It's 2 a.m., and the phone is ringing. The computerized voice on the other end announces, "Reflected power is above the upper limit." Still a little groggy, you start to turn on the deicer but then remember it's the middle of July. A trip to the transmitter reveals that the transmission line pressure is zero and, suspecting that water has accumulated at the base of the line, you drill a tiny hole at the lowest point of the line and water sprays in your face.
Theoretically, transmission lines are completely sealed but that's never true in practice. As every transmission line warms in the sun, internal pressure tends to build in the line causing air to escape. No problem so far. However, as the line cools in the night the internal line pressure can drop below the pressure outside the line causing humid air to force its way in. This cycle is often referred to as line-breathing. Weeks of line-breathing will completely replace the air inside the transmission line with night air. The big problem comes when the temperature of the line itself drops below the dew point of the moist air trapped inside the line, which is when condensation forms. This condensation runs down the transmission line and collects at the lowest point of the line. By pressurizing the line to 6 psi (or so) with dry air or gas, the pressure is always higher inside the line than out so moist air cannot seep in allowing condensation to form.
So what's the best way to pressurize a line? Although they can provide a virtually never-ending supply of dry air, I'm not a big fan of dehydrators. The cycles in pressure they apply to the transmission line inevitably work the seals to the point that the line eventually needs a never-ending supply of air. Yes, now you know, I'm a nitrogen guy. There was a time that we were warned that nitrogen would harm our lines and we should only use dehydrators (but how dehydrators manage to only dry and compress the 21 percent of air that's not nitrogen is still a mystery to me.) Maybe I've just been lucky, but I've never had any ill effects from pressurizing my lines with pure, cheap, reliable nitrogen.
Williams' double regulator in use. The compressor switch, pressure transducer and low-pressure regulator are mounted on a board. A relief valve is installed to protect the transducer in case of excessive pressure.
When using nitrogen to pressurize a line, I've found that it's not sufficient to simply monitor the line pressure; it's also necessary to have an advance warning system in place for when the nitrogen bottle is approaching empty. I set my bottle regulator to provide an output of 100 psi, which I feed into both a second, low-pressure regulator and a standard compressor switch. The compressor switch is set so that when the bottle pressure drops below 80 psi the switch closes allowing the remote control to contact me. Because the low-pressure regulator is set to an output of 6 psi, the line pressurization is not affected until the bottle pressure drops another 74 psi -- which might give you several days or several weeks to get a new bottle in place depending on how leaky your line is. You will also find that this two-stage regulation system is capable of maintaining the line pressure quite accurately since it's not trying to regulate the bottle pressure from 1,000+ psi to 6 psi in just a single stage.
Glenn Williams is the chief engineer of KLJC-FM in Kansas City.