There are four things that affect the behavior of gasses.
1. Mass
2. Volume
3. Temperature
4. Pressure
To solve for anything you need to make 2 of the 4 elements constant and capable of detecting the third to solve for the fourth.
We are interested in pressure, more specifically the head pressure of wort pushing on a column of gas (air). We can measure the pressure using the pressure transducer. This is not a constant.
In a tube with a static column of gas there WILL be a temperature rise. Crap, up to 2 variables. BUT we could compensate if we measure the temperature of the air in the tube, not the wort.
As the temperature changes the bubble expands, and, chit... air is leaking out of the tube. Now the mass of air in the tube has changed. We're up to 3 variables now, 2 if we can compensate for temperature. I dont know how to measure how much air is left in the tube now....
Volume, if the air in the tube begins to change for what ever reason, room temp change for example (Open the garage door, turned on the AC or a fan, the temp in the tube begins to cool. Criminey, the volume in the has changed! We're skahrude
That's four variables. 2 times too many . You might be able to get an accurate result but it will never consistently perform if there is a variance in three conditions.
This is why the bubbler system is better.
Volume IS constant because the tube is always filled with air.
The Temp of the air in the tube isn't constant but it is far more stable than the static tube, as it is constantly being replaced with room temp air (you can use a room temp them to calibrate this if you were dead set on accuracy)
That leaves two variables.
Mass - The amount of air in the tube
Pressure - How much force the wort is pushing down on it.
We don't care about the Mass of air in the tube, we care about the pressure, which we can measure.
So if we don't care about mass what good is this to us? Well, if the only 2 variances in the system is mass and pressure, the pressure reading will always be consistent.
You could address the mass issue in the static tube by running the tube all the way to the top of the kettle, put in a loop or two, and then run it down to the pressure transducer. Mark the tube up to highest point it will contain fluid at all times for all batches. Say 5 gal for a boil kettle or 3 gal for a MLT. Install some kind of bleeder valve by the transducer and leave it open when filling the kettle. Once the level in the kettle matches the fill line on the tube (the liquid in the tube will be at approx the same level) shut the valve and finish filling. oops, see edit at the bottom
If the level is high enough to keep the expanding air in the tube from escaping at peak temps, you have made the mass fairly constant. The volume of air in the tube will be affect by temperature but can be calibrated out if the temp in the tube is known.
For better results the amount of air in the tube above the fill line should be as small as possible. If you are good about not overfilling the tube before shutting off the bleeder valve you could put a loop an inch or two above the tube.
I just realized, you need to measure the vacuum, in the tube for this to work. Just get the DP version of the freescale sensor and plug the reference side into the sensor and leave the other open to ambient pressure. Some one feeling adventurous?
EDIT
Blarhg, scratch that. Leave the vent open on the tube till the kettle is full. The close it. Volume is going to change any way and relative pressure is going to change around the point at which the valve was closed. So as boil off occurs, it will gradually return to 0 psi, if boil off goes below the point the valve was closed you will read a negative relative pressure. Since Volume is going to change any how, and mass will be constant (thats what matters).
