WDTC-20 bad temperature readings?

[Ericryherd]

I keep getting some really bad temperature readings - but only occasionally. I'm plotting these using powertrigger which is how I notice them but it would be really bad if the thermostat (or HS) reacts to these bogus readings.

The temperature I get most often is 0. It's summer and it's clearly not 0F in my house (even in NH). I've also been getting 202 = 0xCA and 18944 = 0x4A00 the interesting thing is that 74 = 0x4A. I am noting the hex version of the temperature to show where the failing bits are in the ZWave packet.

ZWave uses a simple checksum to validate that the packet is good. Checksums are easy to calculate but are not very reliable. There is a 1 in 256 chance that a bad packet will be identified as a good one. And in this case, we can see the failures.

When the temperature is 18944 degress (rougly the surface of the sun) an extra byte of 00 somehow got put into the packet. A checksum won't detect an extra byte of 00. A CRC (which most modern protocols use) would fail.

202 is just a single bit upset in the packet (CA instead of 4A).

I also have 4 HSM100s but their temperature never seems to come back as bad so perhaps its the WDTC-20 that's corrupting the data?

[rmasonjr]

I've only noticed this once on one of my WDTC-20s. HS registered 20deg F, which was inaccurate. The next poll cycle cleared it up. I do wish that it had some 'retry' if the checksum or CRC was out of range or invalid.

[Walt Boudreaux]

I had a reading of 38 degrees F one time and it was actually in the upper 70's. The next time I checked, it was correct.

Walt

[Viper8726]

Bad Temperature Readings from WDC-20

I also have had some bad temperature readings. I get my temperature readings from a Visual Basic routine that I run every 15 minutes. I use that routine to make decisions and/or set flags that reflect temperatures. My work around for bad readings has been to test the value I get and either ignore it, or attempt again to get a valid reading.

[YoYo]

I guess you can also put me in the camp of those who occasionally receive bogus temperature readings from WDTC-20.

In my situation, it seems to happen once every few days, and the reading is just about always 35 degrees F.

Within a minute it receives a correct reading again.

Since I'm using this to send text messages if the temp falls below a threshold, it's kind of a bad deal.

As a result, I'm having to add a new device to hold the "real" temperature and write a script to parse out the bad readings received from the stat.

This really needs to be fixed!