GB2205960A - Duct surface testing for use in optical fibre transmission lines - Google Patents

Abstract

Apparatus for testing ducts for suitability for use in fibre blowing installations comprises a slider 5 that is projected along a sample of the duct 3 to be tested by a burst of air or gas. Velocity measurements are taken e.g. by two pairs of sensors 11, 12 as the slider decelerates due to frictional resistance in the tube and a friction or resistance value is obtained from the deceleration value determined from the velocity measurements. The slider is chosen to have a mass that exerts a force similar to the typical tension on a fibre package during blown installation, and the sample of duct is held in a curved track to present maximum resistance. A further sensor 13 may be provided to help determine the power required by the burst of air, the aim being for the slider to come to rest between sensors 12 and 13. <IMAGE>

Description

DUCT SURFACE TESTING This invention relates to transmission line installation.

Lightweight transmission line packages can be installed in a duct by a procedure termed 'fibre blowing'. The technique involves blowing compressed gas down a duct in which the transmission line is to be installed and feeding the transmission line package into the duct at the same time so that the package is urged along the duct by the viscous drag of the gas flow. The technique is of particular importance to optical fibre transmission lines which can be damaged by the tension that is produced in pulled installation procedures.

However it has been found that for a given transmission line package there is a considerable variation in the effectiveness of the technique for different duct materials, or even between batches of similar ducting, some ducts being virtually unusable because the fibre package is not conveyed a significant distance while in other ducts the fibre package can be blown for much greater distances.

It was originally considered that the variation in 'blowability' of a duct (that is its ability to sustain successful blown installation of a fibre package) was a function of friction and that poor blowability was caused by the fibre package being retarded by the frictional effect of a rough internal surface on the duct. Various methods of testing the friction of ducting surfaces have been tried but these have not shown the actual difference in blowability between tubes, and have typically required tube lengths of several metres.For example using an elastically restrained slider to measure friction as it is dragged over a surface gives low frictional values (as would be expected) for PTFE, and progressively higher values for high, medium and low density polyethylene, from which it was presumed that PTFE ducting would exhibit good blowability and that the lower the density of polyethylene the less favourable would be the ducting made from that material. However when actually tested with fibre blowing it was found that some grades of HDPE were unsatisfactory, some medium density grades were extremely good and PTFE did not perform the best.

Even once a good material for manufacturing ducting is identified by actual fibre blowing tests differences in the manufacturing process can render some lengths of ducting better than others, and it has been found that nominally similar lengths of duct manufactured from the same material but extruded through different dies can differ to the extent that one length is good and the other unusable. Likewise a die that previously produced good ducts can deteriorate and commence producing bad ducts.

Also the blowability of ducts can change due to ageing, in some instances improving as slip agent migrates to the surface. It is not possible to determine by simple inspection whether a length of duct is good or bad, and so at present ducts are tested by actually blowing a fibre through them while they are wound on drums prior to installation, and unsatisfactory ducting is discarded.

Although the wound configuration of duct differs from an installed configuration which will have fewer bends, the drum wound test blowing results correlate well with actual installation blowing results. However an actual blowing test necessitate blowing fibre through the length of duct that is used in installations, since testing a shorter length eg 100 metres does not indicate whether blowing would be achievable for 200 or 500 metres for example.

Thus each 500 metre length (which is a typical maximum -single blowing length) is tested, or at least one 500 metre length from each production length (typically 2.2km). One big disadvantage of the drum blowing test is the time taken, typically at least 15 minutes to pay out fibre and install it in the duct and a further 15 minutes to remove it.

The present invention is directed towards evaluating ducts for blowability without having to use fibre packages or full duct lengths.

Accordingly the invention provides apparatus for testing ducts comprising a slider adapted for insertion into a sample of duct, means for impelling the slider along the sample of duct and means for determining the retardation of the slider.

The invention is now described by way of example with reference to the accompanying drawing in which: Figure 1 shows an embodiment of the invention, and Figure 2 shows a comparison of test results from the invention with drum blowing tests.

Referring to Figure 1 a curved track 1 is formed on a base plate 2; the track shown is 'U' shaped but other configurations are possible. A sample length of duct 3 that is to be tested is placed in the track and held in place by fixing clips 4. Alternatively a Perspex (trade mark) or other transparent cover may be placed over the track to hold it in place, or the duct may be retained in a moulded, recessed track. At one end of the duct 3 a brass slider 5 or slug is positioned in the duct and a loading chamber 6 to which the duct is clamped provides an easy access loading point for the slider and once closed acts as an extension passage to the duct. A compressed air or gas supply 7 is located adjacent the loaded end of the duct and is controlled by a control unit 8 and solenoid valve 9 to inject a burst of air into the loading chamber passageway via a nozzle 10.The burst of air travels from the loading chamber passageway into the duct and propels the slider 5 ahead of it.

The straight section of track that constitutes one arm of the 'u' is arranged to be longer than the maximum acceleration distance of the slider when launched by the burst of air. After the initial acceleration caused by the burst of compressed air the slider 5 begins to decelerate and at this stage enters the curved part of the track and passes a pair of sensors 11, continues round the bend and then passes a second pair of sensors 12. The slider then continues along the second arm of the u and is finally caught by a stop device 14 from which it is subsequently removed for reloading into the next duct to be tested.

A modification that is preferred to improve sensitivity utilises a fifth sensor 13 located after the second pair of sensors 12. When this fifth sensor is included the aim is for the slider to come to rest after the sensor pair 12 and before the fifth sensor 13. This may be regarded as a check on the power of the burst of air and enables greater consistency. The slider can be recovered after the measurement by injecting further burts of gas into the tube to urge the slider to the end. A calculating means that computes the friction, as explained below, is programmed to discount results or provide a 'retest' signal in instances where the slider fails to reach the sensors 12 or passes beyond the fifth sensor 13.In order to get the slider to come to rest between sensor 12 and the fifth sensor, the flow of gas used to propel the slider is adjusted by varying the duration of the burst, using a constant supply pressure, eg 30psi.

At the first pair of sensors 11 the time tl taken for the slider to pass the known distance dl between the sensors is measured and input to a computer which calculates an average velocity 'u' = dl/t which can be assumed to be the velocity of the slider at the mid point of the sensors 11 (this presumes uniform deceleration of the slider). A similar computation is made to establish an average velocity 'v' at the midpoint of the sensors 12 from their separation d2 and the time taken t2 for the slider 5 to pass between that pair of sensors.Continuing with Newtonian equations of motion (to a first approximation) v2-u2 = 2as and F = ma where s is the known distance between the centres of the pairs of sensors and m is the mass of the slider, enables 'a' the deceleration due to frictional resistance F to be calculated and substituting the approximation for frictional force (F=;mg) gives a = ug or ; = a/g The value of may differ from the value obtained in other friction measuring tests, but that is not of significance to the testing application, what is required is a test that gives consistent results and differentiates between ducts that have good and bad blowability, and this is achieved with the blown slider. It is found that the values obtained from the blown slider test agree well with values predicted by maximum blowing length calculations.

Figure 2 shows a comparision of results of friction coefficients obtained from test measurements on a variety of ducts using the apparatus of Figure 1 (dotted curve) and the time taken to blow through a 500 metre length of duct on a drum. In actual practice it is observed that ducts with a slider u between about 0.5 and 1 are good for blowability. The best ducts have u less than 0.5 and it takes less than 30 minutes to complete a 500 metre blow.

Ducts with a value for u of over 1.5 are generally not usable due to excessive time requirement or complete failure to install fibre along a 500 metre length. Duct smoothness influences the slider firction and blowability, but extremely smooth ducts do not always give the lowest value. It is believed that this is because of the increased area of contact: for example a measurement on PTFE resulted in a slider friction value of 0.7. On the other hand extremely rough ducts do exhibit high friction. However, smoothness is not the only factor that is likely to influence the blowing friction1 and elasticity and intermolecular adhesion are also likely to play a part. It is the combined effect under blowing conditions that the present invention evaluates.

The slider is made of brass as this has been found experimentally to provide consistent results with a suitable spread of measured u for good and bad ducts.

Other materials may be used or the brass slider may be coated with other materials, for example with a layer of foamed packaging similar to that on the outside of a fibre package. During fibre blowing installation the tension on the fibre is of the order of 3g per metre equivalent and so the slider is preferably made of about that mass, preferably between 2 and 10g. It is probable that the resistance to installation is increased at bends because the contact between the duct and fibre is likely to be greater than along relatively straight sections, and for this reason a bent test track is preferred, the radius of the bend preferably being close to the minimum bend radius for the duct (or fibre in the event that this is the larger). It may be convenient to form the test track into a circle rather than the u shape of Figure 1. For personnel safety cut-out devices may be incorporated to prevent operation in the event that the slider stop device 14 on loading chamber 6 is open.

The apparatus of the invention may be used to grade ducts by the sensor outputs being input to a computer that provides a friction or retardation value. In some instances the actual value need not be provided, merely an indication of whether the duct lies within usable ranges, eg the measured slider friction lies below unity or the retardation between 1 and 10 my'2.

Claims (15)

1. Apparatus for testing ducts comprising a slider adapted for insertion into a sample of duct, means for impelling the slider along the sample of duct and means for determining the retardation of the slider.

2. Apparatus according to claim 1 in which the means for impelling comprises a burst of compressed gas.

3. Apparatus according to claim 1 or claim 2 in which the means for determining the retardation comprises sensors located to measure the velocity of the slider at two instances during its retardation.

4. Apparatus according to any preceding claim including a track for locating the sample of duct.

5. Apparatus according to any preceding claim in which the sample of duct is held in a bent configuration and the retardation of the slider is measured as it passes round a bent portion.

6. Apparatus according to any preceding claim in which the slider comprises a brass slug.

7. Apparatus according to any preceding claim in which the slider has a mass of the order of 2 to 10 grams.

8. Apparatus according to any preceding claim in which the means for determining the retardation comprises means for indicating whether the retardation falls below a predetermined value.

9. Apparatus according to claim 8 in which the predetermined value is around 10 my~2.

10. Apparatus according to any preceding claim including means for determining the final slider rest position.

11. Apparatus according to claim 10 in which the means for determining the final rest position comprises at least one further sensor disposed after the means for determining retardation of the slider.

12. Apparatus according to claim 11, further comprising means for indicating whether the slider has come to rest in the zone between the means for determining retardation and the further sensor.

13. Apparatus according to claim 12 in which the means for indicating comprises means for discounting measurements when the slider does not come to rest in said zone.

14. Apparatus according to any one of claims 10 to 13, wherein the means for impelling the slider is variable to ensure that the slider comes to rest before a predetermined location.

15. Apparatus substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.