My PFTs are consistent regardless of machine type. Each pulmonary function lab (or whichever facility is using the equipment) adheres to the same protocol for calculating the results.
Here's a more technical explanation of why PFT results are consistent from place to place:
Whenever a spirometer or similar product is going through the process to be approved for use, something called a "validation test" or "equivalency testing"is performed. How this works is someone in a lab takes three (or more) pieces of new equipment and compares it to the equipment that is already available. They run tests to make sure both types of equipment produce the same results within a small enough margin of error. Depending on how rigid the requirements are (and they're rigid for health care equipment!), both types of equipment should have the same results 99.97% of the time. If they don't, they are recalibrated.
I have experience calibrating spirometers and electronic peak flow meters as well as air sampling equipment. There are regulations in place that require that type of equipment to be re-calibrated regularly so as to eliminate variation in the results.