Multi-conductor cables are one of the most common types of electrical cables used today. They are comprised of several insulated wires all bundled together with an exterior insulated jacket. Multi-conductor cables usually include shielding layers, drain wires, braided sleeves, and even filler material to maintain a uniform cross-section. One problem that exists with these types of cables is that since they are so tightly packed together, air cannot easily travel along the inside length of the cable.
For most applications, this isn’t a bad thing, And, it’s preferred since most cables must be waterproof. However, there are instances where engineers and designers need the multi-conductor cable to allow for a small amount of air to pass, equalizing pressure between the two ends of the cable and wherever the cables are installed.
This requirement is usually reserved for applications exposed to cyclic heating and cooling, exposure to direct sunlight, or products placed outdoors for extended periods. The question remains: “Why does the pressure change with a sudden drop in temperature?”
The Ideal Gas Law – PV = nRT
We all learned about the ideal gas law in high-school physics class which states that the pressure, volume, and temperature of an ideal gas are all related. And if you didn’t learn about The Ideal Gas law in high school or college, maybe you remember this from “Deflategate” when New England Patriots quarterback Tom Brady was falsely accused of tampering with footballs. Assuming air behaves as an ideal gas, this law states that if a volume of air the size of a football is pressurized to 14 PSI at an ambient temperature of 78°F, and if the volume of air is rapidly cooled to 39°F, then the pressure will drop to about 13 PSI. This will occur according to the laws of physics and not because anyone lets the air out of the vessel, simply reducing the temperature of an enclosed volume of air will change its air pressure.
The same is true if the volume of air is heated. This relationship is linear and proportional to the amount of temperature change. Again, this equation assumes that there is no change in the “amount” of air within the vessel- meaning the number of molecules remains constant.
This calculation holds true in the ideal gas law equation where:
PV = nRT
- P = Pressure
- V = Volume
- n = Number of molecules, expressed in relation to Avogadro’s number
- R = Universal Gas Constant [8.31 J/mol ⋅ K]
- T = Temperature
This equation can be further simplified if the sealed volume of air remains closed with no changes to the size of the enclosure. Thus, V, n, and R are all constant. The equation then states the ratio of pressure and temperature will always remain the same. Accordingly, pressure and temperature are related as long as the volume is unchanged and the number of moles of air is also unchanged:
Ideal gas relationship of pressure versus temperature with constant volume.
Why is this Important?
For applications that require a waterproof cable to be routed into an IP rated enclosure, there are concerns that changes in temperature over time could create a pressure gradient which could negatively impact how the system works, or worse, damage parts of the system. This is especially true for products such as pressure transducers, electronics with diaphragms, elastomeric keypads, and other types of seals. As the temperature inside the enclosure increases, the housing can balloon out which could damage the housing or sensitive components. Aside from the mechanical stress created from the pressure differential, there is also concern that over time water and moisture could be drawn into the enclosure with continued fluctuations in temperature.
For designs with pressure differential concerns that cannot be solved with the addition of a waterproof vent (i.e. gore vent), adding a small vent tube to a multi-conductor cable is a worthwhile option for designers to safeguard against pressure variations from a sudden change in temperature.
Example of a vent tube within a multi-conductor cable.
Considerations Before Adding a Vent Tube to Your Multi-Conductor Cable
The first step in considering if a vent tube should be added to your multi-conductor cable is to determine if the outer diameter (OD) of your required cable can be increased. If there is space to use a larger diameter cable, then there is hope that this is a viable solution.
Adding a vent tube will increase the rigidity of the bulk cable making it stiffer. If a highly flexible cable is needed, it may not be possible to add the internal tubing necessary without jeopardizing the overall flexibility. Engineers and designers must be cautious on specifying too small of a minimum bend radius since if the vented cable is bent too sharply, the air tube could kink and become damaged blocking the flow of air.
Another consideration to keep in mind when using these style of vent tubes is head loss. Also referred to as a pressure drop due to friction, air behaves like a fluid in these systems and is subject to the same pressure drop as one would see in household plumbing. As air moves along the inner wall of the tube, friction slows the flow and creates a pressure drop that is a function of the diameter, length of the tube, and smoothness of the material. If the tube is too small, or the length of the tube is too long, airflow can become constricted, and the desired venting behavior may not meet expectations.
Material Construction
Tubing is made from countless material types, but vent tubes for multi-conductor cables are typically made from extruded thermoplastics. Semi-rigid polyethylene and TPE are among the most common materials, with some options having a durometer hardness of about 90 Shore A. The wall thickness of the tube itself must be thin enough to be flexible, yet thick enough to be structurally sound preventing it from collapsing in on itself. For these reasons, the wall thickness is recommended to be at least 0.020”.
Flammability requirements are another concern since using the wrong material type could violate the UL flame rating. Thermoplastics such as polyethylene and TPE can be considered as long as they can meet the required VW-1.
Shielding can be added to the multi-conductor cable as a metalized foil shield or as a tinned copper overbraid. Including drain wires is another option for applications that need to terminate the shield to a ground pin.
Example of a vent tube cross section.
Summary
Using a multi-conductor cable with an embedded vent tube is a worthwhile option to consider to solve venting-related design challenges. There are several ways to reduce the effects of pressure differentials on sensitive electronics. especially within sealed enclosures, but the inclusion of a vent tube within a multi-conductor cable is not for every project. For the applications where this does make sense, it remains a low-cost and highly reliable option for engineers and designers to utilize. Better yet, these multi-conductor cables can be completely customized with a complete range of wire jacket materials, colors, sizes, and shielding methodologies.
Furthermore, many of these wire cross sections are UL-rated enabling their use on countless applications across numerous industries. As with any custom product, early engagement with a full-service supplier like Epec is the best way to reduce risk and keep costs down.
