Marine Cables

The Submarine Map of the World produced by TeleGeography, complete with the inclusion of depictions of sea monsters in the spirit of medieval/renaissance cartography - Full Size

Today's trivia is all about the cabling system that makes up 99% of all international data transmitted, an essential part of the Internet, the marine cable network.

The first successful cable attempted was between England, Dover and France, Calais in 1851 and by 1858 there was another successful laying of cable across the Atlantic, from Ireland to Newfoundland. (The 1857 attempt failed in cable break)

From there it became quickly clear that the ability to communicate both securely (as opposed to radio where anyone could receive the message) and quickly was apparent. At the time the company operating the cable was charged at $5 a word, and could transmit at a rate of 6-8 words a minute.

This lead to a boom in the marine cable industry and by 1901 the Eastern Telegraph Company had established a large network around the world.

Map of the Eastern Telegraphy Company - Full Size

Technology

The early cables where a combination of a number of advancements of the time to produce a viable marine cable.

Cut-away of an 1851 cable used in the Dover-Calis cable

Copper was of course the conductor of choice, however uncertain in quality.

Insulator: Gutta Percha, the rubber-like sap of a tree found only in the British Empire provided a resilient and flexible insulator.

Armour: A method of armouring cables with iron wire ropes was developed to work the hoisting machinery in Germany’s deep mines was used.

Hydrography: In 1849, American naval vessels began systematic deep-sea soundings in the Atlantic which allowed the plotting of an ideal route over the sea bed.

Cross section comparison of marine cables

Cables where initially electrical in nature, until around the late 1980s there was a switch to fiber optics as a reliable and faster technology. Modern cables are are typically measured in the hundreds of gigabits per second, or terabits per second.

Fiber optic cables don't have the signal range of electrical cables, so instead they use solid-state optical amplifiers placed at regular intervals and powered by a power line that runs down the code of the cable to amplify the signal and relay it onto the next part of the cable.

Causes of faults

• Fishing with trawl nets is one possible cause for damage to marine cables, apparently a common practice during the Cold War.
• Earthquake and associated tsunami or mudslide is an natural cause for damage of marine cabling. Whether it is movement on the seabed or movement of the water which causes the cable to scrape against rocks, damage can be done to the cable.
• Thieves in 2007 stole 11km of cable in the Vietnamese sea which connects Thailand, Vietnam and Hong Kong. The repair vessel was perplexed when it was sent out to investigate only to find a large section missing. The thieves were later caught trying to sell 100 tons of scrap cable. Cable theft is on the rise.
• Sabotage suspected in 2013 images and a news article were released indicating that three men with diving equipment where responsible for a large scale outage of the SEA-ME-WE 4 marine cable.

Repair

In order to repair a broken cable, it has to be located on the sea bed. Accurate recordings of where the cable was laid are crucial.

A ship like the Pierre de Fermat is custom built for the task. At 100 meters in length and a beam of 21 meters, GPS positioning and thrusters for positioning, it is purpose built for the task of repairing and laying cables.

The Hector 7 ROV being unloaded - Full Size

In the case of cable repairs, a vehicle like this Hector 7 ROV are ideal for the task. The size of a van, it can drive along the sea bed, locate the cable and then attach to the cable before being brought back up to the surface. Then it can find the other end of the cable and an appropriate splice can be performed to join the two ends together.

References

• History
• Submarine Cable Map

Freeze Dried

Today's trivia covers two subjects in one thus making it rather exciting.

What is freeze drying, and what does it do to coffee?

First developed as a method of preserving medical treatment and pharmaceuticals during WWII. Freeze drying is now often applied to food-stuffs as an effective method of preservation that increases the shelf life of food.

In the case of a jar of instant coffee this will extend its life well beyond that of the original bean based product.

The process of freeze drying food removes almost all the water from the food. In doing so the food is preserved by preventing the usual spoiling agents of bacteria and mold from affecting the food stuff.

However as you might expect this also has an effect on flavor in some cases as water is not the only component dried out of the food stuff. Volatile compounds like acetic acid (vinegar) and alcohols are also prone to loss in the drying process.

Another side effect of freeze drying is that it makes the resultant material quite water soluble afterwards as the process leaves microscopic pores in the material. In the case of pharmaceuticals this is quite the benefit as it allows quick re-hydration. We have Clarence Frank Birdseye II to thank for that, yes the same Birdseye as the fish fingers.

Wet books can be recovered using freeze drying as well.

Sublimation

The principle behind the freeze dried process is sublimation, which brings us nicely to the triple point of water. This is the temperature and pressure where three phases (gas, liquid and solid) of that substance coexist in thermodynamic equilibrium.

Graph showing the relationship between the three states of water against pressure and temperature full sized

When water is cooled below its triple point, we can then use pressure changes to force the water to transition from a solid to a gas by a process known as sublimation. In this case the water literally transforms from frozen solid to gaseous form without passing into a liquid in-between.

Industrial Process

So with this useful knowledge we can understand how it might work in an industrial process like the manufacture of instant coffee.

The coffee beans are roasted, ground and brewed as you might expect. However before the freezing process takes place the coffee is reduced to a syrup to ensure it retains the most of its flavour once dried.

Coffee syrup is moved onto a flat conveyor to ensure it is a consistent depth

Then the coffee is moved to freezer room kept at -50C. At this temperature the coffee will freeze very quickly. The speed of the freezing is important to reduce water crystal size and thus damage to the food stuff being freeze dried.

Factory staff dressed in thermal insulation when entering the industrial freezer

Once completely frozen the coffee is then broken up into granules, before being moved to a pressure chambre.

Sheet coffee is broken up into smaller pieces whilst at -50C

From here we move onto the pressure chamber which will reduce the pressure whilst warming the coffee to speed up the process of sublimation. In the image below the coffee takes 5 hours at 60C to sublimate almost all of its water.

The coffee granules are placed on trays and moved into a vacuum chamber

Then as you would expect bottling and packaging commence afterwards.

The factory in this video produces 420 tons of coffee each week.

References

For those that want to see the process in action, this video is informative.

Wikipedia:

• Triple Point
• Freeze Drying

Radioisotope Thermoelectric Generator

In the clean room at KSC’s Payload Hazardous Servicing Facility, technicians prepare the New Horizons spacecraft for a media event. The RTG seen in this picture is not the real flying unit and is only a mockup. The real RTG was installed shortly before launch.

Today's trivia looks at something I have been interested in for a while, the power supplies used on space probes.

The Radioisotope Thermoelectric Generator is a class of electric generator that operates using a thermocouple to generate electricity using the Seebeck effect.

Important details for this class of generator:

• No moving parts
• Powered by heat source and thermocouple
• Very low efficiencies
• Long operational life time
• Consistent steady output

These types of generators are useful for both space and terrestial usage. In space they have been used on a number of the recent space probes, and on Earth they have been used in unmanned remote light houses, beacons and other facilities.

Thermocouple

Before we go further we should of course take a moment to understand how a thermocouples works.

The thermocouple is based on the Seeback effect (Thomas Johann Seebeck, 1821) and occurs when two conductors (typically metals) come into contact and are at different temperatures. When this occurs a positive voltage can be detected at the cooler end.

This principle is widely used in thermometers, where the difference between a reference temperature (room temperature) and the substance being sampled generates an electrical difference into a temperature difference.

K-type thermocouple (chromel–alumel) in the standard thermocouple measurement configuration.

In the case of an RTG, the thermocouple connects the heat source to a heat sink which radiates the heat away, creating the temperature difference to generate energy.

Heat Source

In the case of the RTG used in the New Horizons mission the radioactive element of choice is Plutonium 238.

A pellet of 238PuO2 to be used in an RTG for either the Cassini or Galileo mission

With its half life of 87 years and its tendency to emit mostly alpha radiation it is ideal for the space probes. However by comparison to Plutonium 239 used in nuclear weapons, 238 is considerably more radioactive. Handle with care recommended.

The generator onboard the New Horizons Mission at launch provided 250W 30V DC which by the time it had reached Pluto 9.5 tetras later had dropped to 200W. Enough to power most of the instruments at the same time.

Other materials can be used as well. Strontium-90 for example has been used in Russia to power various terrestrial research stations. As a by product of nuclear power plants it is readily available.

By comparison, Plutonium 238 has to be manufactured specifically, and Russia and the US are the only countries that manufacture it.

Assembly/Design

A radioisotope thermoelectric generator (RTG) assembly and Ulysses undergo a fit-check in 1989.

Given that we are dealing with radioactive decay rather than nuclear fission, the design of a RTG is comparably very simple. A tube containing the heat source is connected to a thermocouple and heatsinking. The voltage is generated is then connected directly to the power distribution parts of the circuit being powered.

Diagram of an RTG used on the Cassini probe. The GPHS (General Purpose Heat Source) is powered by a pellet of Plutonium 238

Future

Will this power source be used on future space missions? Possibly, it is very reliable and easy to manufacture. However the efficiency for mass is a design issue with RTGs, most of which are around 3-7%. The demand on electric systems by scientific instruments will only increase and power output will become a limiting factor.

For example, New Horizons spent the majority of its mission time in hibernation. Only to be very active during the actual fly-by, which is of course a small time window in which all its instruments need to be highly active.

Time lapse photograph of the heater head of a Stirling radioisotope generator undergoing lifetime assessment.

NASA have investigated more efficient alternatives including the Stirling Radioisotope Generator which might offer up to four times the efficiency from the same Plutonium heat source.

Stirling Engines work by using two pistons which are connected together by a tube. The first piston is heated, the second is cooled. The rods of the pistons are connected via a rotary motion to cause the opposing piston to move.