Contributed by Nietzsche:
Something to note for all pH meters is the age of the meter; the gel seal inside the meter is only usually guaranteed to keep its seal for 2 years after being manufactured, although the gel seal may last much longer this.
Digital meters such as the pHscan 1 have the month and year of manufacture on the inside of the battery lid. I suggest checking it when purchasing as some hydro stores may have slow stock turnover.