You should recalibrate your meters every 2 weeks to maintain acuracy. When I calibrate my meters I pour a small amount into a new clean plastic bag to use so I do not contaminate the bottle.
https://www.horticulturesource.com general hydroponics ph 7.0 cal solution 8oz
https://www.horticulturesource.com general hydroponics ph 4.01 cal solution 8oz
pH 7.0 Calibration Solution 8 oz - Access Hydroponic
pH 4.01 Calibration Solution 8 oz - Access Hydroponic
https://www.horticulturesource.com general hydroponics ph 7.0 cal solution 8oz
https://www.horticulturesource.com general hydroponics ph 4.01 cal solution 8oz
pH 7.0 Calibration Solution 8 oz - Access Hydroponic
pH 4.01 Calibration Solution 8 oz - Access Hydroponic