Hysteresis mode short cycling a freezer

[RDH_Texas]

My understanding of reading the Wiki and the forums is that when the hysteresis mode is used for cooling, the BCS should turn on when the top temperature is broken and will not turn off until the bottom temperature is exceeded.
I have my system set up to provide power to an upright freezer with temperature probe in the beer stored in side.
I've seen the system short cycle the freezer around the upper temperature limit, that is if it's at 60 degrees with a 1 degree dead band as the temperature probe bounces between 59.9 and 60 it turns the freezer on and off repeatedly.

I'm working around this by making a process that does what the hysteresis should be doing. Turning on the freezer at 60 and turning off at 59. The question I have is why is the hysteresis mode doing this?
The version I have of the firmware is BCS-460 3.7.0, and the latest version 3.7.1 doesn't mention this as a problem that was fixed.

Not having seen anyone else mention this, I wonder what my problem is?

I have the input and outputs associated correctly, set for cooling. I've tried 0.5 to 4 degree limits.
Output 2 runs to an SSR that controls power to the freezer. Pretty simple setup.
What else is going on that I can troubleshoot?

I also saw the same behavior with ramp mode. as the temperature was bouncing around the set point it would turn the power on and off too quickly.
Any thoughts? I'm a bit frustrated with the system at the moment.

[JonW]

Do you have a ground established between the AC (earth) ground and the BCS DC ground? It sounds like maybe your temp probes are fluctuating too much. If you've extended the wiring on them, it will fluctuate even more.

Have you logged the temps to see if they are bouncing up and down? That's the easiest way to see it.

[RDH_Texas]

I need to look more closely and see if I connected the GND on the BCS to AC earth. It's not clear that I did.
Here's the plot of the most recent. I'm lagering at 30F, and there's points in time where the compressor is being short cycled. Presumably when the upper limit is hit, but the resolution on the plot is not high enough to show the small fluctuations.
I will look in to the grounding issue. Thanks for the reply!

[JonW]

I'm guessing the mash tun temp and setpoint is what you're using for lagering? Why does the setpoint drop at the end of the graph?

You should zoom in the graph so that you can see if there are smaller swings in the temperature.

The pic below is what the fluctuations normally look like when ungrounded. The section towards the end of the graph where it smooths out is right after attaching the grounding.

[RDH_Texas]

I checked the continuity between BCS ground and the AC ground. <0.1 Ohms.
I set the system to log data 1/sec, and set the hysteresis mode to 28.5 F with a 1 deg deadband.

Sorry for the nomenclature. Here's the decoder ring:
The "Mash tun temp" is the beer temp in the freezer being lagered.
The "RIMS heater" is the power being applied to the freezer (0=off 100=on) right hand axis.
"Mash tun setpoint" is the 28.5F desired temp.

You'll notice that the system turns on when the temperature drops below 29.5, the freezer is turned off. When it goes above 29.5 a short time later it goes on again.

[JonW]

Yeah, that's odd. It shouldn't shut off at 29.5, it should shut off at 28.5 and not come back on until 29.5.

For the heck of it, try doing a full integer set temperature like 29.

[RDH_Texas]

Here's an even interval between 29 and 30. when it goes below 30, it shuts off.
The system settings show its set to cool, also.

[JonW]

Brent is going to review the BCS code and see if he can spot anything that could cause this. There are lots of people using hysteresis (including myself) and not seeing this issue.

Can you reset all the field values to defaults in your "PID and Hysteresis Output Control Settings" and test it?

[brahn]

Are you running any other processes when this is happening? Do you have anything configured in ladder logic?

[RDH_Texas]

No other processes; No ladder logic.
Is there a specific way you want me to reset to the default values, or just use the settings I see for the top line (Out0 PID/Hyst)