Since sprint triathlons are not as long is the intesity usually a lot higher?

They are at 100% effort for sure. It's almost like running a 10k road race. I gauge my effort in a sprint by feel (perceived exertion level), while in a half it is so important to moniter the HR and maintain an even pace that will allow you to finish.

Yes, the intensity is much higher. The HRM is the perfect tool, as you easily can spend all your energy during the bike (and I guess during the swim..) and completely "bonk" (hey, you Brits, no laughing! I do know the other meaning of that word!) on the run. You need to be able to stay at or near max during the entire race.