Anyone working in the quality assurance of insulating materials – especially in the field of natural or mineral fibers – is aware of a certain problem: When measuring thermal conductivity (Lambda-value) at the European mean annual temperature of 10°C, condensation of humidity at the cold plates of guarded hot plate instruments and heat-flow-meters can occur. During the further course of the measurement, fiber materials absorb the condensate and the thermal conductivity measurement yields unexpectedly poor results.
A look at corresponding method standards actually further confuses the issue, rather than providing clarification. There are a number of possibilities, from correlation calculations to thermal conductivity values under dry conditions, all of which require extensive effort. From our point of view, there are two simple possibilities for determining thermal conductivity values as quickly as possible in day-to-day quality assurance without having to fear that condensates will falsify the measurement result. Actually, there are three possibilities– the third one, however, is systemically complex.
Have you ever heard of a “lambda room”? No? Well, then let’s start with this possibility – which is also the most technically demanding solution.
Let’s start with the requirements for quality assurance from the standard’s point of view
In the European Standards for Thermal Insulation Products, DIN EN 13162 to DIN EN 13171, it is stipulated that the factory production control (FPC) be carried out without drying cycles on samples in a normal climate at ~23°C/50% relative humidity. At the same time, the method standards for guarded hot plate instruments and heat-flow-meters (ISO 8301, ISO 8302, DIN EN 12664, DIN EN 12667, DIN EN 12939) state that there should be a “safety interval” of 5 K between the plate temperature of the cold plate and the current dew point.
For everyday-use, NETZSCH provides an online dew-point-calculator!
The statistical base value for determination of the thermal conductivity nominal value within the framework of the CE conformity assessment is the Lambda-90/90 value, which is calculated on the basis of the normal Gaussian distribution as a one-sided confidence interval of at least 10 measurement values at a mean temperature of 10°C.
This is the reason why quality assurance in Europe is based on the measured value of the thermal conductivity (lambda-value) at 10°C. To generate a stable heat flow through the sample, which is necessary for the measurement, a temperature gradient of 20 K between the cold and hot plates is established.
Let’s summarize the basics of the standards:
- Measurement under a normal climate without additional drying of the sample
- Safety interval of the cold plate to the dew point is 5 K
- Measurement at a mean temperature of 10°C
- Recommended gradient of 20 K
Under these conditions, the temperature of the cold plate is 0°C; that of the hot plate, 20°C. A mean temperature of 10°C occurs at the sample. The dew point at normal climate is around 12°C to 13°C. With the safety interval of 5 K to the dew point, the coldest possible plate temperature in accordance with the norms would be 17°C to 18°C. The consequence of this calculation is that without taking “additional measures”, the cold plate would fail for every measurement in the laboratory due to humidity.
Set-up of “Lambda Rooms“
That is why we very often find what are known as “lambda rooms” in the field of manufacturers of natural or mineral fibers; these are cooling chambers of around 10°C room temperature with a relative humidity content of 10% – so, not a healthy climate, and a lot of energy costs accrue as well. On top of that, there are guarded-hot-plate-instruments and heat-flow-meter instruments with integrated cooling mechanisms on the market which also blow a lot of waste heat into heavily air-conditioned rooms and thus push up energy costs even further. The heat-flow-meter instruments from the NETZSCH portfolio work entirely without internal cooling systems. We rely on external cooling thermostats which can be placed outside these “lambda rooms” by means of a hose connection. Increased energy requirements due to waste heat from instruments are thus reduced to a minimum. Also, in this way the coolant levels can be checked from outside and refilled if necessary.
These rooms are, of course, energy-intensive, but do ensure measurements under standard conditions – so, they are certainly the “safest” method for manufacturers of natural and mineral wool.
Establishment of a dry measurement environment without complex climatic chambers
One thing in advance: We ALWAYS recommend using an air-conditioned measurement room for operating a heat-flow-meter. But does that always mean installation of a “lambda room”? No – not from our point of view. Nevertheless, it does make sense to pay attention to performance when selecting air-conditioning systems. In some laboratories, we encounter air-conditioning systems causing very heavy room-temperature fluctuations and thus resulting in malfunction of the units. A close loop of control between the actual and target temperatures should be kept and above all, care should be taken that the unit not be exposed to drafts from the air-conditioning systems.
However, much more important than the room temperature is the relative humidity. In a lambda room, the humidity level in the entire room is reduced – but it would actually be sufficient to reduce the humidity level in just the measuring chamber. That is why the heat-flow measuring instruments in our portfolio feature purge gas connections leading to the measuring chamber which can be controlled via digital flow meters. There you can connect dry compressed air, thus reducing the dew point within the measurement instrument to a level of up to -30°C. This way, it is possible ‒ with relatively low technical effort ‒ to carry out series of measurements on the basis of normative requirements.
Extrapolation of the 10°C measurement value – temperature coefficient of the thermal conductivity
While getting familiar with the European standards for insulation materials, measurement at a mean temperature of 10°C is primarily recommended. However, upon taking a closer look, we 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.”!
And – what does that, in turn, mean? It means that quality assurance can also be carried out at other temperatures and the thermal conductivity value can be extrapolated at 10°C. If the thermal conductivity is measured at 30°C, 40°C and 50°C, for example, it can be determined for almost all insulating materials that the thermal conductivity function in this moderate temperature range is linear. The slope in the thermal conductivity function is called the temperature coefficient of the thermal conductivity (alpha lambda). If the temperature coefficient of the thermal conductivity of your product has been determined through measurement and has proven to be stable, you can determine the 10°C value by extrapolation, e.g., with a measurement at 30°C and the temperature coefficient of the thermal conductivity. It is up to you – within the framework of your own internal quality safety agreement ‒ how often you check the slope of the thermal conductivity function and validate the 10°C measurement value by direct measurement, and if necessary, by means of a measurement under purge gas.
Humidity plays a major role in the quality assurance of insulating materials. Ignorance in dealing with condensation and establishing good measurement conditions can result in unexpected measurement results.
Our laboratory staff and experts in setting up appropriate QA methods are available around the clock to help you find the best solution for your product.
You are also welcome to take advantage of our lab measurement service and have your own measurement values verified in our vacuum-tight, absolute-measurement guarded hot plate instrument based on ISO 8302.