I do the pool test strips and you can buy them at your local walmart or any where that sells the pool test strips. You want to get the ones that test levels of the PH you can get a 5 way testing strips wich I paid $12 dollars for and you use the first and third square on there. Then you want to get distilled water which is really cheap and will last you forever. With this method the ratio is 6:1 meaning 6cc's of distilled water to 1cc of milk, if you cant get a full cc and you can manage to get .5cc's then you mix in 3 cc's of water. You want to use something like maybe a shot glass to mix them with or something really clean that wont interfere with the testing. I just took a leftover little plastic Garlic Powder bottle washed it out and cut it down in size to use it as my milking cup. It is really verry accurate. This is how you would read it:
If the calcium level (which will be the first square) reads less that 100ppm you can wait two days before retesting unless she is showing other signs of foaling readiness. Above 100ppm, but less than 200 ppm, check daily, in the evening. Once you hit 200 ppm, you've got colostrum. 90 percent will foal within 24 to 48 hours. Over 500 ppm, do not leave her unattended. Chemetrics is similiar. Test once per day until calcium levels reach 125. After 124 test twice per day, preferably morning and evening. When the calcium level first reaches 200 ppm, about 50 percent of mares will foal within 24 hours. Birth usually is very near when the mare's calcium concentration reaches 300 to 500 ppm.
The pH (which is the third square on the strip) is another good indicator of foaling readiness. It will be relatively unchanged at 7.0 or higher the whole time, then drop below 7.0 within hours of birth. Do not leave her unattended from this point on. Chemetrics does not measure pH, so it is a good idea to use both methods to really narrow down the timing of partruition.
I am using this method now and my mare is at day 302 and it is working fine for me thus far it's showing I have a way to go yet.. Hope this helps.