Time Query

Discussion in 'Diamond Lil's' started by 21_Man, Apr 24, 2008.

Welcome to the Navy Net aka Rum Ration

The UK's largest and busiest UNofficial RN website.

The heart of the site is the forum area, including:

  1. Anyone know what this type stopwatch is/was used for?

    Inner scale 0 - 6 seconds

    Outer scale 0 - 5000 yards

    Wind from top

    It has the following NS No. 0550/6645 99-520-9365 Max Run- 1 Hour 0128/96

    Original value would be interesting to know

    Many thanks
  2. Sounds like the type of stop watch used by the dabbers in the sound room for counting shaft rpm.
    I.e count for 6 seconds then multiply by 10 to get RPM.. Dunno what the distance bit is used for, maybe a UC will be able to help you more.
  3. sgtpepperband

    sgtpepperband War Hero Moderator Book Reviewer

    It's a chronoscope (as demonstrated here, used to measure small increments in time. As JS mentioned above, ASW personnel use it to measure blade rates and other acoustic data using the inner scale, and also ranges of contacts ("Mark, Snap, Mark..." method) using the outer scale.
  4. It's shown in Annex D of DEFSTAN 66-46


    But with no clues to use.

    Aligning with what jollysailor and sgtpepperband, the distance per second equates to half the speed of sound in sea water.

    Aunty Betty paid over £100 for it but less than £200.
  5. Thanks guys for info so far. Bought it out of interest in watches and cost was £40....from Ebay. It's quite 'young' to be on the market, I thought

    Any of you Submariners able to tune in the 'use of'?
  6. sgtpepperband

    sgtpepperband War Hero Moderator Book Reviewer

    £40?! Shit, you should've said - reckon I've got a couple floating around my 'gizzit' box in the loft... :wink:
  7. Sgt P

    BIG & Black and Purrs louder of course ;)

    The speed this would appear to measure would be in the region of Mach 2.2......given the scaling, so maybe turns on a shaft is out??
  8. sgtpepperband

    sgtpepperband War Hero Moderator Book Reviewer

    21_Man: I could tell you - but I'd have to kill you... :threaten: :wink:

  9. If you "ping" at 0 seconds and hear the "dit" at 6 seconds, the underwater target is 4956 yards away and read directly off the watch.
  10. Many thanks
  11. Good post. I've been sat thinking about this for the last 5 mins and a number of puzzles come to mind.

    I take it the 'rule of thumb' outlined by SPB and Loggie is just that, a 'rule of thumb' because the speed of sound will differ according to the salinity, temp and depth of the water.

    I don't know? I'm trying to work it through. But as I thought about this I began to get confused by the fact that since sound does not travel in vacuum, it being a wave and all that, then sound should travel faster through a more dense medium (ie at greater 'salty', depths) ... but ... some old diving lectures from over 20 yr ago are telling me that sound travels less effectively through dense liquids! One wants to say, 'The greater the density, the greater the speed of sound' but diving lectures from 26 or so years ago are surfacing and telling me that is not right!

    Amazing how a thread can set one off wondering ... BZ to its author, SPB, Loggie and others!
  12. For info

    Sound propagation

    Sonar operation is affected by variations in sound speed, particularly in the vertical plane. Sound travels more slowly in fresh water than in sea water, though the difference in speeds between fresh and salt water is small. In all water sound speed (sometimes called velocity though this is incorrect) is determined by its bulk modulus and mass density. The bulk modulus is affected by temperature, dissolved impurities (usually salinity), and pressure. The density effect is small. The speed of sound (in feet per second) is approximately equal to:

    4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet)) + salinity (in parts-per-thousand ).

    This is an empirically derived approximation equation that is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, for a sound originating on one side of the thermocline tends to be bent, or refracted, through the thermocline. The thermocline may be present in shallower coastal waters. However, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation. Increased pressure increases the sound speed, which causes the sound waves to refract away from the area of higher sound speed. The mathematical model of refraction is called Snell's law.

    Sound waves that are radiated down into the deep ocean bend back up to the surface in great arcs due to the increasing pressure (and hence sound speed) with depth. The ocean must be at least 6000 feet (1850 meters) deep, or the sound waves will echo off the bottom instead of refracting back upwards, and the reflection loss at the bottom reduces performance. Under the right conditions these sound waves will then be focused near the surface and refracted back down and repeat another arc. Each focus at the surface is called a convergence zone (CZ). This CZ forms an annulus about the sonar. The distance and width of the CZ depends on the temperature and salinity of the water. In the North Atlantic, for example, CZs are found approximately every 33 nautical miles (61 km), depending on the season. Sounds that can be heard from only a few miles in a direct line can therefore also be detected hundreds of miles away. With powerful sonars the first, second and third CZ are fairly useful; further out than that the signal is too weak, and thermal conditions are too unstable, reducing the reliability of the signals. The signal is naturally attenuated by distance, but modern sonar systems are very sensitive, i.e. can detect despite low signal-to-noise ratios.

    If the sound source is deep and the conditions are right, propagation may occur in the 'deep sound channel'. This provides extremely low propagation loss to a receiver in the channel. This is because of sound trapping in the channel with no losses at the boundaries. Similar propagation can occur in the 'surface duct' under suitable conditions. However in this case there are reflection losses at the surface.

    In shallow water propagation is generally by repeated reflection at the surface and bottom, where considerable losses can occur.

    Sound propagation is also affected by absorption in the water itself as well as at the surface and bottom. This absorption is frequency dependent, with several different mechanisms in sea water. Thus sonars required to operate over long ranges tend to utilise low frequencies to minimise absorption effects.

    The sea contains many sources of noise that interfere with the desired target echo or signature. The main noise sources are due to waves and shipping. The motion of the receiver through the water can also cause low frequency noise, which is speed dependent.

    From WikiP..........and all this fun for £36 + £4 p&p
  13. So in your own words, Royal .. :w00t: :thumright: :w00t:
  14. As you say:

    This is an empirically derived approximation equation that is reasonably accurate for normal temperatures..

    So, ceteris paribus (all things being equal) eg constant salinity, no noise propogation due to reflection from the seabed, no inverse thermal layers, etc, what is the speed of sound through water?
  15. The reality is that consideing the innacuarcies of stating and stopping the watch using any thing other than the nominal speed of sound in sea water is attributing an unachievable accuracy to the indicated range.

    We used to have 3 of these watches in a wooden holder with a bar over the buttons so that with one press you could start one watch stop the next and reset the third.

    These watches can also be used for ping stealing, a natty way of finding the range of the ship trying to find you with active sonar.

    If you are really bored and want to understand the problems more you could always read @Sound in the Sea@ by Urrick, which is the standard textbook
  16. deleted

  17. In fresh water, sound travels at about 1497 m/s at 25 °C.

    In salt water that is free of air bubbles or suspended sediment, sound travels at about 1500 m/s. The speed of sound in seawater depends on pressure (hence depth), temperature (a change of 1 °C ~ 4 m/s), and salinity (a change of 1‰ ~ 1 m/s), and empirical equations have been derived to accurately calculate sound speed from these variables
    [Google derived info]
  18. Thanks....adds to the info and usage
  19. sgtpepperband

    sgtpepperband War Hero Moderator Book Reviewer

    Not many submarines operating in fresh water... 8O :? :wink:

Share This Page