The Guarded Hot Plate instrument measures the thermal conductivity of insulation products.
The GHP principle is based on an absolute measurement method and therefore requires no calibration standards. It offers optimum accuracy in the available temperature range.
The hot plate and the guard ring are sandwiched between two specimens of the same material and approximately the same thickness (Δx). Typically, two specimens are preferred for a measurement, but it is also possible to work with only one test specimen.
Auxiliary heaters (cold plates) are placed above and below the samples. The cold plates are heated such that a precisely defined, user-selectable temperature difference (ΔT) is established between the hot and the cold plates and thus also over the entire sample thickness.
As soon as thermal equilibrium is reached, the power input in the hot plate with area A is measured.
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.