I use a keg for my HLT and I put the thermometer about half way between top and bottom. This means I need a minimum of 7.5 gallons in the HLT for the thermometer to read the temp. If I'm doing a 5 gal batch I may only need 7 or 8 gallons of water total. So after I add my 3 -4 gallons of strike water my water level is now below the thermometer. This means I have to add more water and heat up this entire volume to get my sparge water. If I had to do it over again I would put my thermometer as low as possible without having it to close to the bottom of a direct fired vessel as others have stated.