A 'Dobson Unit' is a measurement of the thickness of what?
Dobson unit squared. Dobson unit The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated, and continues to be primarily used in respect to, atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer. The Dobson Unit is defined as the thickness (in units of 10 µm) of that layer of pure gas which would be formed by the total column amount at standard conditions for temperature and pressure
Ultrasonic thickness measurement standards relating to UTM and related techniques, see the article Ultrasonic testing. Ultrasonic thickness measurement In the field of industrial ultrasonic testing, ultrasonic thickness measurement (UTM) is a method of performing non-destructive measurement (gauging) of the local thickness of a solid element (typically made of metal, if using ultrasound testing for industrial purposes) basing on the time taken by the ultrasound wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauge. Ultrasonic waves have been observed to travel through metals at a constant speed characteristic to a given alloy with minor variations