I was not happy with my current means of testing motors and getting reliable calibrated data from them, so I bit the bullet and obtained four 50 Kg load cells which will allow me to measure up to 200 Kg (over 400lbs) of thrust.

I went ahead and wrote up some software for the microcontroller to do some pretty nice analysis of the motor right after the test. The load cells are polled between 90Hz and 110Hz and implements a simple low latency regressive averaging scheme to get data. The results look very very promising.

10 3.95
21 5.50
32 7.15
42 8.64
53 9.84
---------------
SNIP for Space
---------------
1590 1.99
1601 1.69
1611 1.58
1622 1.42
1633 1.32
1643 1.28
1654 1.10
Fire Complete

Burn Time: 1.67 seconds
Total Samples: 155 (92.54Hz)
Average Thrust: 30.74lbs (136.74N)
Total Impulse: 51.49lbs (229.04N)
Peak Thrust: 59.60lbs (265.11N)
Motor Designation: H136.7

You can see it gives the raw data with a millisecond timestamp, burn time, sampling data, average thrust, total impulse, peak thrust, and a moto designation. All nearly instantly after a test.

I can then copy and paste the raw data into Excel and get beautiful charts:

Thrust in pounds on the left, and time in seconds on the bottom. This graph is from a 'static test' of me pressing down hard on the sensors with a board on top.

You can see how this will really help me scientifically narrow down engine performance and allow me to develop motors safely and efficiently.

I'll be constructing the new test stand shortly to allow use of these electronics on actual motors. Stay tuned!

EDIT

Wired up the board for the electronics and added a wireless transceiver and connections for external battery connections.

Also, started a project page for this: https://hackaday.io/project/7942-rocket-motor-analysis