Just about any liquid as all liquids conduct ultrasound. Gels are generally used because they stay where they are put. There are different types of couplant, water based which can be a problem on steel as unless cleaned off it tends to make the steel go rusty, oil based which is OK on steel but may stain plastics, and silicon based couplant for high temperature use say over 60 degrees C, 140 degrees F.
So in an emergency any liquid will do, a lot of people use spit.
One of the short-comings of multi-echo thickness gauges is that to get a reading you need several strong repeat echoes to come back to the probe (or sensor). This is OK when the metal is clean and smooth but if rusty of tarnished on either side it will severally attenuate the returning echoes. This problem results in a blank reading or - - - being displayed. Multi-echo gauges are often called "good news" gauges because they don't display anything when used on corroded and therefore thinning metal.
It is usually better to use a "first back wall echo" gauge like the Touchstone 1*, as this only needs one echo to get an accurate reading and therefore very rarely shows a - - - and so is much more realistic about the state of the metal.
Two ways; if the material is known i.e. brass or aluminium, then use our sound velocity table*, if it isn't known then take a know thickness of the same material, measure it's thickness (mike it up, from the word micrometer) and then use the Touchstone 1 to measure the speed (in this instance the words velocity and sound can be and are interchanged) by adjusting the velocity (marked VEL on the Touchstone 1) until the reading displayed is the same as the measured thickness.
ABS Classification Society Manufacturing Process Approved
ISO 9001:2008 Certification Number GB13487
IIMS Corporate Membership Number C508
NCAGE Number U0B22
NFPA Membership Number 2800960
FPA Membership Number 27135