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Re: [APD] Milwaukee controller and testing of CO2
PH is a tricky variable to control, since water can get stratified.
Therefore smooth changes and good circulation are mandatory.
The MS 122 is a on/off controller, thus a solenoid is activated when the
probe gives a value below a programmed threshold, and is disconnected when
value surpass another threshold. These values could not be the same for a
correct operation. Industrial controllers can adjust this hysteresis value,
beside the on/off programs.
Note that this has nothing to do with instrument accuracy or the number of
digits the display present. These are different things.
There are other types of more sophisticated controllers, such as
proportional, derivative and PID, but this application does not deserve
them, since a simple on-off can do the job.
Backing up to the probe location, I think the best place to install the
probe is close to the point where CO2 getting into the tank, with good water
movement also. If the CO2 amount is small, the PH rise will be gentle, just
a differential value, and with sufficient recirculation there should be
little stratification among the tank. When the water surrounding the probe
reaches the programmed solenoid off value, there will be still some time for
water to homogenize, so the average value will be smaller than the
programmed off value. This will lead to have a smaller variation through
hours. The counter part is that the on/off cycle will get shortened. But
again, this time depends in the injected CO2 amount. When the value gets
below the on threshold, the process starts again.
>From: "Adam Aronson" <adam at aronson_net>
>Reply-To: aquatic plants digest <aquatic-plants at actwin_com>
>To: "'aquatic plants digest'" <aquatic-plants at actwin_com>
>Subject: Re: [APD] Milwaukee controller and testing of CO2
>Date: Thu, 13 Oct 2005 08:20:09 -0400
>You bring up a point I never considered... What is the best placement for
>the probe? Mine is on the opposite side of the tank from my reactor.
> > -----Original Message-----
> > From: aquatic-plants-bounces at actwin_com
> > [mailto:aquatic-plants-bounces at actwin_com] On Behalf Of
> > Mariano F. Bonfante
> > Sent: Wednesday, September 28, 2005 12:43 PM
> > To: aquatic-plants at actwin_com
> > Subject: Re: [APD] Milwaukee controller and testing of CO2
> > After 10 years of no CO2 controller life, I finally acquire
> > mine past July.
> > Since then, life's easier. To know what's up with PH level I
> > just take a glance to the MS 122 display.
> > I can look for an industrial PH controller with a greater
> > precision, but I think is a matter of solving a problem with
> > the minimum resources.
> > The hysteresis of my system round 0.1. If values are
> > changing more radically there's an other issue, such as too
> > much gas getting to tank, without the sufficient time to
> > equalize the average value, or the probe is to far from the
> > reactor/difusser. This type of controllers (on/off) enhanced
> > they hysteresis when the error signal is getting earlier to
> > the decision circuit. This is done by slowing down the
> > process or by getting actual values asap.
> > Mariano
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