A practical and useful expression is the penetration depth (or power depth) dp. It is defined as the depth below the surface of a thick, plane “irradiated” surface of the material where the power density has diminished to about 37 % of the value at the surface. Since the decay is exponential, also 37 % of the total power will be deposited below this depth. In technical and scientific work, the so-called (complex) permittivity ε is used and consists of the (real) permittivity ε' (the old term “dielectric constant” is deprecated), and the loss factor ε" . A high real permittivity, such as for materials with a high water content, causes the interior wavelength to become times shorter. dp equals about 20×/ε" , in mm at 2450 MHz.
Under most practical circumstances, a dp between about 5 and 100 mm results in a good absorption, but dp up to 500 mm may be used in special industrial systems. – The absorption capability in non-magnetic metals is proportional to dp.
Microwave transparency is, in some standards, defined by ε' <7 and dp > about 3 m. This value is in the practical border region of possible processing (see above). A plate with food is not heated relatively.
The dp of non-magnetic metals is inversely proportional to the square root of the frequency and conductivity.
The table gives dp values in mm, m, and µm, respectively – all at 2450 MHz.
|Examples of penetration depth: absorption (mm)||Examples of penetration depth: transparency (m)||
Examples of penetration depth: metals (µm)
|Water +20 °C||16||Micanite||28||Silver (best of all materials)||0,33|
|Water +100 °C||80|| Quartz, pure
(best of all materials)
|Cooked meat –10 °C||25||Fedspar china||5,8||Brass, 70 % Cu||1,4|
|Cooked meat +40 °C||11||Aluminium oxide ceram, for MW use||30||Zink, pure||1,24|
|Mash. potato +40 °C||8||Polyethene, 25 °C||40||Nickel, pure||1,3|
|Spruce 10 % moist.||77||PTFE (Teflon®)||90||Titanium, pure||3,3|
|Nitrile rubber, natural||650|| Plexiglass,
|2||Stainless steel (304)||4,3|
|Rubber, SBR, vulc.||190||Borosilicate glass, 50 °C||ca 8||See this site for a comprehensive table of metal conductivities|
|Sand soil, dry||2000||Ice –12 °C||12||–||–|