I wanted a better picture of what was going on so I figured out the Acurite's 433 mhz protocol for the 986 sensors. I've added support to rtl_433. The pull request was merged November 20th, 2015.
Now I pump the data into rrdtool and see what's going on. I now see how long and often the compressor is running, and how long it times to warm up between cycles. The spikes are the defrost cycles.
Now I can also monitor/get alerted when I'm out of the house.
I'm pretty happy with the Acurite 986 display and temperature sensors even without the rtl_433 decoding.
Acurite 00986 Refrigerator / Freezer Thermometer Protocol
- Includes two sensors and a display, labeled 1 and 2, by default Sensor 1 - Refrigerator, 2 - Freezer.
- Data format: Pulse Position Modulated (PPM), 5 bytes, sent twice, no gap between repeaters
- Start/sync pulses: two short, with short gaps, followed by 4 long pulse/gaps.
- Data Format - 5 bytes, sent LSB first, reversed
- TT II II SS CC
- TT - Temperature in Fahrenehit
- MSB = sign. Encoding is "Sign and magnitude"
- II - 16 bit sensor ID changes at each power up
- SS - status/sensor type
- 0x01 = Sensor 2 (
- 0x02 = low battery
- CC = CRC (CRC-8 polynomial 0x07, little-endian)