Why would Trichlor SLAM work and Sodium Hypo fail?

[TimboZero]

Last couple of years I have been SLAMing with 9.6% hypochlorite and had no issues.

Last year no matter how much I tried I could not get the pool to hold chlorine and was using 3-5 times more dichlor than a usual year. I had intended this year to drain and restart but in the end decided to give it a last go.
The pool for reference is approx 11k gallons or 42k litres.

I hit the pool with 20litres of hypo at season start and managed to hold it up for approx 24 hours by adding another 10litres (total) at intervals during that 24 hours (i.e. I struggled to hold a shock level of chlor for a day)
For a few weeks the pool consumed chlorine at a normal rate, then as before started to use 3-5 times more.

Recently I have hit it with a fresh shock, this time with a kilo of trichlor. Three days on and it is still holding shock levels of chlorine.

I am well aware that hypo is unstablised, trichlor is stabilsed etc.. Bit why the heck would one burn off so fast and the other last so long ?
for reference the pool has a base CYA of approx 30.

 

[Texas Splash]

Timbo, I can't really answer that one. Maybe one of our other staff members can. Chlorine is chlorine, so regardless of the form, if it was the same amount of actual free chlorine added, you would expect similar results. Perhaps that's the main difference. I suppose in your area you are forced to use that much cal-hypo or trichlor as opposed to liquid chlorine? Maybe chlorine is in short supply because of COVID? Your CYA can handle it, I was just curious.

 

[IceShadow]

Your CYA was probably low. If it’s at 30 after all the trichlor / dichlor then it was much lower before that point.

 

[TimboZero]

Firstly, thank you to both of you for your replies.

Timbo, I can't really answer that one. Maybe one of our other staff members can. Chlorine is chlorine, so regardless of the form, if it was the same amount of actual free chlorine added, you would expect similar results. Perhaps that's the main difference. I suppose in your area you are forced to use that much cal-hypo or trichlor as opposed to liquid chlorine? Maybe chlorine is in short supply because of COVID? Your CYA can handle it, I was just curious.

Early in the season options were a little sparse and I did open the pool about 2 weeks later than ideal, but that can happen anyway.

Your CYA was probably low. If it’s at 30 after all the trichlor / dichlor then it was much lower before that point.

I would agree. To be clear it was at about 20 before treatments, I have since raised it to around 40 (for now, will add more CYA if needed once SLAM complete and pool balanced again).

The pool is still holding at 20+ (I can't test over 20) after 4 days with a trichlor shock of 1kg (1.3 to be exact, and that's a little under 3lbs).
With Sodium Hypo 9.6% (not cal hypo as Texas Splash said) I added 30 litres (that's 8 gallons) and it was gone (zero FC,TC,CC) in just 24 hours. It was the same last year.

What I don't get is why I could add 3 times more liquid chlorine than 'needed' and fail to SLAM (barely even shock for 24 hours) yet standard quantities of trichlor works and holds shock level for days.

Probably should add that other chemical levels are in spec, pH is 7.4, CH is 200, TA is 65. water temp 22 Celcius (72F)

 

[IceShadow]

You killed a lot of whatever it was with the liquid and lost some to the sun. Getting to 30 CYA with stabilizer and sticking with liquid would have had similar results.