal thermal connection. We therefore recommend cleaning the surface of the sensor and the battery with Isopropanol. Then the sensor can be attached by either a double side sticky thermal tape, or by pressing the sensor to the surface using thermal paste or by gluing the sensor to the surface with a thermally conductive epoxy. The sensor needs to be removed carefully. Do not pull at the flex-print! It is better to remove the sensor by a blade and Isopropanol.

Which sensor is better, the XM or the XP?

This depends on the application. The advantage of the XM sensor is its small size of 4mm x 4mm, while with the larger XP sensor a higher resolution can be obtained (below 0.09W/m2). We propose that you test both so you can find out which one is best suited for your application.

Can the sensor be used in water?

We have done several experiments in deionized water without any problems. The highly accelerated stress tests at a humidity of 85% and a temperature 125°C for 100h do not show any sensor degradation.

Could the sensor be used inside the battery?

We do not have any experience with this kind of experiments yet. You can try to do tests at your own risk. However the sensor is very robust against chemicals. For this kind of experiment we suggest to seal the borders and the contacts of the sensors with chemically stable glue. If you need a longer flex-print for insertion into the battery, just ask for an additional flex-print. You can attached this flex print to the flex-print of the sensor by using a solder paste, pressing the tow contacts onto each other and heating it with a solder gun from top. Please make sure that you seal the position of the connection also by chemical inert glue.

How are the sensors calibrated?

The sensors are calibrated in a special measurement setup using NIST traceable thermal reference materials. For further information see here.

Please do not hesitate to contact us if you have any questions.