air velocity analog sensor

I have an application where my customer is a woodworking facility with a dust collector. He basically wants to setup a drive on a PID loop that will monitor the amount of vaccuum so that the dust collector will automatically speed up when more machines come online and the air requirements increase. What sensor would you recommend to produce an analog signal that will monitor the cfm?

I would use a pressure sensor, to keep it simple use a washmachine level sensor, When pressure is too low jus speed up until pressure is good again.
with velocity you will not get a control as the velocity has to change when a valve is opened.

+ 1 on a static duct pressure sensor. That's the way this is done. But how you do it is critical. Since this is a dust collection system, any sort of sensor you use must be located in a CLEAN air plenum, otherwise the dust clogs the sensor (the same would be true for an anemometer (air flow sensor). So before doing anything, you must thoroughly study the entire dust collection system to find an appropriate place to do the sensing. The usual place is in the air feed duct, as far away from the opening as possible, but ahead of the first dust collection inlet.

You normally use a differential pressure transmitter. One sensing line goes on the clean side of the dust collector and one sensing line goes on the dirty side (through a filter).
You set the PID loop to vary the fan speed to maintain a constant differential pressure across the dust collector. You need a minimum air velocity to prevent dust from settling in the collection lines. The overall duct system has to be designed for all machines that will be running at the same time. The only way that what you describe will work would be if every machine had an individual duct that terminated directly at the plenum of the dust collector and the fan was located at the clean side of the dust collector. Wood dust is explosive and there are strict NFPA regulations that apply and MUST be followed. If this is a home system the NFPA regulations do not apply. This will work: https://www.alliedelec.com/m/d/08116605c044641b93cc683e2f7c8307.pdf.
You really need someone experienced in pneumatic conveying to design the dust collection system. A poorly designed system is an accident waiting to happen.

IF you want to control to pressure use a pressure transducer. If you want to control CFM based on how many systems are running, you can get duct averaging flow transmitters. Although, I don't know how they will handle the dust.