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Hardline Cable

Question

I have been tasked with installing a model 6237M70 high temperature accelerometer into a gas turbine engine. I cannot drill any new holes in the turbine; therefore I need to utilize an existing hole. Unfortunately, there are no existing holes available large enough to pass the sensor's head through. Can I cut the sensor's integral hardline cable and then re-weld the cable back together once the sensor head is installed? We have welding equipment here and are very experienced in welding.

Answer

No, Meggitt Sensing Systems does not recommend you cut the hardline cable on the model 6237M70, or the hardline cable on any high temperature accelerometer, for that matter. Despite your advanced welding capabilities, highly specialized techniques and tooling are required such as those developed at Meggitt Sensing Systems are used to manufacture this accelerometer. Cutting the cable and attempting to re-weld it may result in a seriously compromised sensor, with significant negative impacts to both short-term performance and long-term reliability.

Why is this?

The hardline cable is hermetically sealed from the connector end to the transition into the sensor head (The sensor head itself is not hermetically sealed. Full discussion of why is outside the scope of this article). The hardline cable must remain hermetically sealed in order to perform its function as expected. Any breach in this seal will compromise the function of the cable if not repaired under proper conditions.

Hardline cable is constructed from a small diameter metal tube. The conductors inside are supported in the tube by a tightly packed powder made from magnesium oxide (MgO). MgO is hygroscopic and it will readily absorb moisture from the air rapidly.

If there is a breach in the cable that exposes the MgO powder to moisture, the MgO will begin to absorb that moisture immediately. In a matter of a few minutes, enough moisture can be absorbed to cause the insulation resistance (IR) between conductors in the cable to drop to the point where the cable can no longer reliably perform its function.

For a piezoelectric accelerometer to function correctly and reliably, the insulation resistance between signal conductors from the piezoelectric sensor crystal, all the way to the charge amplifier, must be maintained at a very high value. Cutting the cable would no doubt allow a large amount of moisture to be absorbed, seriously compromising the sensor.

How does Meggitt Sensing Systems avoid this problem while manufacturing the sensor?

Before assembly, the hardline cable is heated to drive out the moisture. Insulation resistance is tested to make sure it is at the level required. After heating, the cable is kept in a dry, controlled environment to maintain the high insulation resistance. While the cable is being installed and welded to the sensor head, the entire assembly is maintained in this dry, controlled environment to prevent any possibility of moisture being absorbed.

Insulation resistance is constantly tested to make sure it is maintained at the required levels. It is doubtful these methods of assembly could be duplicated in a user environment such as yours.