well, why don't you buy a PPM meter, actually, this is commonly and properly called/known-as a
TDS meter (Total Dissolved Solids)
a TDS meter is powerful and very useful because it is NOT designed to specifically measure Chlorine Dioxide or to measure any specific chemical, but rather it is designed to measure how MANY particles are there in your **WATER** that are NOT pure water itself !! so if you MIX your water with Chlorine Dioxide, the meter will DETECT how many particles {of Chlorine Dioxide} are MIXED into your water, thereby giving you a measurement of the concentration/dilution of Chlorine Dioxide that is PRESENT in your WATER !
here in South Africa, i am able to buy one for about $7. if you are a skeptic of READINGS/Numbers like me, you can buy TWO of them to get a confirmatory / independent reading from a SECOND/DIFFERENT one
LOL
e.g.
www.bidorbuy.co.za/item/469111330/TDS_Meter_Tester_LOCAL_STOCK.html
they are so common i'm pretty sure you'll find them ANYWHERE in the world !
there are sophisticated ones that cost MORE money, and they can measure
1. PPM (Non-water particles dissolved/mixed into your WATER), as well as
2. pH level (acidity/alkalinity of your water), as well as
3. Temperature (of your WATER)
but for your purposes (MMS and specifically Chlorine Dioxide), you really do NOT need to buy any such fancy/sophisticated ones. ALL you want to know, assuming that you have used the recommended "distilled" water to prepare your Chlorine Dioxide mixture, ALL you want to know is that the amount of Chlorine Dioxide particles in your mixture is between 3 000 PPM and 3 700 PPM. you want to know that your mixture is WITHIN RANGE (Not too strong and Not too weak, but a little weaker, e.g. 2 990 PPM is always SAFER than too strong = e.g. 4 600 PPM, but you're STILL Not going to die or get sick from 4 600 PPM . . . it is still a concentration that is FAR from toxic, toxic would be somewhere in the range of 60 000 PPM upwards !!)
so DON'T waste your money on buying an expensive one thinking that it would be "useful" for you to also measure the water pH level and water temperature ! the pH of your water really does NOT matter for MMS/Chlorine Dioxide, if the meter was perhaps measuring the pH in your BODY or the pH of your BLOOD then such a pH reading would perhaps be of some use to you, so just a plain cheap TDS meter (or TWO of them) that measure (give you a reading of) the PPM of your Chlorine Dioxide mixture is Simple, Enough and PERFECT !!
the thing about test strips is that they give a RELATIVE reading (color-coding). such a color-code reading could be OUT of RANGE by over 500 or 800 units, so I find them a little inaccurate, because again, i am a skeptic when it comes to numbers/readings ! LOL
i do accept an error-margin, but when i am measuring something, i prefer to see an ACTUAL quantity or absolute number of THINGS found/detected inside my water/mixture on the reading, Not a color-code that i must now go somewhere else to match it BY EYE to some OTHER reference color !