Salt and sea water blocking for ‘totally dry’ terrestrial optical cables

Over the last decade many optical cable producers have promoted the use of so called dry blocked cables for outdoor use. These cable designs bypass oils, gels, and/or other flooding compounds and instead use super absorbent powder (SAP) infused into thread or other cable elements to protect against intrusion of water. In the presence of water, SAP expands (see Figure 1), filling possible water pathways and preventing water intrusion. Such dry blocking technology is often used under the cable jacket, under metal armor, around buffer tubes, and even inside buffer tubes in direct contact with optical fibers. Cables that totally eliminate gels or glues in favor of dry blocking technology are sometimes referred to as “totally dry” fiber-optic cables.

SAP technology has several advantages, but one major drawback: In the presence of salt typical commercially available SAP chemistries suffer greatly reduced water blocking performance. For this reason a number of network operators have opted not to deploy such cables, or to only deploy dry cables in applications where there is no chance of contact with salt/sea water. However, a new way of measuring the effectiveness of dry cable technology may provide better insight into how SAP-enabled dry cables perform in the presence of salt water and open the door to spreading the benefits of dry cables to more applications.SAP basics SAP is typically infused by different means onto polyester, aramid, or fiberglass threads, yarns, or tapes that are then used in the basic construction of the optical cable. This type of dual-purpose use can combine water-blocking technology with cable strength materials, aiding in cable design and reducing cable weight and cost. The primary advantage of dry cables versus traditional gel-filled cables is the reduced time and expense of cable cleanup during preparation for connection and/or fiber splicing. The use of dry cables avoids the labor necessary to remove gel and oil from each fiber in a buffer tube, or from optical ribbons in the case of ribbon-style cables. In one published study a 432-fiber dry cable preparation saved around 1 hour per cable end.¹ The study considered that over a 20-km route using ten 2-km-length 432-fiber cables, a total of 20 preparation ends would be required. The study concluded the installation crew would save 23 crew hours for the route. According to an estimate from the FTTH Council, labor is the single most expensive element of FTTH deployment, comprising 46% of the total installation cost.² So any labor time savings can offer a significant benefit to those paying for cable installation. Secondary advantages of dry cables can include:

• Elimination of the need to purchase solvents and wipes to remove gel and oil
• Tools, cabinets, and closures stay cleaner without exposure to gels and oils
• Dry cables are in general lighter in weight than gel-filled cables. This has the added advantages of making the cables easier to handle and adds less load onto aerial infrastructure.

Too much salt
Despite these advantages, the problems SAP encounters with salt water has limited the use of dry cables. An informal poll in Q2 2011 of three major U.S. fiber-optic cable companies revealed that perhaps 40% of the U.S. optical cable market comprises dry cable, while about 60% of customers still purchase gel-filled cables.

SAP’s salt problems don’t end with absolute water absorption. The speed with which the SAP absorbs water is greatly reduced as well. This latter point is also very important in that if SAP reacts too slowly, water may penetrate far inside a cable before absorption occurs. SAP technologies have an inherent “delay” in absorbing once initial contact with water is made. So, it is important to specify not only how much water a dry technology can absorb, but perhaps more importantly how quickly the technology reacts to block water.

Table 1: Absorption speed (gr/gr/min) and absolute absorption (gr/gr) of coated aramid fibers from three major vendors of current SAP technology.

	Average Absorption Speed in De-ionized Water	Average Absorption Speed in Sea Water	Average Absolute Absorption in De-ionized Water	Average Absolute Absorption in Seawater
Supplier A	6	2	50	20
Supplier B	15	2	75	40
Supplier C	30	5	100	60

The table above represents an average of many samples from the same size (denier) water-blocking aramid threads from three leading vendors of water-blocked thread commercially sold in the U.S. for use in dry-blocking fiber-optic cables. Seawater was generated by use of commercially available seawater kits such as those sold for salt water fish aquariums. By definition these kits incorporate salt (and other ingredients to lesser extent) with approximately 3.4% incorporation into water. Target salinity is 27 to 35 parts per thousand.

While space does not allow a complete description of the test procedures (you can contact the author for these), here is an overview:

1. To measure absorption speed a 1-m length of thread is weighed dry. Then, the thread is placed in a water solution for 1 minute. The thread is removed and again weighed. Subtracting the beginning and ending weights yields the amount of water absorbed, equating to gram per gram per minute (gr/gr/min) absorption speed.
2. A similar method is used to calculate absolute absorption, simply with a long time period (long enough for the threads to reach a point at which they no longer absorb).

Parameters for successful sea/salt water performance
It can be seen from the test data in the table that even the best (fastest absorbing) product on the market exhibits greatly reduced absorption speed in seawater versus de-ionized water. This particular product exhibited borderline water penetration performance when tested in actual optical cable (GR20 FOTP-82 one meter water penetration test). That is especially interesting considering Supplier C’s absolute absorption in seawater is higher than Supplier A’s absorption in de-ionized water, again highlighting the importance of absorption speed as a measure of blocking seawater.

Supplier A on the other hand successfully sells this product on the U.S. and global markets for dry blocking deionized water in fiber-optic cable. So an average of 6 gr/gr/min absorption speed is assumed as a minimum level that would be required for producing a quality seawater blocking product.

With this in mind a suggested target value for average absorption speed in seawater might be something above 6 gr/gr/min. Owing to the borderline seawater blocking performance in cable at 5 gr/gr/min absorption speed, one might consider a value of 7 to 8 gr/gr/min to give some margin.

While absolute absorption is less obvious to specify a minimum acceptable performance level, certainly something above 50 to 60 gr/gr is needed based on current industry minimum performance levels. A figure such as 70 to 80 gr/gr might be considered as a minimum acceptable average absorption.

Summary
Knowing what performance levels (absorption and absorption speed) are currently considered successful when blocking de-ionized water gives an indication of what the targets should be for blocking sea/salt water in dry optical cables. With these values understood a cable can then be produced to successfully block sea/salt water and then deployed anywhere a gel-filled cable is currently used.

Ryan Chappell manages global sales, fiber optic products at Coats North America, a division of Coats plc. He can be reached at [email protected].

References:
1. National Fiber Optics Engineers Conference 2003, Peter A. Weimann, Richard H. Norris, Howard M. Kemp, Richard G. Gravely, Swati Neogi, “A New Family of Totally Dry Outside Plant Cables,” pp. 3 (2003).
2. FTTH Council, www.ftthcouncil.org, “Fiber 101 - Deployment Cost Models."