DC vs AC gas exchange

DC vs AC

The most significant point of departure from the air-conditioned termite nest idea is in the mechanism of gas exchange. This has commonly involved some variant on so called flow through mechanisms (see Gas exchange 1). These do not seem consistent with the behavior of termites, which is to block flow.

What seems to be at work in these colonies is a radically diffferent form of gas exchange, something more akin to what happens in the lung (which is also not a flow-through system).

The difference is best described by analogy to electrical systems. Flow through ventilation is a type of "DC" gas exchange, where bulk flows are driven by a steady or quasi-steady pressure difference, Termite colonies and lungs are both examples of "AC" exchangers, where exchange are driven by an unsteady ("AC") energy source.

What "DC" and "AC" mean

This is easiest to see with the energy sources we use most often: batteries, and wall voltage to power our electronic devices. All these devices use a potential energy source (voltage) to power a device by the use of electrical current.

Suppose you have a battery powered device. The device works by tapping the steady voltage provided by the battery. This drives an electrical current through the load (essentially a resistor), which does work. In this instance, the steady voltage powers a steady (or direct) current: hence DC.

Now suppose a device powered by wall current. The voltage in your plug oscillates between two voltages: +60 volts and -60 volts (in the Americas that is: everywhere else it's +120 volts and -120 volts) at a frequency of 50 to 60 cycles per second (or hertz, Hz). Now the current that does the work is alternating, flowing first one way (from +60V to ground), then the other (-60 volts to ground). There's still current, but it's alternating: hence AC.

This example happens to use electrical current, but it can apply to any physical system that uses potential energy to do work. This can include wind. Wind is air, which has mass moving at a certain velocity. The product of mass and the square of the velocity is energy.

Natural wind is almost always turbulent, which means it has both DC and AC components. Look at the trace below. The wind has an average speed, which is the wind's DC component, what it would be if it were averaged to a steady speed. One can build a device, like a wind turbine, that captures this DC component and uses it to do the work of producing electricity.

ACDC

The wind velocity varies about this mean, though. Its speed varies and so does the direction. This variation is the AC component. One can do an analysis (called a Fourier analysis) that takes this random variation and parses into a series of well-behaved oscillations at several frequencies. This produces a frequency spectrum that shows turbulent wind consisting of a series of wind oscillating at different frequencies. Turbulent wind typically has a broad brand spectrum: the energy is not concentrated in any one frequency.

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Filtering of AC wind energy

The function of the mound turns crucially on capturing and filtering this AC energy in wind.

Unfiltered wind sounds on a windy day

The AC energy in turbulent wind is most easily captured with a microphone placed directly in the wind. This produces a rumbling sound: the rumbling comes from turbulent vortices being generated around the microphone. You can hear this by playing the audio clip.

Filtered wind sounds on the same windy day

The turbulence can be filtered by surrounding the microphone with a porous barrier (in this case an open weave net, with hole sizes about 2 mm in diameter. The difference is audible.

This difference can be visualized with a so-called spectrogram, below. Here, the x-axis is time, the y-axis plots frequency, and the color indicates the sound intensity at that frequency. The spectrogram below analyzes the two audio clips above.

Spectrograms

Note how the unfiltered gusts contain energy in the higher frequencies (including harmonics). The filter, however, seems to selectively reduce the higher frequency components, while allowing the lower frequency components through. This filtering is at the heart of the termite mound's function.

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The problem for the termite colony

drivers

The problem for gas exchange in the termite colony may now be properly framed.

The nest is a center of metabolic activity. As a consequence, there is a steady (DC) potential energy difference in the partial pressures of both oxygen and carbon dioxide. Oxygen partial pressure is steadily about 2kPa lower than the atmosphere's oxygen partial pressure. This drives a flux of oxygen from the atmoshpere into the nest. This is a DC flux. The same dynamic (but opposite in direction) applies to carbon dioxide. By itself, the DC gradients in partial pressure cannot supply oxygen at a sufficient rate.

The mound captures AC energy in turbulent wind. The AC component in the atmosphere is unfiltered and substantial. In the nest, the AC component is virtually nil.

The mound, then, captures this AC energy and filters it in such a way that it powers a flux that supplements the inadequate DC flux.

The interaction of AC and DC fluxes is the basis of the mound's function as a wind-driven lung.

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Termite pages

Termite home

Structure

Endocasting

Social homeostasis

Nest temperature

Water homeostasis 1

Water homeostasis 2

Water homeostasis 3

Fungal symbiosis

Fungal symbiosis and water 1

Fungi and water homeostasis 2

Gas exchange 1

DC vs AC Gas Exchange

Gas exchange 2

Gas exchange 3

Gas exchange 4

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