Unexpected ORP results

[sixn3is7]

I know what the general consensus of this forum is regarding ORP (at least now I do). Unlike most things on the internet, I trust what I read here and I wish I had read before I invested.

I picked up a OEM refurbished Etatron DLXB Rx/M ORP metering pump on eBay for $99 (good price). Picked up a Hanna ORP electrode and was on my way to good things . Mind you, I used to maintain 3 indoor "pools", with ORP controllers, for the U.S. Navy, about 200,000 gal total with ridiculous BOD (another story for another day).

My set up: Pressure side of main pump into electrode cell back to suction side of main pump. Bleach is injected after the sand filter. I have the metering pump operating slowly because I realize it takes time for the bleach to circulate before it makes it back to the electrode.

My unexpected result is that if I set the metering pump to inject fast enough, as the water circulates the ORP reading drops. This does not make sense and I was wondering if anybody knows why :?:

Most likely, after experiencing some of the ORP issues I should have read here first, I will set my target ORP very high and set the dispensing rate slow enough to let it run all day thereby turning it into a continual feed system. I am just interested in knowing why.

Current reading.
FC 33.5 (fighting algae)
cc 0 (passed OCLT this morning and will double check tomorrow morning)
ph 7.3
ta 110
ch 60
cya 70 (yes, I know )
borates 50~60
temp 88

 

[Patrick_B]

ORP is so affected by so many factors, it really makes it tough to control FC with it accurately. After many years of trying it in all sorts of water with the best equipment on the market, I am resigned to the fact I''ll never figure it out. Sorry to be of little help. For what little it may be worth, I've worked nuke sub guys that gave up on it too. Sometimes you get lucky with some consistency, but it hasn't been my fortune so far.

 

[JasonLion]

Do you have a PH automation system? Without PH regulation, ORP is completely useless (as opposed to mostly useless). ORP varies dramatically with PH. Adding liquid chlorine tends to temporarily raise the PH, which will lower the ORP reading.

 

[KenGood]

Your CYA may be affecting the reading too. The suggested CYA for ORP I think is 30.

 

[sixn3is7]

Adding liquid chlorine tends to temporarily raise the PH, which will lower the ORP reading.

This makes sense and is exactly what I am seeing. I was just interested in understanding what was happening.

I am making it a manual pump by replacing the electrode with a resistor so I always have a fixed reading. Then I will adjust set point and % feed rate to get the optimum daily dose.


Thanks for the replies.