Subject: [PTS] a very prius-like hack Okay, today was a weird one, but lots of fun. I recently got one of the new super-efficient heat pump systems installed for the house, the type with an inverter that drives the compressor at a large variable speed range to pretty much exactly match the capacity needed at any given time. It's a three-phase motor driven by a rack of power transistors -- I've said this before -- just like a Prius. There are some limited diagnostics obtainable at the thermostat / controller but more about the indoor half of the system than the outdoor, and nothing to indicate whether the refrigerant charge inside is up to snuff or not. At a first approximation, the presence or absence of the right amount of refrigerant in the system is determined by pressure, measured with gauges connected at Schrader fittings to the high and low sides and taking the temperatures of each coil into account since that changes the saturation pressure of the refrigerant. Due to a bit of an installation screwup the system's lineset had a small leak at a flare connection, which later got fixed and I watched the HVAC guy add more R410A back in with the thing running until his low-side gauge read about what he thought was "right". Fortunately, these newer systems are much more adaptible and lenient about slight over or under charge so his guesswork in this regard is probably okay for the moment. It's working again, at any rate. While looking at the outdoor unit schematic for the Nth time I suddenly realized that it has *pressure sensors*. Not just an overpressure trip switch, but actual transducers that report the working pressure on either side of the compressor as a voltage between 0 and 5. By peering into the unit I could see them, brazed into small tees on the piping, and observed the straightforward way that they're wired over the control board. The afternoon turned into a familiar scene: oscilloscope and meters set up next to the unit, wires hanging out all over the place, and ultimately the addition of a small diagnostic connector spliced into the pressure-sensor leads in the usual slit-n-tap manner. But the lab-slab victim wasn't a car this time, it was my new heating and cooling system. The sensors aren't identical, as one is designed for a much larger range than the other. But under steady-state cooling at modest demand both settled in just shy of 2 volts, giving me a nice benchmark as to what "normal" pressure readings should be. Now the theory is that I can go check on this important bit of status with a couple of cheap voltmeters, rather than having to buy an expensive set of gauges and risk losing a little bit of the "juice" every time they're hooked in. More observations will be made later under other ambient conditions, in heating mode, etc. Hooray for modern consumer goods heavily instrumented in the interest of energy efficiency. And as with the Prius, it would have been nice if the manufacturer wrote the control software to give a *whole* lot more visibility into the system's workings to those who bother to ask. They make a token nod toward it, but don't really cater to the geek side or even the service technician. A measly few more lines of code could have easily filled that in. _H*