As far as I know, there are no test strips available to consumers for pools/spas that test for Calcium Hardness (CH). Test strips can only test for Total Hardness (TH) which includes magnesium in addition to calcium and is irrelevant with regard to the saturation index since scaling occurs with calcium carbonate, not with magnesium (at levels found in natural water) and it is saturation with calcium carbonate that protects dissolving/pitting of plaster.
In addition to the lack of a CH test, the CYA test strip seems to be the most problematic and inconsistent. Also, the resolution and accuracy of test strips compared to the drop-based test isn't even close as you can see below (though LaMotte has added some wider range chlorine tests recently that I also describe after the table):
....................................... <---------------------------------- TEST STRIPS -------------------------------------> . <--------------- DROP-BASED ---------------->
........................................
LaMotte Insta-Test 6 ..........
Taylor sureTrack 6 .......
AquaChek Select/7-way ........
Taylor K-2006 ..............
TFTestkits TF-100
Free Chlorine (FC) .......... 0, 0.5, 1.0, 3.0, 5.0, 10 ........ 0, 1, 2, 3, 5, 10 ........... 0, 0.5, 1.0, 3.0, 5.0, 10.0 ....... every 0.2 ppm* ............. every 0.2 ppm*
Combined Chlorine (CC) ..... calculate from TC-FC ......... not available ................ calculate from TC-FC .......... every 0.2 ppm* ............. every 0.2 ppm*
Total Chlorine (TC) ......... 0, 0.5, 1.0, 3.0, 5.0, 10 .......... not available ............. 0, 0.5, 1.0, 3.0, 5.0, 10.0 ...... calc. from FC+CC .......... calc. from FC+CC
pH ................................. 6.2, 6.8, 7.2, 7.8, 8.4 ... 6.4, 6.8, 7.2, 7.5, 7.8, 8.4 ..... 6.2, 6.8, 7.2, 7.8, 8.4 .... 7.0, 7.2, 7.4, 7.6, 7.8, 8.0 .. 6.8, 7.2, 7.5, 7.8, 8.2
Total Alkalinity (TA) ...... 0, 40, 80, 120, 180, 240 .... 0, 40, 80, 120, 180, 240 .. 0, 40, 80, 120, 180, 240 ...... every 10 ppm ................ every 10 ppm
Calcium Hardness (CH) ......... not available .................... not available .................... not available .................... every 10 ppm ................ every 10 ppm
Total Hardness (TH) ......... 50, 100, 250, 450, 800 ..... 0, 100, 200, 400, 800 .... 0, 100, 250, 500, 1000 ... not available (or needed) .. not available (or needed)
Cyanuric Acid (CYA) .......... 0, 40, 100, 150, 250 ..... 0, 30-50, 100, 200, 300 ... 0, 30-50, 100, 150, 300 .... 30-100 every 10 ppm ... 20-100 every 10 ppm
*The resolution is dependent on sample size where a 10 ml sample has 0.5 ppm, a 25 ml sample has 0.2 ppm.
The newer wide-range test strips (
Wide Range Total Chlorine & pH - click on Specialty Test Strips tab) measure 0, 1, 5, 10, 20, 50 ppm Total Chlorine (not Free Chlorine) and 4, 5, 6, 7, 8, 9, 10 for pH.
A doubling of Total Alkalinity (TA) from 40 to 80 ppm (at a CYA level of 30 ppm) increases the saturation index by 0.37 which is large and not insignificant. A doubling of Calcium Hardness increases the saturation index by 0.3, though you can't even measure this with test strips (you can only measure Total Hardness which prevents you from calculating a real saturation index). A change in pH on the test strips of 0.4 is twice as large as that measured in the drop-based test of 0.2 and affects the saturation index by these same amounts. A misreading on test strips of a pH of 7.8 as 7.2, a TA of 80 as 40, and a CH of 250 as 100 (assuming you could even measure CH with test strips, which you can't), is an error in the saturation index of over 1.3 -- even a half-way misread would still be an error of nearly 0.6. A misreading with drop-based tests of the pH of 7.8 as 7.6, a TA of 80 as 70, and a CH of 250 as 240 is an error in the saturation index of under 0.3. If one has a vinyl pool with no need for calcium carbonate saturation and the CH is known to be low, then this error is not a problem.
Since the active chlorine level is roughly proportional to the FC/CYA ratio, with test strips a misreading of an FC of 3.0 as 5.0 and a CYA of 100 as 40 misreads an FC/CYA ratio of 0.03 as 0.125 which is a huge error that can lead to algae growth. With drop-based tests even using the 10 ml sample size, a misreading of an FC of 3.0 as 3.5 and a CYA of 100 as 80 misreads an FC/CYA ratio of 0.03 as 0.044 which is not that much of an error and would alert one to too low an FC relative to the CYA level.
One cannot do an accurate overnight chlorine loss test with test strips, especially when shocking the pool.
The drop-based test kits are easier to see as well since the FAS-DPD chlorine test goes from pink/red to clear as you add drops. The TA test transition from green to red is easy to see. The CH transition from red to blue is more difficult. The CYA turbidity test can also take a little getting used to.
Once you know your pool you usually test mostly chlorine and possibly pH so I can see that once you get to that point you can use test strips for a quick check if you've calibrated such measurements against a more accurate test, but should still probably do a more accurate verification (of chlorine and pH) once a week or so just to be sure. However, if you are keeping your pool on the edge of a minimum FC relative to the CYA level, the test strips could throw you off.
Richard