Under Pressure: Nitrogen, Watch Out
and Cindy Cavell of Cavell, Mertz & Associates, Paul Shulins of
Greater Media Boston, Mary Ann Seidler of Tieline and I led a
day-long RF “boot camp” aimed at IT engineers at the NAB Show.
Fig. 1: A typical regulator shows both line pressure and volume.
was one of many efforts that the SBE and NAB are taking to help
educate IT professionals. More than 60 folks showed up to learn the
ins and outs of what happens beyond the studio.
like this remind us of things we may take for granted. Yes, today’s
equipment is much more reliable, but systems still need to be checked
if you haven’t scheduled some quality time at your transmitter site
in a while, you’re probably overdue. Let’s start with a subject
that was of particular interest to the attendees: focusing on FM and
nitrogen pressure on the transmission line.
pressure gauge, as seen in Fig. 1, can help you avoid disasters. This
gauge usually is located at the transmission line input. Monitor it
for at least three psi (pounds per square inch) pressure on the line.
The nitrogen provides dry, positive pressure to the inside of the
transmission line to keep moisture out.
at the site, check the volume of the nitrogen tank. Full tanks have a
volume of around 2,200 psi. Log both the pressure and the nitrogen
volume; a dropping tank volume will signify a leak in the line. Keep
a clipboard with this information right at the tank. Also include the
phone number and address of the nitrogen supplier, so you can order
replacement tanks easily.
our boot camp, we discussed tank safety, too, because 2,200 pounds of
pressure per square inch is enough to blow a hole through your hand
or, if at eye level, blind you … even kill you. Never open a tank
without a regulator attached.
the nozzles of unused tanks covered with their metal screw-on covers,
pictured in Fig. 2. Make sure tanks are secured against the wall.
Chain or commercial tank supports work best. This is an OSHA
requirement. You can find the tank supports online or at a local
welding supply shop.
Fig. 2: Keep nitrogen tanks capped when not in use.
A fire marshal once told me about an unsecured tank that fell. The
valve assembly broke off and the tank tuned into a missile, blasting
through two cinder block walls and landing out in a field. You can’t
be too careful around nitrogen tanks!
If the transmission line is not pressurized and moisture develops
inside, VSWR (reflected power) can increase. If the condition is
ignored, a costly flashover inside the line can be the result. This
usually results in
line replacement, as seen in Fig. 3. The situation is not only costly
but keeps you off the air until it is repaired.
Some stations may use unpressurized rigid copper line sections of
transmission line inside the building. Since the outer conductor is
at ground potential, use your hand to feel for any hot spots along
the line. Ninety-degree elbows may be warm, but should not be hot. A
hot elbow could be the result of a bad connection or split bullet.
Remember, heat will eventually destroy components, so don’t ignore
this type of warning.
Here’s a good question from one participant, “What do I do if the
line pressure is falling?”
First grab a bottle of Formula 409 or Fantastic spray cleaner. With
the tank valve and regulator on, spritz the soapy cleaner around the
regulator fittings, then along the hose leading up to the
transmission line, and finally at the gas inlet port on the
transmission line itself. If there is a leak, hundreds of tiny
bubbles will appear as the gas escapes.
There’s your leak — and you didn’t have to call in a tower
Tell this to the manager. Let him or her know when you save the
station money through preventive maintenance. Usually, tightening the
fitting or replacing the hose will solve the problem.
Fig. 3: The line at left is contaminated by soot.
If you don’t find any leaks leading up to the transmission line,
calling in a tower crew may be necessary. They’ll inspect the line
all the way up to the antenna, checking all fittings just like you
did but also looking for bullet holes or other damage to the
Some stations may use a dehydrator instead of, or in addition to,
nitrogen tanks. Two things to check on these devices: First, do they
seem to be running all the time? Even twice an hour is too much. More
typical is once every hour or two. More activity could indicate a
The second thing to check is that the moisture-absorbing crystals are
blue. These crystals turn pink when they absorb moisture. Check with
your manufacturer about replacement. Moisture-laden crystals can be
rejuvenated in a warm oven, so don’t throw them away.
More transmitter site maintenance tips next time.
Contribute to Workbench. You’ll help your fellow engineers and
qualify for SBE recertification credit. Send Workbench tips to
email@example.com. Fax to (603) 472-4944.
Author John Bisset has spent 44 years in the broadcasting industry
and is still learning. He handles West Coast sales for the Telos
Alliance. He is SBE certified and is a past recipient of the SBE’s
Educator of the Year Award.