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  • Helium Measurement

    Hi,

    I have a customer who is trying to find out how much helium he is putting into a telephone cable in order to locate a leak in the cable's protective sheath. His measurement device is a portable flow rater, which is simple tool designed to measure air flow rates between 0 and 20 scfh. His helium tank input pressure into the flow rater is set to approximately 10 psi.

    He can read air flow pretty accurately with the device, but is there an offset or multiplier that he would need to apply at different psi outputs to determine the flow rate of the helium?

    I'd appreciate any help you could provide with this. Thanks.

  • #2
    Re: Helium Measurement

    Originally posted by PHWebber View Post
    Hi,

    I have a customer who is trying to find out how much helium he is putting into a telephone cable in order to locate a leak in the cable's protective sheath. His measurement device is a portable flow rater, which is simple tool designed to measure air flow rates between 0 and 20 scfh. His helium tank input pressure into the flow rater is set to approximately 10 psi.

    He can read air flow pretty accurately with the device, but is there an offset or multiplier that he would need to apply at different psi outputs to determine the flow rate of the helium?

    I'd appreciate any help you could provide with this. Thanks.
    Like this one?
    http://phonetx.com/25627500.html

    - I couldn't find how it works, sorry maybe someone else has seen them in action? - Why are they using helium instead of air?

    Comment


    • #3
      Re: Helium Measurement

      Originally posted by Mrs X View Post
      Like this one?
      http://phonetx.com/25627500.html

      - I couldn't find how it works, sorry maybe someone else has seen them in action? - Why are they using helium instead of air?
      Helium leaks well, and it is easy to detect around the leak. Detecting air in air is hard except for gross leaks where you hear hissing. Helium testing is even more sensitive than testing for air leaks underwater, using the bubbles.

      But I don't understand the request. You have to flood the volume of the cable with helium to a specified pressure, and then detect leaks with a helium sniffer. If the leak is gross enough to to require continuous flow in cubic feet per hour, use air, and either detect hissing or bubbles.

      Comment


      • #4
        Re: Helium Measurement

        Here's the deal -- my customer is trying to locate an air leak in a section of pressurized telephone cable. Telephone companies force 10 psi of dry air into their direct-buried and underground cables (the ones that have copper conductor pairs, that is) as a means of keeping moisture out. Water causes electrical shorts, cross talk, and other conductor faults that screw up communications.

        So, this pressurized air is supplied continually to the cables from large air compressor/dehydrators. As long as there is adequate air pressure inside a telephone cable (0.5 psi for every foot of water above the cable), the positive pressure will keep the conductor insulation dry.

        In order to find an air leak in a section of air pressurized cable, some phone techs inject helium into the cable and search for the escaping helium with a gas detector. My client wants to know the flow rate in SCFH of the helium that he is injecting into the cable. Problem is, he's using a tool calibrated to measure air flow rates in standard cubic feet per hour at 10 psi, not helium flow rates at 10 psi.

        Is there a conversion factor or formula he could use with his air flow tool to find out the flow rate of helium into the cable?

        Thanks for your help.

        Comment


        • #5
          Re: Helium Measurement

          Originally posted by PHWebber View Post
          Here's the deal -- my customer is trying to locate an air leak in a section of pressurized telephone cable. Telephone companies force 10 psi of dry air into their direct-buried and underground cables (the ones that have copper conductor pairs, that is) as a means of keeping moisture out. Water causes electrical shorts, cross talk, and other conductor faults that screw up communications.

          So, this pressurized air is supplied continually to the cables from large air compressor/dehydrators. As long as there is adequate air pressure inside a telephone cable (0.5 psi for every foot of water above the cable), the positive pressure will keep the conductor insulation dry.

          In order to find an air leak in a section of air pressurized cable, some phone techs inject helium into the cable and search for the escaping helium with a gas detector. My client wants to know the flow rate in SCFH of the helium that he is injecting into the cable. Problem is, he's using a tool calibrated to measure air flow rates in standard cubic feet per hour at 10 psi, not helium flow rates at 10 psi.

          Is there a conversion factor or formula he could use with his air flow tool to find out the flow rate of helium into the cable?

          Thanks for your help.
          It would depend on the sensing mechanism. If it is proportional to density, obviously Helium is much lighter, in the ratio 4/29. However, I don't know if the sensing mechanism would be proportional to density. It would depend on the model and how it senses. I don't think there would be a universal correction.

          Helium is expensive, and often a mix of air and helium is used as the medium. You still use a helium sniffer, and because of the air, the sensitivity is reduced. But it sounds like he has a gross leak. If you used 10% helium, 90% air, the sensitivity of the flow meter would not be much affected.

          Comment


          • #6
            Re: Helium Measurement

            The flow-rater instrument is lacking at best even for dry air flow measurement as the balls in the unit become worn over use. Every telephone cable has a unique resistance to air flow based upon the manufacturing and many sections of cable in the same electrically connected cable run may be of different types.

            Knowing what flow is leaving the tank is helpful, but without succinct cable makeup records, it may not be useful in determining the time required for helium to get to point B. EXAMPLE: If the first 1000' section of cable is pulp insulated 3600 pr. cable, followed by pic insulated cable; the helium flow will be slow regardless of pressure because the pulp resistance to airflow is much greater than PIC. When helium reaches the pic cable sections the movement in it is much faster (at a lower pressure.) But you won't know that if you are testing from the tank.

            Two companies make gauges and in-line apparatus to test pressure differential air flow. As helium is a lighter gas, using these tools requires a volume doubling to be on the safe side.

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