Testing pH of thick fat based dressings?

Trying to test the pH of a thick fat based dressing but I'm getting different pHs each time. I've tried three different pH meters (all calibrated) and would get pH 4.10, 4.34 and 4.44.

I also tried diluting the dressing with DI water to make it thinner (have been told that diluting with DI water shouldnnot affect the pH) but I would have a 0.20 difference between diluted and undiluted.

Does anyone have any recommendations on how to test the pH of thick fat based dressings?