Is there a way to cap the inside differential input to get to an absolute measurement? The cap would have to bleed very slowly to correct for atmospheric pressure changes. Advice encouraged!
Could you provide more data on the idea you have in mind so we can try to come up with more detailed answers? What do you mean with “absolute measurement” in this case?
Absolute vs. gauge pressure is a source of great confusion. Imagine an old black and white movie, where the crusty old engineer down in the engine room, looks at an old analog brass pressure gauge on the rusty boiler and shouts “Captain, she’s gonna blow!” He is probably measuring the “gauge” pressure between the interior of the boiler (probably hundreds of pounds per square inch, or millions of Pascals) vs. the ambient atmospheric pressure of 14.1 PSI or 101 kP, which are relatively small. So small we can ignore them vs. the boiler’s interior pressure. The measurement is the difference between the two. Changes in the atmospheric pressure, cause minor differences in the measurement.
Many years ago our company was using an ultrasonic sensor that used a thin mylar membrane’s motion to create and detect the ultrasonic signals. It worked great at sea level, but when we took it up about 600 meters the film would expand and cause the unit to fail. We learned how to vent the unit to prevent the problem without bringing in water vapor.
So absolute pressure is measured against a constant pressure, which raises the issue of temperature effects. ( i.e. PV=NRT problems). So in software you can temperature compensate to get rid of the problem, or control the temperature. The other effect is changes in atmospheric pressure, which can be eliminated by a very slow (pinhole aperture) to equalize the pressure.
I haven’t looked hard enough at the boom unit to understand the pressure system. Your guidance would be most helpful.
Am I right in assuming that you want to do this to eliminate the effect of opening and closing the door to your instrument room?
Perhaps reading the section of the manual relating to the mechanical filters might help to clarify things?: Raspberry Boom (RBOOM/ RS&BOOM) Infrasound Monitors
Many thanks! I missed this completely. I’ll take a harder look at how the external tube is attached.
With Shakes and Booms,
I wanted to send you the link to the manual, but I see that sheeny has rightly anticipated me. I hope you will find it helpful.
As for your request, thank you very much for the details (I loved the “rusty boiler” analogy!) that made what you want to achieve more understandable. I personally cannot offer much more as this is beyond my knowledge, but I’ll ask to see if any of my colleagues can help better.
I definitely think that there will be some DIY involved to achieve what you have in mind, so keep us abreast of your progress!
First, please forgive me for not reading the manual in more detail. Shame on me for my behavioral lapse. It sounds like the 20 second filter might be exactly what I’m looking for. Can I buy one from you or DIY one easily? I want to keep the RBoom protected from the elements in the instrumentation room, and so would have to suffer the door swings. If I take 8 seconds to open and close the door it avoids the problem, but I rarely have the patience…
This is an interesting problem… given the pressure differential the Boom measures is set up internally to the Boom i.e. across the highpass filter in one of the two tubes connecting the signal port to the sensor. The external tube you have connected to the shake should mean that the pressure changes inside the room shouldn’t matter i.e. the boom should not see them. I’d appreciate someone else either confirming or disputing my thinking on this.
This suggests to me that something else is not right. Possible problems that might cause this are:
a possible leak or disconnection of one of the tubes inside the Boom;
a leak or disconnection of the external tube close inside the room;
the tube is too flexible (doesn’t separate the inside and outside pressures effectively;
a blockage in the external tube such that pressure changes inside the room transfer through the tube wall into the tube to affect the signal. This might be due to condensation, kinking or squeezing of the tube close to the open end perhaps?
I’m not convinced the 20s mechanical filter is the solution you are looking for. Being a highpass filter, I would expect it to pass all frequencies above 20s period (i.e. 0.05Hz) and this won’t effect the pressure waves from the door.
Other than the external tube (which you’ve done) I’m not sure what else could be done to filter out the door pressure waves without losing a lot of valuable bandwidth in the Boom signal.
Sometimes the simplest answer is the best. I cracked a window open and the venting gets rid of the problem.
It’s great to hear that you were able to solve the issue, and thank you (and all the others involved) for all the interesting feedback and reasoning that was posted in this thread!