Constitution and Function
The Pipe-Thermal Conductivity Test Tool λ-Meter RP1000 is designed as compact desktop tool. A test chamber which can be accessed from above is situated in the upper part of the tool. The test arrangement can be inserted into this chamber from above.
This arrangement consists of a test pipe with a heat controlled front and rear end, the actual specimen surrounding the test pipe and an outer test sheath also heater controlled covering the specimen on the outside. The outer test sheath is thermally isolated from the air in the test chamber by an insulating sheet. Using air-cooled Peltier elements the test chamber can be thermally controlled and brought to any temperature between 0 °C and 35 °C. Thermal conductivity tests are possible for mean specimen temperatures in the range of 10 °C and 40 °C at a temperature difference of 10 K to 15 K between the specimen surfaces. The entire electronics control unit is below the test chamber. The processor board with the memory circuits for the customised tool software can be accessed by removing the enclosure on the right hand side of the tool. The remaining parts of the electronic control can be reached by removing the enclosure at the bottom of the tool.
The principal structure of the test arrangement is as follows:
The test pipe consists of three interconnected thick-walled aluminium pipe sections. The thickness of the walls facilitates an even distribution of the heat within the individual heated sections. The mechanical connection over the gap is essentially thin-walled PVC piping. It serves to thermally isolate the central section, i.e. the test area from the back and front end sections.
The test area has a length of 600 mm, taken from gap centre to gap centre. The back and front end sections are 200 mm in length each and serve to seal off the test area endwise.
To this end thermocouple chains in the interconnecting areas serve as heat flux sensors for the gap control. The aluminium pipe sections contain temperature sensor films and heating films 70 µm thick and covered by a protective sheath 25 µm in thickness. The heating films of the middle pipe section, the actual heating conducts the heating energy to the specimen and builds up the desired temperature. The outer two heating films of the test pipe are operated such that endwise no heat is being conveyed over the gaps. This operational arrangement ensures that the entire heating energy will flow through the specimen.
The temperature sensor film is on the outside of this arrangement. It integrates and measures the mean temperature along the entire test area of the inside of the specimen. This provides higher test accuracy than the spot tests done by thermocouples on other tools.
The drop in temperature on the outer protective cover of the test pipe (and the outer test sheath) will be taken into account during test for working out the real temperature difference between the specimen surfaces. It will amount to a few mK and prevent an error in the test result.
The connecting cables for the heater and the sensors are separate in order to prevent electromagnetic disturbance and run along the inside of the test pipe. They exit the pipe to one end leaving the other end of the test pipe for the specimen material to be slipped over.
The test pipe is now enclosed by the specimen. In a second assembly step it will be wrapped by the outer test sheath. The outer test sheath is a thin film with a flat outspread thermal sensor on the inside. This sensor will measure the surface temperature of the specimen. The outside of the sheath contains a flat-layered heater covered by an insulating layer (about 4mm foam rubber mats). The heater will provide the desired temperature to the outer test sheath, which is about 10…15 K below the central temperature.
The ambient temperature of the test arrangement must be lower than on the outer test sheath in order to dissipate the heat of the heater evenly. This is achieved by the assembly of the test arrangement inside a thermally controlled test chamber, which allows you to maintain the required temperature constant over time.
The principle of a heat controlled outer test sheath is vital for maintaining the desired test temperature and temperature difference constant and accurate over time. It is a precondition for reliable thermal conductivity testing.
The entire heat energy q produced by the heater will evenly dissipate into the surrounding specimen away from the pipe. This will lead to a temperature difference between the inside and outside of the specimen. Thermal conductivity of the specimen material can therefore be evaluated as a function of the temperature difference Δϑ, the length of the test area L and the inner as well as the outer diameter (da, di respectively). The following equation will be applied:
The Pipe-Thermal Conductivity Test Tool λ-Meter RP1000 can be used for testing tubular specimens of 1m in length and various inner and outer diameters. The inner diameter of the specimen must match the diameter of the test pipe to allow for airtight alignment.
The outer test sheath has to be big enough in size, esp. in width to fully cover the outside of the specimen, which of course may vary according to outer diameter.
The specimen can therefore only be tested if its dimensions, esp. inner and outer diameter correspond to the existing test pipe and sheath size.