WranglerJohn,
Actually, chronograph data is a VERY good indication of pressure. Like a strain gauge or a piezo transducer, it IS another measure of pressure.
I have visited professional pressure labs, and even helped in one. It is astonishing how closely pressure and velocity are tied together, the reason many professional ballisticians also suggest that a chronograph is the best affordable home test of pressure.
Now, you can run into trouble with a chronograph if it's giving false readings, and some do, especially under certain conditions such as bright sunlight. Which is why we should always remain cautious--and as Big Redhead pointed out, we still have to be observant of traditional signs of excessive pressure.
But I have been using a chronograph as my primary method of pressure measurement for several decades now. While I've run into signs of high pressure now and then, most were also accompanied by too-high velocities.
The exceptions that I can remember were rare instances of too-soft brass, and a few bolt-faces that had little machining ridges around the ejector hole that left a mark on the case heads even with mild loads.
As the last pressure-lab ballistician I visited (just a few months ago at the Western Powders lab) said during our afternoon: "The chronograph is the handloader's best friend!"
Yes, I use a chronograph during every load development session. However, during my career in the water and wastewater treatment field, I installed and operated various electronic and mechanical pressure, flow, temperature, turbidity and analytical instruments. Each different type of device has a range of accuracy related to percent of scale. Many accuracy standards range between 2% to 10% of full scale value, 2% across middle half of scale, etc. Operational instruments such as bourdon tube pressure gauges are less accurate than precision calibrated electronic transducer gauges, but are sufficient for the purpose. Online real time analytical instruments are usually less accurate than laboratory instruments calibrated at time of use.
In the case of chronograph data, there are too many variables to use for other than determining trends in pressure based on velocity. While there is a interrelationship between charge density increase, percent of velocity gain and pressure, the accuracy of all instruments involved determines the result, as you point out.
My main concern is that to make the test requires discharge of a cartridge that may or may not go to failure. If this test is conducted by live firing, it exposes the shooter to a hazardous condition.
While the advanced loader can find the chronograph a useful instrument in recognizing velocity and pressure trends relative to increases in charge density, he or she is still not measuring actual pressure or pressure/time curves, but determining the point where additional powder is not producing velocity gain. Regardless of the velocity he or she is able to say that load is near maximum based on efficiency, but not the actual pressure.
So I believe the novice or disinterested loader is better served by using tested data from reliable sources.
