Two of the non-invasive meters I use at work can measure everything I need in a single shot: kV, exposure, exposure time, filtration, half value layer (HVL).
Even though the meters will calculate the HVL of the x-ray beam for me from one exposure, I still measure it with different thicknesses of aluminum and calculate the HVL. Just from first order impressions of using the meters over the years, I always felt that the HVL determined by the meter was never as accurate as doing the actual measurement.
It's not anything I've ever tested or looked closely at though. With all the HVL data I have from the past few years, I can take a closer look at how the meter HVL and my calculated HVL compare. There's at least a couple of years worth of HVL as a function of kV, so a plot of HVLmeter vs HVLcalc should give me a straight line with a slope of 1 if the meter HVL is useful.
I'll need to go back to my spreadsheets and extract the meter derived HVLs and collect the corresponding measured HVL so it could be a while before I find a big enough chunk of free time to do that.
If it turns out the meter derived HVL is close to my calculated HVL, that means I could probably modify my data acquisition and do my testing with fewer exposures.