you can buy them here
https://www.aliexpress.com/item/Heat-Meter-High-precision-DN20-Ultrasonic-Heat-Meter-Accurate-Measurement-Household-Heat-Meter/32955180217.html?spm=a2g0s.9042311.0.0.73bd4c4dbltopg
I never tried your suggestion as of yet was just about to hit the hay myself just thought I provide you with a link first. and I let you know what happens tomorrow