The lambda value depends on temperature. This makes it necessary to carry out measurements at a minimum of three different temperatures.
The measurements are carried out on a 25-metre-long roll (approx.) of FlexPipe. The exact dimensions depend on the type of service pipe used in the FlexPipe roll. The length of pipe is taken from a whole roll from the normal production line.
The temperature of the surroundings (ta) is kept constant while the measurements are being carried out.
Because of the size and length of the section of pipe being measured, it is not possible to keep the service pipe at a constant temperature. Instead, water with a constant inlet temperature (ti) is circulated through the pipe.
The temperature of the casing pipe is measured when the inlet temperature (ti) and the outlet temperature (tu) in the service pipe are constant. Measurements are taken at three different service pipe temperatures.
The lambda values for the three temperatures can then be calculated on the basis of the inlet temperature (ti), the outlet temperature (tu), the temperature at the casing pipe and the temperature of the surroundings.
The lambda value for pre-insulated district heating pipe is always given by an average temperature in the polyurethane foam at a temperature of 50oC.
A straight line can be plotted for the calculated lambda values, because the lambda value is directly proportional to temperature.
The lambda value at 50oC can then be read off from the graph showing the temperature-related lambda values.