Comparison between the APEX WVR measurements and the PWV from tipper TA-3 for 2011–2014, when both instruments coincided in both site and time. The cyan circles are the PWV quantiles used in linear regression. The red circles indicate the quartiles (25%, 50%, and 75%). Error bars are y-axis standard deviation for each quantile. As noted in the text, TA-3 shows a small offset with APEX, which shows the measuring independence of the tipper instrument when the proposed conversion is used.