OGPSS - The weather, corn, ethanol and oil production

News of the future was, in my youth, something that one found by crossing the palm of a lady in a dark tent with a piece or two of silver (or the modern equivalent) at one of the fairs that came to town. Such opportunities still exist, with all the caveats that existed back then likely still being in force. However projecting the future, whether of the weather, the likely corn crop this year in the United States, or the production of crude oil by the nations of the world has become a much bigger business with copious tables, graphs and theories replacing the rather worn pack of cards or crystal ball of my youthful experience.

Our part of the world underwent a drought last year severe enough to kill several trees in our yard, for example, as well as hurting the corn crop. This year, corn plantings have been severely impacted by the heavy rains and cold weather, so that decisions on crop plantings have become more complicated and delayed, with follow-on impacts on the ultimate yield in a number of Mid-Western states. Corn yield apparently falls at an average rate of 2.3 bushels per acre per day of delay in Northern Wisconsin. These changing conditions make it difficult to assess how much ethanol, for example, will be available to meet demand, although the latest EIA TWIP holds out some optimism for this year.

The impact of the drought on corn prices, and the consequent fall in ethanol production, as production costs rose, are directly visible from their plot of the two over the last year.


Figure 1. A comparison of corn prices and ethanol production in the USA (EIA TWIP )

However, with the weather impacts still being assessed it is already being concluded that the US corn crop is unlikely to reach the record level of close to 14.6 billion bushels that were earlier projected. It still, however, has the potential to reach around 12.3 billion bushels, which would satisfy the just under 5 billion bushel need for ethanol, as well as other demands of the market. By May 12th only 28% of this year's expected crop had been planted, in contrast with a normal year where 65% would be in the ground. Thus even the relatively short-term projections of the EIA could yet be in trouble for this year.

Moving to the slightly longer-term the nations that form OPEC must try and estimate global demand for their products, and the amount that other non-OPEC nations will produce, so that they can balance supply and demand at such a level that will sustain prices at a level they are comfortable with. Their estimates come out as Monthly Oil Market Reports and in the latest (May) version they continue to expect global demand to increase by 0.8 mbd over 2013, but are beginning to hedge that bet, as the global economy continues to appear anemic, with Russian and Asian economies slowing. Yet by the fourth quarter of the year they anticipate that global demand will reach 90.9 mbd.


Figure 2. Global oil demand by region (OPEC MOMR)

OPEC anticipates that, with the major increase coming from the Americas, that non-OPEC oil production will increase by just under 1 mbd to reach an a level of 54.41 mbd in the fourth quarter of the year. The majority of that growth (some 0.59 mbd) will come from the United States, with the Permian, Bakken and Eagle Ford being cited as the anticipated source of these gains. OPEC, having looked at current rig counts, project that these numbers may be revised upwards over the course of the year. And yet it is worth noting this:

On a quarterly basis, US oil supply is seen to average 10.62 mb/d, 10.67 mb/d, 10.62 mb/d and 10.61 mb/d respectively.

The sustained gain in North American production comes about because:

On a quarterly basis, Canada’s production is anticipated to average 4.02mb/d, 3.97 mb/d, 4.02 mb/d and 4.12 mb/d respectively.

Russia is expected to continue to lead in oil production over the course of the year, although it is not longer expected to increase production above current levels.

On a quarterly basis, Russian oil supply is seen to average 10.45 mb/d, 10.43 mb/d, 10.43 mb/d and 10.43 mb/d respectively.

And this brings us back around to OPEC as they try and balance their production against the gap between global demand and non-OPEC supply. As has been the case for a while, OPEC produced two separate tables showing production, as reported by secondary sources, as well as those directly reported by the countries themselves.


Figure 3. OPEC member production as reported by secondary sources (OPEC MOMR)


Figure 4. OPEC member production as reported directly (OPEC MOMR)

It would appear, with Manifa coming on line, that Saudi Arabia is increasing production again, while Venezuela and Iran would have you believe they are producing more than they are, and Iraq, which is now producing above 3 mbd, is directly reporting less (though that could be because some of that production is coming from the north, and there are some communication problems between there and Baghdad).

As long as OPEC has available reserves it can continue this balance to keep enough oil available at an acceptable price to allow the world economy to continue at its present pace. And with that ongoing adjustment available, their projections for this year of a relatively stable price would seem fairly founded, absent some major change in one of the larger producing states.

Iraq overtook Iran as the second largest producer in OPEC last year (according to secondary sources) and expects that with production from Majnoon, it will increase production capability by upwards of 200 kbd by the end of the year. Ultimately the goal is to achieve a target production of 1.8 mbd. However, as overall production levels increase, Iraq may join with the Kingdom in controlling production to maintain price.

Yet even with those abilities OPEC is becoming cautious over predicting that their estimate of the demand:supply balance numbers for this year will be accurate over that time interval.

With these uncertainties in even short-term projections of future production whether it be corn, ethanol or crude it is perhaps wise to continue a somewhat cynical view of projections over a longer time period. Although the bounding bar of a decline in existing field production continues to exist and will continue to require an offset in increased production from new wells to offset. Perhaps that lady in the tent of my youth may prove as prescient as some of the more optimistic forecasts that we continue to see.

Waterjetting 9b - the effect of standoff distance.

One of the problems with relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.


Figure 1. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target, under the jet, is not eroded, but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width, and will be subject to high lateral jet flows.


Figure 2. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.


Figure 3. Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff the central section, though still present, is being eroded.


Figure 4. Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point the cavity is starting to get noticeably deeper.


Figure 5. Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time the central section of the jet is beginning to break down into, initially short strings, that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.


Figure 6. Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

By this time the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.


Figure 7. Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size, and condition as a function of distance, and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used, if the droplets are generated properly, and used within the relatively narrow window that they exist, to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.


Figure 8. The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

There are ways of getting around this problem, but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

More rumblings in Iceland

At the beginning of April there was a burst of earthquake activity just off the northern coast of Iceland, and while it occurred offshore there was no evident major disruption. Now there has been a similar flurry of activity down along the southwest corner. (Ed note This was corrected on May 13, h/t to Steve Golson for catching the error which had east and west mixed up). This contained a number of quakes that ran up to around 4, but it also now seems to be slowing down a little.


Figure 1. Earthquakes in the last 24 hours around Iceland (Icelandic Met Office )

Jón Frímann has written a post pointing out that the activity seems somewhat cyclic, and that the last significant events were in either the 18th or 19th centuries, with no indications of any imminent danger.

Both the current events and the ones at the top of the island lie on the Mid-Atlantic rift that runs through the island, and marks where two of the earth’s plates are slowly drifting apart.

NOTE: This post was updated on Sunday, May 12th.

Figure 2. Map of Iceland showing major volcanoes (The Times of London)

However, you may note on the earthquake map that there is also a little current activity around Katla, which is the volcano that I have been expecting to be the next to erupt, after the pattern of quakes that has occurred there over the past couple of years.

Over at Volcano Café the suggestion has been made that, instead of Katla, it will be the neighbor Hekla that will go next, since there was sufficient activity there to warrant a warning to the public in March. That has since been withdrawn, after the region returned to quiescence. Yet there continues to be some activity in the Katla caldera.

I must continue to remember that imminent in geologic terms does not necessarily mean this month.


Figure 3. Relative positions of Hekla and Katla (which is under Mydralsjokull) (Icelandic Met Office)

UPDATE (Sunday 12th May). While the overall intensity of the quakes it diminishing, I can't help note that they are now extending along the rift line, and taking a straight shot at Hekla.


Figure 4. Earthquakes in Iceland leading up to noon 12th May 2013 (Icelandic Met Office)

Waterjetting 9a - the instant of contact

Plain high-pressure waterjets penetrate into material in a different way than that which occurs when abrasive is used to make cutting easier. And even with abrasive there are different ways in which the target will react depending on how brittle that it is. In this next segment I will write just about the stages that occur as water alone cuts into a target.

In its simplest form consider first a spherical drop of water, moving at very high speed, which suddenly strikes a flat surface.


Figure 1. Droplet striking a flat surface

As the droplet impacts the surface, but can’t penetrate it, so the water that comes into contact with the surface tries to flow away along the surface, to get out of the way of the volume of water striking the surface behind it.

But in the early stages of the impact (see inset) the edges of the droplet ahead of that lateral flow are coming down onto the surface faster than the water can move that is trying to escape. In this range of activity the distance that the edge of the droplet must travel, L, remains smaller than the distance, D, that the water must move to escape.

This instantly traps the water and with confinement comes a very rapid increase in pressure along the edge of the drop. This pressure also acts on the target surface, so that it is pushed down a little. This pressure was first measured by John Field at the Cavendish Lab in Cambridge, UK, who found that it could exceed three times the water hammer pressure that the water might otherwise exert.

For those not that familiar with the term, the water hammer pressure is also sometimes called the hydraulic shock pressure, and it can occur when a valve is suddenly closed in a feed line, and this sends a shock or pressure wave back up the line. (This is what can sometimes cause banging in feed pipes). Often there is a small air cushion built into water lines to act as a sponge, when such a shock occurs, since otherwise the repetitive shocks can cause parts to fail.

This becomes more of a problem with higher pressures because the equation for the pressure that is generated is given by the equation:

Pressure = fluid density x impact velocity x sound speed in the fluid

Compare this with the impact pressure when a shock is not generated:

Pressure = 0.5 x fluid density x (impact velocity)^2

As a very rough rule of thumb, the speed of sound in water is roughly 4,800 ft per second.

If a waterjet is driven out of a nozzle at a pressure of 40,000 psi then the speed at which it is moving can be roughly calculated as:

Jet velocity (ft/sec) = 12 x Square root (Pressure)

The jet velocity, to a first approximation, is thus 12 x 200 = 2,400 ft/sec.

The Water Hammer Pressure is thus 2 x (4,800/2,400) = 2 x 2 = 4 times the pressure exerted by the water more conventionally. Since that driving pressure was, in this case, 40,000 psi, then the water hammer pressure would be 160,000 psi. With the multiplier that Dr. Field found, this can take that pressure up to around 500,000 psi for that instant of contact.

It is, however, only applied to the target at that instant of impact, and where there is the spherical end of the drop to cause the pressure accumulation across the face.

It does, however, cause a very high lateral jet to shoot out of the jet, at about the point that the droplet curvature no longer provides confinement (at about 1/3 of the droplet diameter measured radially from the center of contact).

John Brunton, also at the Cavendish, has provided photographs of the damage done in that instant of contact.


Figure 2. Droplet impact damage on a sheet of Plexiglas (Brunton “High Speed Liquid Impact” Proc Royal Soc London, 1965. P 79 - 85.)

Part of the damage comes from the high lateral velocity of the released water running into the wall of material not compressed under the generated pressure. Mike Rochester found that the diameter of this ring crack closely followed the diameter of the nozzle from which the droplet was released.


Figure 3. Relative size of the ring crack to that of the originating nozzle (jet head) ( M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

In our case, however, the jet is not a single droplet, but rather, at least close to the nozzle, a steady stream with the pressure constant across the diameter.

Thus, in the microseconds after the first impact, as the jet continues to flow down onto the target, so it is flowing out across the damaged zone created by that first impact. The resulting pattern of erosion, which we captured in aluminum, changes as the target moves away from the nozzle. Close to the nozzle the wear pattern looks like this:


Figure 4. Damage pattern around the impact point of a jet on aluminum, target close to the nozzle.

The pattern, close to the nozzle, shows that directly under the jet the pressure is relatively even on the surface of the metal. With no differential pressure across the grain boundaries in that region, the metal is uniformly compressed, and suffers no erosion. At the edges of the jet, however, there is not only the original ring crack damage created on the instant of impact, but also there is a differential pressure along the edges of the jet, which helps to dislodge those initial grains, and provide crack loci for the water to exploit and remove material as it moves away from the original contact surface. The greatest portion of the damage, at this point lies outside the edges of the impacting jet as the laterally flowing jet erodes material as the jet continues to flow.

As the target is moved further from the nozzle, the pressure profile changes from one with a constant pressure over the jet, to one where the central constant pressure region starts to decline in size. Rehbinder calculated the two components of the pressure in the target at the beginning of this erosion process at that point and provided the following mathematical plot.


Figure 5. Impact pressures calculated for the pressure into the target and that along it, during waterjet flow. (Rehbinder, G., "Erosion Resistance of Rock," paper E1, 4th International Symposium on Jet Cutting Technology, Canterbury, UK, April, 1978, pp. E1-1 - E1-10.)

The result of this change in the pressure profile of the jet as it moves away from the nozzle can be seen in the change in the erosion patterns of the jet as it strikes an aluminum target, and that will be the topic for the next post.

OGPSS - The dangers of complacency

Perceptions based, perhaps on too small a collection of information, can lead into opinions that, on investigation, turn out to be incorrect. Just recently a couple of friends had mentioned that charities that they are associated with were seeing a decline in donations. I built this into a picture of the general public being less able to afford earlier levels of giving, perhaps because of the continued impact of higher costs of fuel. The perception is, however, as a general statement wrong, and (Via the National Park Service from The Giving Institute I learn that:

Americans gave more than $298.42 billion in 2011 to their favorite causes despite the economic conditions. Total giving was up 4 percent from $286.91 in 2010. This slight increase is reflective of recovering economic confidence.

The greatest portion of charitable giving, $217.79 billion, was given by individuals or household donors. Gifts from individuals represented 73 percent of all contributed dollars, similar to figures for 2010.

In the perception that is becoming increasingly prevalent on the future of energy supplies, and particularly crude oil, the current adequacy of supply is projected forward to anticipate no problems with supply in the future. Peak oil is now being suggested to occur, not because the supply is limited, but because, with the increasing use of renewable energy, demand will peak, and then decline. Bloomberg New Energy Finance founder Michael Liebreich is quoted as projecting that the growth in fossil fuel use will almost stop by 2030, while Citi Commodity Researchers are suggesting that the increases in prices will drive increases in efficiency that will bring a peak in oil demand “much sooner than the market expects.”


Figure 1. Projected changes in global oil demand from Citi Commodity Researchers)

This anticipation of future gains in efficiency of use is a common thread to pictures of the future from the three major oil companies that I recently reviewed. All three, ExxonMobil, Shell and BP expect that energy efficiency gains will have a major impact on demand. BP, for example, anticipates that through 2030 energy demand will increase 36%, but that without this improvement in efficiency global energy would have to double by 2030.

One of the problems in assessing the changes in efficiency over time is that, when looking at the past decade, one has to recognize the significant impact of the recession. For example, the Odyssee project looked at energy use in Europe and clearly showed the impact of the recession on demand.


Figure 2. Changes in electricity use in the countries of Europe following the start of the recession. (Odyssee)

What also caught my attention in looking where most of the energy savings were occurring was that it was in countries catching up to Western Europe, rather than in the more established West, and that when the overall savings are totaled these appear to have slowed significantly.


Figure 3. Overall energy savings in the EU relative to a 2000 baseline (Odyssee)

The second problem with the curve that Citi projects lies in the rate at which vehicles are switched from diesel and gasoline to natural gas power. There is currently an economic incentive in parts of the world to make this change, it currently sells at around the equivalent of $2.10/gallon in the USA. Yet it requires both infrastructure and an investment of capital to make the change at any level of significance. Nevertheless it remains a key ingredient of the Pickens Plan that Boone Pickens has been selling around the country for a number of years now.

The fact that Clean Energy Fuels can list all 22 stations that added natural gas pumps along the “Natural Gas Highway” in the November-January period, does not indicate a great rush to build that infrastructure. It is easier to change the local distributor networks, with companies such as Waste Management indicating that they will use CNG in 80% of their new trucks, than it is to see the rapid change of the longer distance haulers, and for passenger vehicles. A recent article in the Washington Post noted that only 20,381 vehicles ran on natural gas of the 14.5 million new cars and trucks sold last year. Further not only does a CNG vehicle cost more to purchase, it also has a lower range, although for some applications that may not be much of a handicap.


Figure 4. Average Annual Vehicle miles travelled by category (Alternate Fuels Data Center )

Yet, at the moment, it is the use of ethanol that is having the most impact on alternate fuel use. Other than that there has been little indication of much change in the market.


Figure 5. Alternate Fuel Vehicles in use from 1995 to 2010. (Alternate Fuels Data Center )

And in this regard Europe has also seen little movement toward the use of natural gas, in contrast with the use of biofuels, and neither has made large gains.


Figure 6. Comparative penetration of liquid fuels market in Europe by biofuels and natural gas (Odyssee)

The problem, of course, is that if these improvements in efficiency and switches to alternate fuels do not occur, then the demand will continue along the Business-As-Usual line, and, as BP forecasts, demand will double by 2030.

The question as to what will be available to meet that enhanced demand remains one of the great imponderables that folk seem, again, unwilling to face. Certainly with a steadily increasing demand, and the constraints on supply that these pages have continued to document over the years, it becomes very difficult to see how price stability can be maintained, where demand exceeds supply at a given price. The problems that this will bring, particularly those nations that now subsidize fuel, a policy that is unlikely to change in Asia, are likely to be major. Yet for countries such as India, which last year has spent the allocated fuel subsidy budget for the year by the end of July the political costs of change remain very high and could well remain in place until the financial burden becomes intolerable. Unfortunately, with the current complacency, at that point it will then be too late to start searching for alternate answers.

Waterjetting 8d - Choosing angles

How times change! I was reading a column in the British Farmer’s Weekly, and came upon this, where the author is discussing the need for a generator.:

It will also be vital to keep the fuel flowing into the tractors, and power the pressure washer, and light the security lights, and all the other essentials of an average arable farm.

It is an indication of how far the use of pressurized water has come, that it is now seen, at the lower end of its application, as a vital farming tool. Which is a good introduction to talk a little further about the use of cleaning streams, and how to interact with differing target materials.

There was an initial first step, when someone would send the lab a mystery block of material and asked – how do I cut it? Generally the samples were small, but we would find a flat surface on the material, and carefully point a jet nozzle perpendicular to this surface. (In the early stages this was hand-held). When a jet strikes a surface, but can’t penetrate it, then it will flow out laterally around the impact point, under the driving force of the following water.

The test began with the jet at low pressure, and this was slowly raised, until the point was reached when the pressure was high enough to just start cutting into the material. At this point the jet had made a small hole in the target, and so the water flowing into that hole had to get out of the way of the water following. The sides of the hole stop it flowing laterally, and so it now shoots back along the original jet path. This spray can hit the lance operator if the nozzle is hand-held, but it is a fairly graphic way of determining the threshold pressure at which the material starts to cut. (and I’ll get into what happens as the pressure continues to go up in a future series of posts).

But for the purpose of cleaning, the jet has to move over the surface, once it has made that initial hole, at pressure. But, in many materials, if the jet comes vertically down onto the target, then only the material directly under the jet will be removed. And so the jet has to be played on every square inch of the surface in order to ensure that it is cleaned, or that the coating/layer is removed. In some sandstones, for example, two jet paths could be laid down, almost touching one another, and yet the rib of material between them would remain standing.


Figure 1. Adjacent jet passes in sandstone, the cuts are about an inch deep, but note that even though the narrowest rib is about 1/8th of an inch wide, it is only when the cuts touch that the intervening material is removed.

Yet that rib of material was, in that case, so weak that it was easy to break it off with a finger. (This turns out to be a weakness in making delicate sculptures out of rock). To use the full pressure of the water can be a waste of energy, if the material is very thick, since it all must be eroded with such a direct attack.

Yet the minimum amount of material that needs to be removed is that that attaches the layer to the underlying material (the substrate concrete, steel etc) and this can be quite thin. Thus, in attacking a softer material, particularly one that can be cut with a fan jet, a shallow angle directed at the edge of the substrate can be more effective.


Figure 2. Round v fan cleaning from Hughes (2nd US Waterjet Conference)

Because there is a balance between cutting down through the material to be removed, and cutting along the edge to grow the separation crack between the materials, some practice is needed to find, for a given condition, what that angle would be.


Figure 3. Choice of angle from Hughes (2nd Waterjet Conference)

The more brittle the material, then the greater the angle to the surface, since rather than just erode the material, the jet may also shatter the layer into fragments that extend beyond the cut path. But otherwise using an angled jet to the surface can be more effective. Hughes (from whose paper at the 2nd Waterjet Conference I took these illustrations) has a simple test for orifice choice.


Figure 4. How target response influences nozzle selection. (Hughes 2nd Waterjet Conference)

Some of the more advanced cutting heads use a series of nozzles that spin within an outer protective cover, as they remove anything from layers of damaged concrete to thin layers of paint from ship hulls. Increasingly these are connected to vacuum systems that will draw away the spent water and debris from within the contained space, without it entering the work space, and creating problems for the worker.

In order to reduce any collateral damage to the surroundings these jets are often made very small (thousandths of an inch in diameter) so that their range is short, and they are inclined outward to cut to the edges of the confining shield.

We have had some success in turning those angles the other way, so that they cut into the shield, rather than away from the center, and also so that each jet is directed towards the path of the next jet around the circumference. The intent in this case is to allow the use of a slightly larger jet, with a greater cutting range. In this case the individual cleaning/cutting path is a little larger, but because the jet at then end of the cut moves into the range of the adjacent jet, then any remaining energy that it and the dislodged debris still have, will not be enough to get through this second jet.


Figure 5. Inclined jet and shroud design.

The action of each jet then becomes not only to cut into and remove material, but also to contain the spent material from the other jets dispersed around the cutting arm, and to hold the debris in the center of the confinement for the very short time needed for it to be caught up in the vacuum line.

In all cases the choice of pressure, nozzle size, and operational factors such as angle of attack, come down to the target materials, those that have to be removed, and those that need to be left undamaged. And it is why it is useful, at the start of any new job, to take the time to do a little testing first, to make sure that the right choices of nozzle and angle have been made to get the job done quickly and efficiently.

Incidentally the idea behind the test of effective pressure, that the jet flows laterally when it hits something it can’t cut, can help, for example in easing the meat from the bone when a jet cuts a deer leg.


Figure 6. Cut across a deer leg, note how the jet has cleaned off the meat from the bone, undercutting the flesh.

Fixated on Katla

It has been more than a couple of years since I started to write on the potential for an eruption of the volcano at Katla in Iceland, following the eruption of its neighbor at Eyjafjallajokull, and the resulting disruption of air traffic.

One of the reasons that I continue to do so, apart from the historic pattern that Katla often erupts following Eyjafjallajokull, is the ongoing earthquake activity around the caldera. I noticed it clearly again today where with most of the zones of Iceland quiet, there is still that steady pattern. And this activity has persisted at a relatively low, but varying level, across this time. Doesn't say it will happen soon, but does continue to suggest that it is, nevertheless, going to happen. Patience is a good word.


Earthquakes in Iceland in the last 24 hours (Icelandic Met Office) Hopefully there won't be much more news for a while.