Thanks J5 I am going to need all the encouragement I can get!
It's a Fieldpiece SMAN3, which is basically a two port digital manifold with an integrated micron gauge and a pair of thermocouples that precisely measure temperature.
I considered going with the basic setup which is fine in 99% of cases. But I want to be sure I get a vacuum of 40 microns before I charge (an old Porsche HVAC mentor, Jim Sims, formerly of the Los Alamos National Laboratory, insisted on that level) which means using a Micron gauge. A decent micron gauge is a couple hundred bucks, so by the time you add that to the manifold and hoses I was money ahead with this eBay purchase.
The really neat thing is the thermocouples and the built-in pressure/temperature database. To measure superheat, which is the temperature beyond the boiling point of the particular refrigerant at a given pressure, you clip one of the thermocouple leads to the outlet of the evaporator, and then set the correct refrigerant using the buttons. You can see in the photo above that I have set it to R134a. The box then automatically measures the difference between the measured temperature and the "ideal" boiling point from the p/t chart. You want the refrigerant to continue to heat up after leaving the evaporator-- if the temperature is too cold you run the risk of slugging the compressor with liquid refrigerant.
Subcooling is similar, except you're measuring how many degrees cooler than the boiling point the refrigerant is as it leaves the condenser. The concern is that if it's too cold, liquid refrigerant could back up into the compressor, destroying it.
So by looking at superheat and subcooling you can tell if the refrigerant charge is too much or too little. Suppose that 10F is the right amount of subcooling-- for measured values below this number refrigerant can added until the target subcooling is stable at that number.
And of course only the subcooling method will work for a system controlled by a TXV, like ours is.
Now I need some REAL HVAC technicians to chime in!