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Hysteresis control and one-wire probe with linear offset temp problem  

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(@angus_grant)
Eminent Member
Joined: 2 months ago
Posts: 21
March 9, 2019 3:11 am  

Hi,

I have a one-wire DS18B20 with a linear offset of -1 as it is strapped to the outside of my fermentor. An iSpindel inside my fermentor is submitting temp readings one degree lower hence the reason for the adjustment.

A hysteresis control using theDS18B20 is controlling the temp and seems to be controlling the temp at the unadjusted value.

So currently my raw probe reading is 17 degrees and adjusted to 16 degrees.
My hysteresis target is set to 17 degrees, but the temp is being controlled to 16.

I have a feeling the hysteresis control is reading the unadjusted temp value instead of the adjusted value.

Edit: the red line on graph is target and green is logged temp

Thanks,
Angus.

This topic was modified 2 weeks ago 2 times by Angus

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(@angus_grant)
Eminent Member
Joined: 2 months ago
Posts: 21
March 9, 2019 3:12 am  

And a screenshot of the linear offset applied to DS18B20


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(@angus_grant)
Eminent Member
Joined: 2 months ago
Posts: 21
March 9, 2019 7:25 am  

As I guessed, setting the hysteresis target to 18 has given me the actual temperature of 17 with the raw reading of the DS18B20 being 18 degrees celsius


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(@pbruno3)
Estimable Member Admin
Joined: 2 years ago
Posts: 212
March 9, 2019 2:59 pm  

Yes, sorry. 1-wire sensors cannot be calibrated back to the sensor. In the case of thermistors and RTD's, the temp calculation is made by the application. In the case of 1-wire, the temp is determined by the sensor itself, so there isn't a way to tell it to be different. We are contemplating a fix, but this is the current status.


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(@angus_grant)
Eminent Member
Joined: 2 months ago
Posts: 21
March 12, 2019 8:05 pm  

Not a problem in this case as the production install will be using PT100's.

Once I get those wired in, I'll be able to confirm the calibrations work for the hysteresis element.


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