My mind still thinks in RPN, so it's painful to use this piece of crap. Mostly I just use it for trig functions these days.
If I ever get another job in engineering, I will have to replace it with a real calculator.
When I was a startup engineer at the Cholla#4 power plant (Joseph City, AZ), I used my HP-45 to do some curve fitting, which was needed to calibrate the "percent load" gage for the steam turbines. It was really cool.
I think that was the only time in my entire career that I actually used any thermodynamics, and also the only time I had to use steam tables.
Surprised the hell out of me when the gage in the control room actually worked!
At that same plant, I saw an H-P instrument that made me want to fall on my knees in reverence: The Structural Dynamics Analyzer. It was a real-time Laplace Transformer.
They had this instrument hooked up to monitor deviations in the shaft frequency of the main turbine/generator, and fed "pink noise" into the exciter. This H-P instrument integrated this data over a period of days, and displayed a frequency response plot of the massive rotating equipment from zero to 60 Hz. They were looking for resonant points that would have to be "tuned out" in the exciter to prevent sub-synchronous oscillation.
If that wasn't impressive enough, you could, with a few keystrokes, enter your compensation parameters and re-plot the frequency response as it would look with the compensation in place.
I have NEVER seen a piece of test equipment so powerful, or so far ahead of it's time.