From the HBD Archive
From: Pete Soper <soper@maxzilla.encore.com>
Subject: Re: detent
Date: 1989-09-22 16:29:18 GMT

In HBD #261 att!iwtio!korz@hplabs.HP.COM (Al) says:

>> The only thing the Honeywell unit has that I wish the Hunter had is an
>>adjustable "span". I know I've probably used the wrong term. What I mean
>>is a way of saying "turn on X degrees above and turn off Y degrees below
>>the set point".

>What you're thinking of is called "detent."

The terms I couldn't remember are called "dead band" for on/off
controllers and "proportional band" for proportional and PID controllers.
Think of it as hysteresis.
My thought is that without control over this, depending upon the thermal
inertia of the wort, interaction with heat production by yeast, etc. you
might not be able to use the *wort* temperature to control the fridge
without suffering from wide and constant temperature swings.
I plan to make an alternative probe or probe carrier for the Hunter
unit and put this inside the fermenter so the wort temperature and not
that of the area around the fermenter is what is held constant. If
anybody has already done this I'd very much like to hear about it.
Incidently, I have a controller accident report to make. Several weeks
ago I got some wires tangled and left the probe of my Hunter unit trapped
outside the fridge. I came back a day later and the Hunter reported 80
degrees (garage temp) while my Radio Shack thermometer (probe inside the
fridge) indicated 30. The fridge was humming merrily away, on its way to
its 10 degree lower limit. The yeast didn't just go to sleep, it moved
to Wisconsin. Even after warming back to proper temperature the
fermentation stayed dead until I pitched another starter.
As long as I've shared this embarrassment, let me tell you about one
more. Have you ever had the inside nut holding the drum tap of your
bottling vessel loosen after you've racked 3 gallons of beer into it?
I hate it when that happens.

--Pete Soper

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