On nearly every refueler truck and hydrant cart which is capable of underwing (pressure) refueling there is a gauge marked “nozzle pressure”. Since it is not connected to the nozzle, how can it really show actual nozzle pressure? There is no magic involved. Actually, it does not show nozzle pressure! What it shows is simulated nozzle pressure. The gauge is connected to a device called a VENTURI. It is usually mounted at the inlet of the hose reel. Few people understand what the venturi does or how to adjust it properly. A large number of the venturis in the world are not properly adjusted.

To understand a venturi, we have to understand energy. In a flowing pipe, the energy at any point depends on velocity and pressure. The energy level remains the same except for gradual decreases caused by friction as the fluid passes through valves, fittings and pipe. In Figure 1, the pressures at A, B and C will be exactly the same if there is no flow. If flow takes place, the pressure at C will be nearly the same as at A, less a friction loss. However, the pressure at B will be much lower than at C. The reason is that we turned some of the pressure energy into velocity energy to get the fluid through the small opening. After the restriction, the velocity energy decreases so the pressure energy must increase accordingly.

Giovanni Venturi, the Italian scientist, discovered 200 years ago that he could make an excellent flow meter with very little pressure loss if he carefully designed the outlet after the restriction to avoid turbulence and friction. See Figure 2. The pressure recovery zone is a long gradual taper back to the original pipe size. The pressure drop (energy loss due to friction) in his design is much less than in Figure 1.

The purpose of a venturi in a fuel system is not to measure flow rate but to provide a simulation of nozzle pressure at B and to use that pressure as the control signal for the pressure control valve. When fuel is pumped to an aircraft, you want maximum flow, but pressure must be kept below 50 psi maximum aircraft fueling pressure to achieve maximum flow rate into the aircraft. See Figure 3. Such a system may require 80 psi at point C in order to result in 50 psi at the aircraft (D), assuming a 30 psi pressure drop in the reel and hose. The problem is that when the aircraft is full, there is no flow and therefore no pressure drop. Without a venturi such a system would, at no flow, put 80 psi on the aircraft. This could cause damage to the aircraft fuel system. If set at 50 psi, the actual fueling rate would be very slow, no where near the design rate because the pressure drop between points C and D would use up much of that 50 psi.

FIGURE 3

(The above example shows a simple, non-compensated single pressure control valve system, for simplicity.)

The venturi system (Figure 4) solves this problem. By locating the venturi just before the hose reel, pressure drop across the filter and meter is ignored by the pressure controller. The venturi’s job is to compensate for pressure drop in the hose reel, hose and nozzle. As flow increases, the venturi “fools” the pressure control valve into delivering enough pressure so that there is 50 psi at the low pressure point (B). The venturi is “tuned” so that B and D match. Under full flow conditions, supply pressure (C) may be 90 psi and pressure at point A may be 70 psi, but pressure at the nozzle and at point B is 50 psi. When the aircraft is full and flow stops, pressure A, B, C and D return to 50 psi.

FIGURE 4

Venturis are not designed to match each system because it would be far too expensive to custom machine each one. Instead, a venturi is selected that has too much compensation. The Whittaker (formerly Thiem) F505 provides up to a 34 psi pressure depression at 600 gpm. If you only need 27 psi of depression to compensate for the pressure drop through the hose reel, hose and nozzle, you must “bleed off” some of the signal. This is done by adjusting a needle valve (Figure 5) so that some fuel from the high pressure zone at the venturi inlet will flow to the venturi throat. The resulting “mixed” pressure signal (27 psi below system pressure) is then fed back to the control valve as its “command”. When correctly adjusted, the pressures at gauges B and D read the same. This should be checked at no flow and at full system flow rate.

FIGURE 5

• When properly adjusted, the system allows maximum flow rate without danger to the aircraft. If you change to a different hose length or size (diameter), the venturi must be re-adjusted. We recommend that this adjustment be checked at least once a month. In this way, a possible problem with the pressure control system may be detected and maximum safe flow rate is assured.
• If a hose-end pressure control valve (HECV) is mounted on the nozzle, it must be either removed or “blocked out” to adjust the venturi “nozzle pressure”. When finished testing the system, remove the block-out device and re-test the system. (The HECV vent will spray some fuel when next used!) Bear in mind that if the HECV is removed for this test, its own pressure drop will not be compensated for by the venturi.
• Most specifications for fueling equipment now require dual pressure controls, one primary and one secondary or “back up”. This can be achieved by various combinations of inline, bypass and hose-end controls. (We will address this subject in a future Gamgram). A common method has the deadman valve serve as a primary pressure controller with the bypass valve as a secondary controller. Figure 6 shows how this is done. Both valves must not be adjusted to control the same pressure because they will “fight” one another with resulting instability. In any system, the primary regulator controls pressure 5 to 15 psi lower than the secondary. This may be done either by setting the regulators at different pressures, or by sending the regulators differently tuned venturi signals. In this example, the in-line (primary) controller is set at 45 psi, (pressure E). The bypass (secondary) controller is set at 50 psi (pressure B). With this arrangement, if the aircraft valve closes suddenly, the secondary valve backs up the primary valve by opening quickly to relieve pressure on the primary. (Primary pressure may be set as high as 55 psi).

FIGURE 6

Many systems on refueling trucks use air pressure as a reference using an air pressure regulator. It is a good idea to use a locking regulator (See Bulletin 70) to prevent unauthorized or “accidental” adjustment. On a Whittaker regulator, this will be 16 or 25 psi higher than the desired fuel pressure. This difference is called the “bias” pressure. The purpose of this bias pressure is to allow smooth operation of the controller. To better understand this system, think of it this way: The air reference pressure is on one side of a sensing pilot piston with the venturi pressure on the other side with a bias spring. If the total of spring force and venturi pressure is greater than the air reference pressure, the piston moves slightly toward the air side; this results in the control valve closing slightly to reduce system pressure and make the sensing piston return to a balanced position.

This issue of the GamGram is written out of total frustration. The problem is that it seems necessary for a person to work with tubing, pipe & pipe threads for at least 5 years before understanding them, and then he orders the wrong size fitting half the time.

Customer: “I ordered ten (10) 3/8 inch toggle valves, and you sent me these big things with threads that are almost 3/4 inch in diameter”.

Answer: “Well, 3/8 inch pipe has an OD that is about 3/4 inch — only 0.075 inches less, so we sent you what you ordered.”

Customer: “But I measured my pipe and it was about 3/8 inches OD. How can you say that your 3/4 inch is what I ordered.”

Answer: “If you had told us what your pipe measured, we would have sent you 1/8 inch valves. Pipe that 1/8 inch size has an OD just 0.03 inches greater than 3/8 inch.”

Customer: “You guys are nuts!”

The customer is frustrated, and so are we. So now we are going to tell you the “facts of life” about pipe sizes. We do not want to appear to justify the peculiarities of the system – all we will do is explain it.

Pipe was originally made by casting processes. The inside diameter measured 3/8 inch or 1 inch or 3 inch, whatever the size was supposed to be. Within the last 120 years, wrought pipe was perfected, but it was found unnecessary to use the heavy wall thicknesses that were required to avoid problems of cast pipe. Wrought materials such as steel and brass could withstand far greater pressures than cast pipe. Obviously, the wall thickness had to be reduced to save metal, but the question was whether the OD should be reduced or the ID increased. Someone decided that the ID should be increased. This way, the wrought pipe could be connected to the same fittings that had been made for cast pipe. This is how the world became confused about pipe sizes.

The following chart gives the sizes of standard schedule 40 pipe. Manufacturing tolerances are not shown. Schedule numbers that are less than 40 or greater than 40 all have the same OD; only the ID changes. Pipe that is made for high pressure has a thicker wall, and the schedule number is greater than 40, such as 80, 120 or 160. Light duty pipe has thicker walls and smaller schedule numbers, such as 20, 10, or 5.

The lesson to learn is that it is impossible to find a dimension on a fitting or a pipe that tells you the actual size until you get up to 14 inch pipe. The OD of 14 inch pipe is really 14 inches; the OD of 20 inch pipe is 20 inches, etc.

So now if all of our good customers would please measure their pipe or pipe fittings, our problem will be solved forever — but we don’t really believe that. Why? Because then we develop confusion about tubing sizes.

Tubing

Tubing is measured by its OD not its ID; 3/8 inch tubing really measures 3/8 inch on its OD, but its ID is usually only about 0.3. Where we have a tubing problem is overseas where metric sizes are used; American fractional inch tubing fittings will not fit. That is why we have to insist that our overseas customers actually take very close measurements. This will insure that we provide the right metric sized tubing fitting.

Pipe Threads

Most pipe threads in petroleum service outside of North America are straight, not tapered. A gasket does the sealing. When an American product reaches an overseas destination, the mechanic thinks that the fitting should be tightened until no threads can be seen on the pipe. The result is usually that the fitting splits open because of the tremendous forces that are created when two tapered parts are driven together. Regrettably, there is no standard torque that should be used. Generally speaking, from 4 to 7 turns will produce a tight joint if a thread sealant, such as Teflon tape, is used. Our experience shows that most manufacturers of stainless steel fittings do not adhere to the standards which specify that hand tightness is reached in 4 – 5 turns. You are lucky to have 2 turns. The number of turns after hand tightening should be 2 or 3.

An exceptionally severe problem develops when stainless steel fittings are screwed into aluminum castings with Teflon tape as the sealant. Although the Teflon provides some lubrication, it is not consistent. If a thread is rough at one point, the Teflon tears away; the mechanic feels the resistance and thinks the joint is tight. It leaks. We have learned over the years that adding petroleum jelly (Vaseline) to the tape after it is applied, makes “all the difference in the world”. But we warn you, the very low friction can easily result in a split fitting. Little more than hand tight will make a leakproof connection.

British Pipe Threads

Although British tapered pipe threads are used in foreign industry, the petroleum industry in all areas out of North America, seem to have standardized on the British parallel or straight pipe thread. A gasket makes the seal. We refer to it as BSPP for British Standard Parallel Pipe threads, but we are criticized for adding the second P. The reason we insist on doing it is because BSP can mean either tapered or parallel threads.

The dimensions of BSPP threads are remarkably close to NPT as the chart shows, but the thread form is 55° instead of 60° and the number of threads per inch is different. Nevertheless, some mechanics consistently make tight connections with Teflon tape when installing NPT male fittings into female BSPP threads if one material is soft, like aluminum.

Summary

The only safe way to know what you are doing when you order pipe fittings is to measure with a caliper or a micrometer. We surely do hate to ship parts half way around the world and then find the customer ordered the wrong size.

NOTE: Just to clarify, OD stands for Outside Diameter, and ID stands for Inside Diameter.

When an engineer specifies equipment that has little technical foundation, we often call it, “Seat of the pants engineering”. This is a perfect description of the way people specify heaters for filter separators. Call it intuition, guesswork, hocus-pocus or pure charlatanism; there really is no apparent technical basis for the way most heater systems are designed for filter separators.

Filter separator salesmen are known to make stupid statements, such as, “A heater keeps the filter separator warm.” We recommend that you throw the guy out, because he simply does not know what he is talking about. Consider the fact that if your filter separator operates at 600 gpm, and the incoming fuel is at 20°F (-6.7°C), the amount of heat required to raise the temperature to 32°F (0°C) is 24,000 BTU/min.. This requires a heat input of 421 KW. Even if you totally ignore the incredibly high heat loss from the vessel surface to the atmosphere, you can easily see that the typical sump heater rated at 1 KW achieves nothing toward heating the vessel. It is like trying to melt steel with a match!

There are really two separate problems in operating filter separators in cold weather:

1. Preventing the vessel drain fitting from freezing. (How can you change elements if you are unable to drain it?)
2. Maintaining fuel flow through elements that are blocked with ice.

The first problem is the most common one. The second problem is restricted to regions where fuel in storage tanks drops well below the freezing point of water.

To keep the drain fitting clear of ice, an immersion heater provides a simple answer, but it takes some careful thought to insure that the heat will be located where it is needed. Figure 1 is an example of thousands of poorly designed installations. The drain valve is located for operator convenience, but the heater will not prevent freezing at the valve. The design in Figure 2 is good because the drain valve is as close to the heater as possible, and the heater is directly under the drain connection. Convective recirculation up through the drain into the vessel tends to prevent ice formation that might bridge over the drain entrance in the vessel. Heaters rated 300 to 750 watts have been found suitable.

To assist the drain line heater, a sump heater is often used. However, many of them are installed as in Figure 3. Clearly, a heater at this location in the vessel is worthless, because heat will not be located where it is needed, at the drain entrance. In Figure 4, the heater is located so that it can assist the drain line heater in maintaining a clear path for water to leave the vessel. There is no way that the entire deck-plate can be heated, so it must be resolved that ice will exist in areas located away from the inlet of the drain. Obviously, most of the heat from such a heater will rise upward, away from the water that collects, so it is very important that the heater be located as close to the sump surface as possible — hopefully no more than ½ inch (12mm) above it.

We now return to the problem of dealing with ice that forms in fuel before the water has settled to the sump. Underground storage tanks almost never have very low temperatures, except possibly in arctic regions. The fuel in above-ground storage tanks can drop far below the freezing point in many parts of the world. As the temperature of fuel drops below the freezing point, water that was dissolved in the fuel appears as ice crystals in the form of microscopic needles that do not tend to settle, because they are too small. Coalescer elements become hopelessly blocked by these slivers of ice.

Dealing with this problem is extremely difficult and there are few alternatives. All are very expensive and none are simple:

1. Use anti-icing additives.
2. Install the filter equipment in a heated building. When the elements become plugged with ice, stop the flow and wait until the ice melts. If you have extra money, install two parallel filter systems so you can switch from one to the other to allow thawing.
3. Heat the fuel storage tank to keep ice from forming.

In case you would like to know some other methods that have been used in the last 40 years, consider these:

1. One filter separator manufacturer had a specially designed unit that you buried in the ground with only the cover exposed. Earth is a great insulator!
2. Hundreds of filter separators were installed inside of plywood boxes having electrical space heaters inside. It was necessary to dismantle the box to change the elements.
3. Hundreds of installations were made with the sump and drain piping thermally insulated (lagged). Rain water always leaked into the insulation making the heating task far worse; many vessels rusted on the outside nearly to the point of structural failure.

We know of two airports that were kept in operation in a recent winter, because the ice-blocked coalescer elements were periodically removed and thawed in a heated building. Fueling of aircraft was maintained by alternately using two sets of coalescers. There are no low-cost solutions to this problem.

In conclusion, concentrate on keeping drain systems clear of ice, and then hope and pray that you will not have to deal with wet batches of jet fuel.

Did you know that there are 5 times more aviation accidents caused by water and dirt in the fuel than from misfueling (pumping the wrong fuel into an airplane)? We heard a supposedly responsible representative of an aircraft manufacturer say that this means the industry should concentrate on the water and dirt problem instead of the misfueling problem. What a monkey!

We already do many things to keep fuel clean and dry but what has been done in the past to prevent misfueling? The answer is almost nothing. Oh yes, we do label trucks, tanks, pipes and fill stands, but not always. The aircraft are supposed to have placards stating what fuel to use, but they do not always. Bottom loading equipment can easily be keyed with product selectors that have been on the market for 30 years but they are not always installed. In fact, only a small fraction of the apparatus is so equipped.

Misfueling accidents all have one thing in common — human error.

After an air show, an experienced and highly professional pilot watched his light twin being refueled with a truck marked “jet fuel”. Fortunately, he crash-landed safely.

A very young, untrained line serviceman put jet fuel in a cabin twin because he saw the word TURBO in the name of the airplane. Seven people were killed.

A dealer had a practice of keeping the unmarked jet fuel truck in one location and the unmarked avgas truck in another. Guess what happened. Someone made a mistake in parking the jet truck. The accident was inevitable.

A corporate pilot arrived at one of the world’s largest airports and deliberately specified that he did not want fuel. He got it anyhow, jet fuel in his avgas cabin twin. The dealer caught this error before the pilot departed.

So you see, it is always human error. Fortunately, someone decides to do something positive about this crazy situation. The General Aviation Manufacturers Association promoted a campaign which is summarized below:

1. The word TURBO has been eliminated from airplane names.
2. Decals were designed and distributed for labeling filler caps.
3. Bands were designed for labeling all overwing refueling nozzles.
4. A keying system was devised to prevent a large jet fuel nozzle filler spout from entering a smaller avgas filler opening.

The fourth action is the one that is the most important, because it overcomes human error. Research showed that 74% of avgas aircraft have filler openings that are less than 2.3” in diameter. Therefore, only 26% of the aircraft fleet needed to be modified with smaller openings if all jet fuel spouts on overwing nozzles were made larger. Through some very clever design, Shaw Aero, the largest manufacturer of caps and filler openings, developed an insert to reduce the size of large openings. These kits are offered by the airframe manufacturers, such as Piper, Cessna, Beech, Mooney and Aero Commander. All new avgas aircraft manufactured after early 1984 have a small filler opening.

The spouts on all overwing nozzles prior to 1984 were sized to fit through the smallest filler openings. However, to make the interference system work, every overwing nozzle that dispensed jet fuel had to have a new spout that is large enough so it would not enter an avgas opening. The problem was that some jet fueled aircraft have a “D” shaped opening; they are not round. The solution was a spout that is oval shaped. It measures 2.6” at the largest point so it will not enter a 2.3” opening, but it will enter the “D” shaped opening.

So now you may think that the problem of misfueling is solved. Unfortunately, it is not solved because of several other problems:

1. The FAA has not developed a clear, consistent policy. For example, owners of Cessna 300 and 400 series aircraft received AD’s that mandated installation of restrictor kits but owners of Beechcraft twin engined aircraft only received Service Bulletins; compliance is not mandatory. The Piper AD only applies to Navajo and Aerostars, not to Aztecs! Very Confusing!!
2. There is no law that forces a dealer, FBO or oil company to install the new oval spouts on their jet fuel nozzles.
3. After all of the heavy research that went into the design of the system, some aircraft that use jet fuel have been found to have filler openings that are too small for the oval spout. Fortunately, some of these can be modified to the large size but others remain a problem. More about these cases will be found later in this article.
4. The real “zinger” in the program is people who “bad-mouth” the idea, just because it is different from what they are accustomed to. For Example:
   1. Some line personnel said that the new oval spout causes excessive splash-back in the Cessna Citations and Beech King Airs. To investigate this, tests were run that showed that round spouts also will cause the same splash-back problem at the same flow rate. The difficulty is that the fuel cannot flow down the slope (dihedral) of the wing rapidly because of the small holes in the wing structure.
   2. Some people say absolutely that the shape of the oval spout causes a spray pattern that results in splash-back. This is pure “bunk” – look at these photographs of the fuel flow pattern.
      
   3. Some people have stated that the flow rate is reduced by the new spout, as compared to the 1½” diameter spout it replaces. Tests by Cessna and OPW prove this is simply not true.
   4. An article was printed stating that the flow in a round spout is laminar but not in the oval spout. Fantastic!! Try to find a qualified engineer who will agree that laminar flow can exist at 9 feet per second downstream of a cone strainer. That is the velocity in a 1½” round spout at 50 gpm. Man! — That is turbulent flow, not “laminar”.

“People” problems are an aggravation but hopefully they can be solved by real facts. Big problems? Yes, there are a few. The biggest one is the Hawker Siddley 125. There are 200 of these aircraft in the U.S.A. (through the 600 Series) that have filler necks that are about 1/4” too small for the oval shaped spout to enter. Obviously, these executive jets must be refueled somehow but FBO’s resisted installing the new spout “just in case” a HS125 landed at their airport. This problem was solved by British Aerospace (the manufacturer) when they developed an adapter spout for HS125 operators. The crew simply hands this adapter to the refueler who slips it on to his oval spout.

But what is to be done about the helicopters that have been found to have small filler necks? These are the Wessex, Boelkows, Puma, Dauphin, Gazelle, Twin Star and the extended range mods for the Hiller OV-12 and the Bell Jet Ranger. Eventually, it is hoped they will all be modified with large filler necks. There is an adapter available from Fjord Aviation Fueling Products. Clearly, this is not a solution to the misfueling problem because someone could use the adapter and put jet fuel in an avgas airplane. However, the adapter was purposely planned to be awkward, to encourage operators to remove it unless it is absolutely needed to refuel one of the listed aircraft.

The end of the story is liability!!!

Regardless of aggravations and inefficiencies that can be blamed on the oval spout, do you think that a misfueling law suit can be won by an FBO because the new spout was not installed? The reason “it would not fit in 200 executive jets and helicopters” that represent possibly 1 percent of the jet fleet in the USA would mean nothing to a jury.

All is not lost! Think of it this way. Any jet fuel nozzle that is equipped with the oval spout will not be capable of putting jet fuel in 74% of the avgas aircraft fleet. That is a real accomplishment!

In conclusion, we feel that the new spouts simply must be used regardless of aggravations they create. People who refuel aircraft do make mistakes, like all other people in this world. No FBO can afford not to install the new spouts and no owner of an aircraft with a large filler opening can afford not to have the restrictors installed.

This is a totally revised issue of GamGram No. 28. Rewriting it became essential because during the 12 years since the original issue was published many changes have occurred.

The basic problem has been that the industry requires more exacting information, better test reproducibility and data that relate more closely to the filter separators that are being used in aviation today. This has resulted in the development of better test procedures and refined testing apparatus. The original title was “How to Measure WSIM.”

Most jet fuel supply systems include a piece of equipment known as a filter separator. Monitor filters are also used but they are not the subject of this GamGram. Unfortunately, very few people know the conditions that must exist for the filter separator to do its water removal job properly. If the water could always be expected to lie in the bottom of a tank with the fuel on top, it would be a simple job to drain it away, and most operators do that regularly. But fuel with water in it that goes through a centrifugal pump becomes an emulsion of literally millions of tiny water drops that do not settle to the bottom of the tank for long periods of time. It is this emulsion that the coalescer elements in the filter separator must deal with. They must gather the tiny water drops together so that they will become large drops (coalesce) that can rapidly settle to the bottom and be drained away.

The enemy of a coalescer element is “surfactant” or surface active agents that prevent water drops from gathering together into large drops. They are chemical molecules that seek and influence a surface. The particular surface they “like” is the surface of a water drop in the fuel. The reason they like a water surface is because they have 2 “heads”. One head likes fuel; the other head likes water. So, if the fuel contains surfactants and if water is present, those 2- headed molecules “zoom” to the surface of the water drop just like bees go for honey. The fuel “heads” orient themselves to stay in the fuel and the water heads are captured by the water. Ultimately, the entire water drop is surrounded by a surfactant film making it impossible for 2 water drops to coalesce together because they cannot come into contact.

In the early days of jet fuel handling, it became obvious that a test was necessary to find out if a batch of fuel was contaminated with surfactants to an extent that coalescer performance would be jeopardized. The Water Separometer Index (WSI) test was developed and after modification it became the WSIM test (pronounced “wiz-um”). A reading of 100 was excellent, meaning that coalescers would perform very well. If the reading was as low as 70 the fuel was considered very poor. Extremely contaminated fuel could be “zero”.

The modern instrument that measures the surfactant contamination of the fuel is currently called the Micro-Separometer®. It is a highly refined version of the original equipment. The reading is still 100 for the best fuel but instead of referring to it as the WSIM rating, it is called MSEP (pronounced Em-sep).

Both the WSIM and the MSEP equipment are based on the same idea; an emulsion of water and the fuel sample is forced through a pad of fiberglass coalescing media. An optical device measures the haze in the effluent. The less haze detected, the higher the rating and vice versa. While precision (repeatability) has never been very good for either test, MSEP has proven to be superior to WSIM. Another big problem has been that the test over-reacts to Stadis 450, the additive that improves fuel conductivity. In other words, a low MSEP fuel may perform quite adequately in a real coalescing performance test. Considerable pressure from users has influenced great effort to overcome these problems.

Possibly the most important variable that has been investigated has been the replacement of fiberglass with the same coalescing media that is used in manufacturing modern coalescer elements that have passed the tests that are specified in API 1581, Revision 3. The new material looks somewhat like heavy paper; it contains very, very fine glass fibers. Fiberglass insulation is such an inconsistent material that coalescer manufacturers were forced to find better media several years ago. The device that holds the fiberglass pads in the current version of the Micro Separometer is an aluminum capsule called the Alumicel®. So what we are saying is that in the future new Alumicels are expected to contain a paper-like coalescing material instead of fiberglass. Meanwhile, the currently available Alumicels must be considered valid. As of June 1996, encouraging test results show that the instrument itself will probably not have to be revised. This is very good news for owners of the model known as Mark V Deluxe.

This review of tests that attempt to determine the effect of surfactants on jet fuel would not be complete without a comment on the technical property that is involved. That property is “interfacial tension”, and in our business it means “strength of the interface between the fuel and water.” If the film of molecules at the interface is strong, large water drops can exist. As the interfacial film decreases in strength, the smaller the water drops will be until the mixture of water and fuel becomes an emulsion. The measurement of interface strength is performed in the laboratory by a delicate instrument called a “tensiometer”. It is definitely not a field instrument but a kit has recently entered the market that performs this measurement in the field. It is called “SWIFT KIT” and is marketed by Velcon Filters, Inc. This kit is particularly useful in checking the performance of clay treaters that are used in our industry to capture and remove surfactants that cause the interfacial tension to decrease; clay adsorbs the surfactant molecules as described in GamGram No. 14. Therefore, by checking the interfacial tension (IFT) before and after the fuel has passed through the clay, the operator can quickly assess the performance of the clay. This can also be determined with a Micro-Separometer but that is a more time consuming and expensive test.

In conclusion, the Micro-Separometer has proven to be the most reliable instrument for evaluating the ability of a fuel sample to have its water removed by a filter separator. A program is currently underway to improve repeatability, and we will further revise this GamGram to reflect the results of that investigation when it has been completed.