usually I sit down and spell out the cheapest solution... but I have like six meters and I only use one... Hanna Combo Waterproof pH/EC/TDS/Temp HI-98129 Hanna Combo Waterproof pH/EC/TDS/Temp HI-98129 its 144 dollars... I know its a arm and a leg, but its worth it... PH/EC/PPM/temp compensation.... auto calibration... waterproof seriously the best pen thats not stationary... alot of people will offer cheaper solutions but nothing beats this... im telling you.. its a investment that pays for itself in the long run... quality
^agreed... I haven't had to calibrate mine in a month, still tests 6.99-7.01 OP, you can find them for $120 on google shopping