The Guarded Hot Plate instrument measures the thermal conductivity of insulation products.
The hot plate and the guard ring are sandwiched between two samples of the same material and approximately the same thickness, d.
Cold plates are placed above and below the specimens. The plate temperatures are controlled such that a well-defined, user-selectable temperature difference, ΔT, is established between the hot and the cold plates (across the specimen thickness).
The guard ring is maintained exactly at hot plate temperature in order to minimize lateral heat losses.
The great advantage of the GHP method is that it is an absolute method; i.e., no calibration or correction is required at all. The thermal conductivity values result in the stationary state simply from the:
- precisely measured total power input into the hot plate, Q,
- average sample thickness, d,
- measurement area, A, and
- mean temperature difference, ΔT, along the sample or the two samples, as the case may be (the factor 2 results for two samples):
- How is a particular insulation material performing?
- How can cryo tanks be insulated in the best possible way?
- What is the optimum insulation for furnaces operating at different temperature, gas or pressure conditions?
- Is the thermal conductivity low enough to prevent thermal bridges?
- How is the R-value reduced when my sample has a given amount of water absorbed?
Insulation of modern house roofs, cryo-tanks or even ships requires materials featuring both low thermal conductivity and high mechanical stability. Polyurethane (PUR) foams offer these properties.
This plot presents a comparison of a test with a heat flow meter (HFM) at room temperature and a GHP test down to -160°C. The two sets of results agree perfectly. Additionally, the GHP result shows the impact of cell-gas condensation between -50°C and -125°C.