A while ago, I got a call from a customer who was having trouble with a differential pressure transmitter. He was using a draft range DP transmitter to measure the pressure in a combustion chamber, so it could be controlled with a damper. He had one port connected to the combustion chamber with impulse tubing, and the other (low side) was left open to the atmosphere.
He’d noticed that when a fork truck or other vehicle sped past the furnace – the transmitter was mounted next to a traffic lane — it cause the furnace pressure to momentarily dip downward, and cause the damper to oscillate.
He figured out that the air movement provided by the passing vehicle was creating a momentary pressure pulse on the low side port. These air movements were creating difficulty in maintaining furnace pressure.
So, he asked me, “How can we dampen the effect of the momentary pressure pulse?”
I offered a few possible suggestions:
Suggestion One:
Use the transmitter’s electronics to digitally average or dampen the pressure (PV) signal. In the ST3000 draft range DP transmitter, PV damping is set by default to 0 seconds, but could be easily adjusted to fit this task.
Suggestion Two:
Use a length of capillary tubing attached to the low side port to physically dampen the pressure pulse. This would have the same effect as putting a needle valve on a gauge dampener.
Suggestion Three:
Use a pneumatic exhaust muffler to dampen the pressure pulse. It’s a metal sintered “filter” with a quarter-inch NPT fitting.
Ultimately, since it was inexpensive (currently listing at around $8.00), and so easy to install the muffler onto the threaded port of the DP transmitter, the customer chose this option.
And it worked like a charm! Pressure changes caused by the passing vehicles disappeared and the furnace pressure remained stable.
An exhaust muffler can also be used as an inexpensive way to protect any open-to-the-atmosphere port on a DP transmitter from the environment. Customers have told us they use the same exhaust mufflers to protect open ports from insects that would fill the cavities with mud to make their nests.
Looking for an exhaust muffler? You can buy them online at Lesman.com
Lesman Tech Tips 