RFX vs Cast Iron, RFX vs Foil: Radio Transmission Basics for the Home Cook

The introduction of ThermoWorks’ RFX™ system has been very well received. People love the fact that our sub-gigahertz transmission technology Connects—and stays connected—better than BlueTooth^®-connected cooking probes. But customers are sometimes perplexed when RFX’s transmits through their smoker walls or a cast iron pot but loses signal when the probe is wrapped in foil—say, during a smoke braise or the crutching step of a BBQ cook.

“How is it possible,” they ask, “that the signal from the RFX MEAT™ probe can go through a cast iron pot, but it can’t go through a couple layers of aluminum foil?” It’s a natural question, as it seems that a heavy pot should block the signal much more effectively! The full and comprehensive answer to this question is long, very mathematical, and requires a good understanding of Maxwell’s and Helmholtz’s equations. But here’s we aim to explain what’s happening in a shorter form that should help people understand what’s happening. Let’s take a look.

Electromagnetic waves: the basis of signal communication.

All radio waves, Bluetooth and wifi signals, x-rays, UV rays, and even visible light itself are all waves of various frequencies on the electromagnetic (EM) spectrum. We call it the “electromagnetic” spectrum because each wave is composed of an electric field wobbling, which creates a magnetic field wobbling counter to it. The magnetic field then creates an electric field wobbling counter to it, etc, etc. This is how it propagates through space—two fields moving along at right angles to each other. 

To carry information, like songs on the radio or temperature data from a probe, those waves vary either their signal strength (amplitude, as in Amplitude Modulation [AM] radio), or they vary their frequency slightly ( as in Frequency Modulation [FM] radio). The pulses of those variations can be received in an analog fashion, with the waveforms directly corresponding to sounds, or digitally, with the waves transmitting in binary, which is then decoded and processed by a computer.

Signal reduction in materials

When those moving electromagnetic wavefronts encounter a substance that is electrically or magnetically resistive, some of the energy of the fields gets absorbed by the material. How much gets absorbed depends on the magnetic and electrical properties of the substance it’s going through—some materials absorb certain frequencies more readily than others. So glass is easy for EM waves, including radio signals, to pass through, but gold is harder to pass through.

The thickness of a material that absorbs ~37% of the signal is called the “skin depth.” At that thickness there is very rapid signal loss, and the rate of loss increases in as the thickness of the materials increases.  

We can pause here to say that the degree of attenuation (how much of the wave’s power is lost) is also dependent on the frequency of the EM wave itself. Higher-frequency waves are absorbed more quickly and in thinner material than lower-frequency waves. That’s important for comparing different signals, but when we’re talking only about why our signal acts a certain way, it can be disregarded, and we can compare skin depth directly between two materials. The equation that approximately describes skin depth is

𝛿 = 1/√(σπƒµ₀µ))

Where 𝛿 is skin depth, σ is the conductivity coefficient value of the material the wave is passing through, ƒ is the frequency in Hertz, µ₀ is the magnetic permeability of free space, and µ is the permeability of the material. Because π, ƒ, and µ₀ are all constant when comparing one frequency across various materials, we can cancel them out—doing so will not give us a true skin depth, but will give us a skin depth ratio between two materials. Further, we’ll use the inverse of conductivity, resistivity. Resistivity, ρ = 1/σ. With those cancelations and substitutions, we can simplify to a relative skin depth equation:

𝛿_r = √(ρ/µ)

Relative skin depths of aluminum foil and cast iron

The equation above doesn’t tell us the actual skin depth, but it provides us an accurate coefficient, a ratio, for comparing the skin depths of two materials blocking the same frequency signal. If we put in the values for aluminum foil and the values for cast iron, we can find how the skin depths for the same frequency compare to each other.

So by using accepted values for the alloy that is found in almost all aluminum foil1 and the values for “unpurified” iron2 (cast iron is not pure, it contains several percents of silica and carbon which dramatically change the values from pure iron—purified iron lies in the range of 99.85% pure, and this is in the range of 94%), we can evaluate the relative skin depths of these materials.

Aluminum has a resistivity of 0.0000000365 (3.65×10^-8), and a permeability of 0.00000126 (1.26×10^-6). Unpurified iron has a resistivity of 0.001 (1.00×10^-3) and a permeability of 0.000189 (1.89×10^-4). If you do that math, you find that cast iron’s skin depth is about 13.5 times thicker than aluminum. That means that 1mm of aluminum has the same “stopping power” as 13.5mm of cast iron! 

And yes, a cast iron pot may be more than 13.5 times thicker than a sheet of aluminum foil, but by understanding this concept, you can see how it is that something thick and tough can let through more signal than something thin flimsy. (There are other confounding factors that may interfere with the foil or not interfere with the iron.)

Solutions for the foil-blocking problem

So, yes, foil is a more effective signal blocker than cast iron. What is one to do? What we recommend, then, is that you simply stick your RFX MEAT probe through the foil into your meat, letting the antenna wave free in the breeze. Then you don’t have to worry about the Maxwell or the Helmholtz equations!

A note on frequency, Bluetooth vs RFX signals

The reason RFX performs better and stays connected better than Bluetooth has to do with the same skin-depth equation. But in this case we can factor out the things that are the same (resistivity and permeability) and leave in our varying frequencies.

𝛿 = 1/√ƒ

Bluetooth communicates at 2.4Ghz (2,400,000,000 waves/second), and RFX communicates at 433MHz (433,000,000 waves/second). Let’s go ahead and factor out a million form each of those frequencies, giving us 2,400 and 433 respectively. So the relative skin depth of a Bluetooth signal is 2.04124 and the relative skin depth for RFX is 4.80569. An RFX signal can travel through SAME material as a bluetooth signal, but that is 2.35 times thicker for the same signal attenuation! That’s why id stays connected better. Nothing works like ThermoWorks.