Redox reading erratic

[almost]

The nitrareductor filter depends on low level of O2.
It needs to be operated in -250,
From the manual of Aquamedic nitratereductor:


"If it exceeds -50 mV, the denitrification reaction may stop at the nitrite stage!
If it falls below -300 mV, all the nitrate is reduced. The bacteria then start to use sulphate. This is a very undesired process because the end product of this reaction is Hydrogensulfide.
Hydrogensulfide (H2S) is toxic and smells very strange like fouling eggs."


I have set it to work, in area of -250.
So, in one sec it is above the next sec is bellow.
I changed the hysterisis so that powerbar is not allways in a on/off status.
But, when it goes to -260mV, it opens the flow of the filter, when the flow stops (pump connected to powerbar) I see readings of -150mV.
It should stop the flow of the filter earlier, but due to hysterisis and that values changes +-5 all the times, P3 stops the flow much later than I wanted.

My goal was to open on -250 and close on -230 and I can't do that (untill you help me ).

[Matthias]

The hysteresis covers the range between the upper switching point to the lower switching point.
This means
if you have a set value of -250 and a hysteresis of 50 the upward control will be activated at -250 - (50/2) = -275, the downward control will be activated at -250 + (50/2) = -225, both controls (up and down) will be activated when the wished value is reached.

-> hysteresis must be large enough to reduce frequent switching (in your example due to the +/-5mV)
-> and must be small enough in order not to switch too late (in your example: -150mV)

if you want to open on -250mV and close on -230mV you need these settings:
nominal value -240mV (center of wished range), hysteresis 40mV (effective hysteresis for upward control = 40mV / 2 = 20mV)

if it does still switch too late:
Consider the time lag of your system (so-called "overshot"). Switching off will not immediately stop the raising of the redox!

[almost]

O.K. Matthias, I just put the settings you told me, I will inform about the readings of mV (about the correct controlling of the nitratereductor).

PS: I just saw that when I put -240mV and press save, it saves the value -239mV, so I put -241mV and with save it went to -240mV. I do not know if this is critical or not, I just thought I should share it since it made me wonder why this happens (in possitive values, whatever I put, pressing save doesn't change it).