CASE AND BRACELET
1. Frequently, chronographs have scales printed on their dials or feature a bezel with a decal called “Tachymeter” and/or “Telemeter”. Explain the use of these two scales. A tachymeter bezel is a fixed bezel with a non-linear scale that measures the occurrence of an event over an elapsed time. Most commonly this is used for measuring average speed per mile when driving or rate of units of production.
A telemeter bezel is fixed bezel graduated in distance (Kilometers or miles). It is used to calculate the distance of an occurrence based on the sound it emits. This was used in many military watches. A soldier could hear gunfire and see where it was happening then use their watch to calculate the distance and give a report based on the findings. It can also be used to calculate how far away lightning is based on the difference between the flash and the sound of thunder.
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2. Name three types of crystal materials used. List the advantages and the disadvantages of each. Acrylic crystal are flexible, cheap, easy to manufacture, easily polished, shatter resistant and don’t require a gasket. The downside is they scratch very easily.
Mineral glass is hard, cheap and easy to manufacture. The downside is they can shatter and they can scratch but can’t be polished.
Sapphire is very hard and nearly scratch proof. The downside is they are expensive, hard to manufacture and the can shatter.
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3. What are the principal differences between a dry and a wet waterproof tester? Elaborate on the advantages and disadvantages of the two principles. Which one is better suited for diagnostic purposes?
Dry testers simulate water pressure using air in a dry environment. Wet testers apply air pressure to a watch followed by the pressure in the chamber being released while the watch is submerged in water. Dry testers are safer for testing because if the watch fails no moisture will get in the watch. The wet tester shows the user where the leak is so it is better for diagnostic purposes.
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4. Describe the purpose and the working principle of a helium relief valve. How does one pressure test a watch that is equipped with a helium relief valve? Why?
A helium relief valve or helium escape valve, allows helium that has built up in the case of the watch to escape when ascending from a dive as the air pressure drops. It is a one way automatic valve that is pushed out as the air pressure decreases. It is spring loaded so it will be pulled back into the case under the spring’s tension.
5. Describe the forces acting upon a watch during the process of running a dry water resistance test.
Dry tests operate by sealing the watch in a very thick dome or tube. A small sensor is applied to the crystal of the watch that will detect minute deformations in the case as pressure is supplied via a manual crank or an air compressor to simulate the pressure present at a corresponding depth. As the chamber fills with air the case will steadily deform. If the deformation remains constant as the chamber fills and then stops once pressure is reached and remains in that shape for the duration of the test the watch will be considered water resistant. If a leak is present, at the point air enters the case, the case will begin to expand instead of contract as the case inflates. At this point the pressure tester will automatically stop and tell the user it has failed the test.
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6. Why do you apply special lubricants like Fomblin or Silicon 7 on case backs or crowns? Why not leave these areas dry? Why not use regular grease?
Grease allows smooth operation of parts where a tight rubber on metal friction would otherwise cause parts to bind. In addition, these greases act as a lubricant for threads so they slide smoothly over one another and prevent galling or cross threading. These greases also prevent water from penetrating the case by acting as a hydrophobic barrier. Lastly, silicon or fomblin protects the rubber seals from deteriorating due to age and exposure to various elements.
The reason you would use these greases vs some other grease is that they are specially formulated to operate under a wide variety of temperatures and pressures without being displaced by water.
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7. List all the areas water resistance may be compromised in a typical 3 piece case. Describe the assembly of a 3 piece case, detailing all concerns and products used.
Crystal, case back, tube and crowns and pushers.
The crystal is pressed into the case with a gasket as a friction joint unless it is glued in. The case back is threaded or pressed into the back of the case with a gasket acting as a water seal. If the caseback is threaded it should have silicon 7 or Fomblin grease on the threads. The case tube is frictioned or threaded into the case with Loctite acting as a sealant. The crown is either friction fit or threaded onto the tube with a rubber gasket acting as a seal. . If the crown is threaded it should have silicon 7 or Fomblin grease on the threads. All rubber gaskets should also have silicon 7 or Fomblin on them.
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8. For a stainless steel case with threaded case back and screw down crown, describe in detail where you apply thread lockers and lubricants. What kind of thread lockers? What kind of lubricants? Why?
High strength grade (~260) thread locker should be applied to the tube where it meets the case and the crown where it threads onto the stem. Silicon 7 or Fomblin should be applied to the case back threads and gaskets and the crown threads and gaskets.
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9. The diagram of this water-resistant crown is a standard design for water resistant watch cases. Name each component indicated by the arrow, and ways in which this component can fail.
A. Case tube loose, bent, cracked, wrong size
B. Case Improperly sealed, loose tube.
C. Crown gasket cracked, flattened, cut, not greased, wrong size
D. Stem Broken, bent, too long
E. Crown Cracked, bent, wrong size
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11. What parameters do you need to observe when replacing a crown for a water resistant watch?
The internal diameter of the crown in relation to the external diameter of the tube, the height of the crown in relation to the tube, the size of the crown gasket and how snugly it fits against the tube.
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