Interference with Calcium Hardness Test

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H

hoshiemotto

0
Aug 29, 2014
12
East Northport, NY
I just started up my spa and the last thing I need to get in range is the calcium hardness. I am using the LaMotte Color Q and Taylor Calcium Hardness tests. Per the pool math calculator I am supposed to add 16 oz to my 450 gallon spa to get to a 300 ppm hardness. After adding that my LaMotte test is showing 200 and the Taylor test keeps proving a fading end-point and I cannot get an accurate reading with it. I even tried Taylor's suggestion of adding 5-6 drops of the r-0012 first if metal interference is occurring but that did not help the fading end-point. How can I get an accurate test reading if metals are present? I have test strips for iron and copper and it looks like I have .03-.06 cooper in the spa, but the strip is hard to read. I added Leisure Time's Metal Gon at start-up (not sure if this has anything to do with it).

Any help on how I can accurately test the calcium hardness given my current situation, and any advice on how to remove the metals if they are in fact present in the spa would be greatly appreciated.
 
The LT Metal Gon MSDS says that it contains 1-Hydroxyethane-1,1-diphosphonic Acid (HEDP). It is a sequestering agent. It is very likely that this additional sequestering agent is also interfering with the calcium test as it will bind up Ca ions as well as metal ions. That probably explains your vanishing endpoint.
 
To answer your last question, metals cannot be removed from water by chemical additives. The only sure-fire way to eliminate metals is through water exchange (drain and refill). However, your fill water must be metal free as well.

If your fill water has metal in it, you can try to use a garden hose attachment to filter out the metals before filling. Also, you can look up "Culator" on this forum as some users have had success.

http://www.culator.com/


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hoshiemotto said:
Spa construction is acrylic. I decided to go to 300 ppm as it is in the middle of the 200-400 ppm range for a spa. Does anyone know of a reliable test I can perform to test the calcium hardness in the mean time?
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Given that you have added sequestrant to your spa water, it may be impossible to get an accurate read of the calcium level. What I might suggest you do is call Taylor Technologies (1-800-TES-TKIT) see if they have any recommendations on how you can test for calcium. They're actually very helpful.



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SunnyOptimism said:
Given that you have added sequestrant to your spa water, it may be impossible to get an accurate read of the calcium level. What I might suggest you do is call Taylor Technologies (1-800-TES-TKIT) see if they have any recommendations on how you can test for calcium. They're actually very helpful.
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That is a good idea, I will give them a call today and report back
 
At 300-ppm calcium, you will probably scale out your spa. Your heating element will probably get scaled and overheat. I wouldn't add calcium to an acrylic spa.

What is the calcium of your fill water?
 
SunnyOptimism said:
The LT Metal Gon MSDS says that it contains 1-Hydroxyethane-1,1-diphosphonic Acid (HEDP). It is a sequestering agent. It is very likely that this additional sequestering agent is also interfering with the calcium test as it will bind up Ca ions as well as metal ions. That probably explains your vanishing endpoint.
Click to expand...

I must amend this statement - it is unknown if HEDP would interfere with the CH test. The levels of HEDP needed to sequester metals like iron or copper would be in the single digit ppm range so it is unlikely to affect the measured CH value to any significant degree. As well, the interaction of HEDP with the indicator dye (a variant if Eriochrome Black) and the titrant (EDTA) all depends on the relative strength of each in solution. If HEDP is less active than EDTA in binding calcium, than it should not matter if the HEDP is present or not.

Many thanks to @chemgeek for his explanations of the underlying chemistry of the CH Test in this thread -

http://www.troublefreepool.com/showpost.php?p=730596




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JamesW said:
At 300-ppm calcium, you will probably scale out your spa. Your heating element will probably get scaled and overheat. I wouldn't add calcium to an acrylic spa.

What is the calcium of your fill water?
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Not 100% sure what my shell is made out of, I am assuming acrylic as that is the most common. I received the hot tub for free a few months ago, all I know is that it is a 2003 Vita Spa. I included the picture below, hopefully that can help someone figure out what the shell is made up of.

photo.jpg

I tried to get the calcium up to 300ppm as that was the suggestion of Leslies pool store. The fill water is approx. 60ppm of calcium hardness. What would you guys recommend the hardness level be?

Also can someone explain why there are different hardness level recommendations based on the construction of the shell and why/how the hardness level would affect components such as the heating element?
 

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SunnyOptimism said:
I must amend this statement - it is unknown if HEDP would interfere with the CH test. The levels of HEDP needed to sequester metals like iron or copper would be in the single digit ppm range so it is unlikely to affect the measured CH value to any significant degree. As well, the interaction of HEDP with the indicator dye (a variant if Eriochrome Black) and the titrant (EDTA) all depends on the relative strength of each in solution. If HEDP is less active than EDTA in binding calcium, than it should not matter if the HEDP is present or not.

Many thanks to @chemgeek for his explanations of the underlying chemistry of the CH Test in this thread -

http://www.troublefreepool.com/showpost.php?p=730596




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Click to expand...

Is it possible that metals came in the spa water from the hot tub components as I did not increase the calcium hardness until day 3 after the fill as I was trying to balance the alk, pH and borates first? I was constantly running the jets to mix in the chemicals during this time. The calcium hardness at fill up was 60ppm
 
Calcium carbonate solubility is inversely proportional to the water temperature. The hotter the water, the less soluble calcium carbonate is. The water in a hot tub is more susceptible to scaling due to the higher temperature. The water near the heater element is even hotter and more likely to scale.

Calcium carbonate saturation is important to surfaces that contain calcium, such as plaster and grout. Acrylic does not contain calcium.
 
JamesW said:
Calcium carbonate solubility is inversely proportional to the water temperature. The hotter the water, the less soluble calcium carbonate is. The water in a hot tub is more susceptible to scaling due to the higher temperature. The water near the heater element is even hotter and more likely to scale.

Calcium carbonate saturation is important to surfaces that contain calcium, such as plaster and grout. Acrylic does not contain calcium.
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Ok, so I do not have to worry about the water trying to take the calcium from my shell since it is not plaster or grout? Is that what I would be concerned about?

- - - Updated - - -

chem geek said:
We do recommend some CH to prevent foaming in spas, but it's 120-150 ppm which with the Dichlor-then-bleach method that has low TA has very little risk of scaling.
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I thought CH was also used to harden the water so it would not corrode the components of the hot tub (such as the heating element). As I mentioned both Leslies and my Baquaspa/Leisure Time guide suggest the CH to be in the range of 200-400. Why would this be suggested if it can potentially hurt components in the hot tub and cause scaling?
 
hoshiemotto said:
I thought CH was also used to harden the water so it would not corrode the components of the hot tub (such as the heating element). As I mentioned both Leslies and my Baquaspa/Leisure Time guide suggest the CH to be in the range of 200-400. Why would this be suggested if it can potentially hurt components in the hot tub and cause scaling?
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That's one of those misconceptions in the pool/spa industry. They suggest this CH range because that is the standard range suggested everywhere, including for pools. They are idiots for not changing the range for higher water temperatures even if one had plaster spas. Also, the industry often says CH is needed even when there is no plaster or grout to protect (i.e. no calcium carbonate surfaces).

The idea that saturating the water with calcium carbonate will prevent metal corrosion is not at all reliable. Low pH is what accelerates metal corrosion as does strong oxidizers. There are other factors as well, but trying to form a thin coat of calcium carbonate for protection is almost impossible in an environment where water flow rates and temperatures are changing so dramatically, especially in the heat exchanger of a gas heater. See this link for a discussion among corrosion experts.

There are many, many incorrect recommendations in the pool/spa industry because it is not driven by science but by profit and over-simplifying. We don't do that. You can see some of these differences even in one of the best courses around by looking at the thread Certified Pool Operator (CPO) training -- What is not taught.
 
Well I am glad that I posted about this because if I had not I would have been trying to keep my CH level at around 300ppm. Thank you Chem geek for taking to the time to explain this to me. Since I use biguanide in my spa would you be able to recommend the chemical levels that you would prefer if you had my 450 gallon acrylic tub?

I am thinking these ranges may be good, let me know what you think.

pH = 7.2 - 7.8 (preferably 7.4)
ALK = 80 -120 (preferable 100)
CH = 80 - 200 (preferably 120)
Biguanide = 30 - 50 (preferably 35)
Peroxide Oxidizer = 30 - 80 (preferably 50)
Borates = 50 - 80 (preferably 60) - I know the recommended borate level for chlorine is lower than biguanide
 
The hydrogen peroxide oxidizer you periodically add is essentially pH neutral in its usage so you might want to target a lower TA if you pH tends to rise -- as low as 50 ppm if necessary for a stable pH. The rest of your ranges sound OK except that borates should be 50 ppm -- no need to go any higher than that and the EPA limit is technically 50. The CH should be 120-150 ppm to prevent foaming and I wouldn't get it much above 150 ppm so that you prevent scaling in the heater.
 
chem geek said:
The hydrogen peroxide oxidizer you periodically add is essentially pH neutral in its usage so you might want to target a lower TA if you pH tends to rise -- as low as 50 ppm if necessary for a stable pH. The rest of your ranges sound OK except that borates should be 50 ppm -- no need to go any higher than that and the EPA limit is technically 50. The CH should be 120-150 ppm to prevent foaming and I wouldn't get it much above 150 ppm so that you prevent scaling in the heater.
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My pH does tend to drift to the 7.8-7.9 range and stay there. Is 50 ppm ALK safe for the tub since it's well below the suggested range?

Also, I went to pool store today and they said there are no metals present in the water and that the Taylor calcium test may be getting thrown off by biguanide, is anyone aware of this?
 
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