Humidity plays a huge part in the quality assurance of insulating materials. Unexpected measurement results can be due to ignorance in dealing with condensation and creating good measurement conditions. This article describes techniques that help to overcome condensation issues in the quality assurance of thermal insulation materials.
Any person working in the quality assurance of insulating materials is aware of a certain problem, particularly those in the field of mineral or natural fibers. The problem is that condensation of humidity at the cold plates of guarded hot plate instruments and heat-flow-meters can happen when measuring thermal conductivity (Lambda-value) at the European mean annual temperature of 10 °C.
Fiber materials absorb the condensate during the further course of the measurement, so the thermal conductivity measurement gives unexpectedly poor results. Observing corresponding method standards actually confuses the issue further, instead of giving clarification. There are a number of possibilities, from thermal conductivity values under dry conditions to correlation calculations, all of which take huge effort.
NETZSCH believes that there are two simple possibilities for establishing thermal conductivity values as fast as possible in day-to-day quality assurance without having to fear that the measurement result will falsify because of condensates. However, there are actually three possibilities but the third one is systemically complex. The third possibility is the ‘Lambda room’, which is also the most technically demanding solution.
Requirements for Quality Assurance from the Standard’s Point of View
In DIN EN 13162 to DIN EN 13171, the European Standards for Thermal Insulation Products, it is stated that the factory production control (FPC) be performed in a normal climate at ~23 °C/50 % relative humidity without drying cycles on samples.
Additionally, the method standards for heat-flow-meters and guarded hot plate instruments (ISO 8301, ISO 8302, DIN EN 12664, DIN EN 12667, DIN EN 12939) say that between the plate temperature of the cold plate and the current dew point there must be a ‘safety interval’ of 5 K.
The Lambda-90/90 value is the statistical base value to establish the thermal conductivity nominal value within the framework of the CE conformity assessment. It is determined on the basis of the normal Gaussian distribution as a one-sided confidence interval of a minimum of 10 measurement values at a mean temperature of 10 °C.
Lambda 90/90 statistics.
For this reason quality assurance in Europe is based on the measured value of the thermal conductivity (lambda-value) at 10 °C. A temperature gradient of 20 K between the cold and hot plates is established to produce a stable heat flow through the sample, which is crucial for the measurement.
Summary of the Basics of the Standards
- Recommended gradient of 20 K
- Safety interval of the cold plate to the dew point is 5 K
- Measurement under a normal climate without additional drying of the sample
- Measurement at a mean temperature of 10 °C
The cold plate temperature is 0 °C and the hot plate is 20 °C under these conditions, a mean temperature of 10 °C occurs at the sample. The dew point at normal climate is usually between 12 °C and 13 °C.
The coldest plate temperature possible in accordance with the norms would be 17 °C to 18 °C if performing the safety interval of 5 K to the dew point. Due to humidity, the result of this calculation is that without taking ‘additional measures’, the cold plate would fail for every measurement in the laboratory.
Set-up of ‘Lambda Rooms’
That is the reason we frequently see what are known as ‘lambda rooms’ in the field of manufacturers of mineral or natural fibers; these are cooling chambers of approximately 10 °C room temperature that have a relative humidity content of 10 % – not a healthy climate, and big energy costs.
Furthermore, there are heat-flow-meter instruments and guarded-hot-plate-instruments with integrated cooling mechanisms on the market. These also blow a lot of waste heat into heavily air-conditioned rooms and so heighten energy costs even more.
NETZSCH supplies heat-flow-meter instruments which work entirely without internal cooling systems. They depend on external cooling thermostats which can be placed outside these “lambda rooms” using a hose connection. So, increased energy requirements because of waste heat from instruments are decreased to a minimum. In addition, using this method the coolant levels can be monitored from outside and refilled if needed.
Obviously these rooms are energy-intensive, but they ensure measurements under standard conditions, meaning they are the ‘safest’ technique for manufacturers of mineral and natural wool.
Employee in Front of a Lambda-Room.
Establishing a Dry Measurement Environment Without Complex Climatic Chambers
For operating a heat-flow-meter, NETZSCH always recommends utilizing an air-conditioned measurement room, that does not always mean the installation of a ‘lambda room’. Yet, when selecting air-conditioning systems it does make sense to consider performance.
We encounter air-conditioning systems in some laboratories, which cause extreme room-temperature fluctuations and cause the malfunction of the units. A close loop of control between the target and actual temperatures should be kept and above all, care should be taken that the unit not be exposed to drafts from the air-conditioning systems.
Yet, the relative humidity is much more important than the room temperature. The humidity level of the whole room is decreased in a lambda room – but actually, it would be sufficient to only decrease the humidity level in the measuring chamber.
That is why NETZSCH heat-flow measuring instruments have purge gas connections leading to the measuring chamber which can be controlled via digital flow meters. Using that, it is possible to connect dry compressed air, consequently reducing the dew point within the measurement instrument to a level of up to -30 °C. In this way, with relatively low technical effort, it is possible to perform a series of measurements on the basis of normative requirements.
For standard industrial environments, NETZSCH HFM’s can be flushed with dry air to keep the measurement chamber as dry as possible.
Extrapolation of the 10 °C Measurement Value – Temperature Coefficient of the Thermal Conductivity
Measurement at a mean temperature of 10 °C is primarily recommended whilst becoming familiar with the European standards for insulation materials. Yet, upon closer examination, we can see that it actually reads, “… at 10 °C or at other temperatures if the relationship between temperature and the heat transfer property can be assumed to be known.”
Extrapolation of Lambda
This means that quality assurance can also be performed at other temperatures and the thermal conductivity value can be extrapolated at 10 °C. For example, if the thermal conductivity is measured at 30 °C, 40 °C and 50 °C, it can be established for most insulating materials that the thermal conductivity function is linear in this moderate temperature range.
The slope in the thermal conductivity function is known as the temperature coefficient of the thermal conductivity (alpha lambda). If the temperature coefficient of the thermal conductivity of your product has proven to be stable and has been established through measurement, you can establish the 10 °C value by extrapolation, for example using a measurement at 30 °C and the temperature coefficient of the thermal conductivity.
It is the user’s decision, within the framework of their internal quality safety agreement, as to the frequency of which to check the slope of the thermal conductivity function and validate the 10 °C measurement value by direct measurement, and by means of a measurement under purge gas if necessary.
Humidity has a key part in the quality assurance of insulating materials. Ignorance in handling condensation and establishing good measurement conditions can lead to measurement results which are unexpected.
NETZSCH GHP 456 Titan® Guarded Hot Plate Instrument.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
For more information on this source, please visit NETZSCH-Gerätebau GmbH.