The operating instructions on page 10 (https://hammer.freshdesk.com/helpdesk...) state the following:
"Pulse: A 5kHz chest strap must be used. Reception of the pulse signal is signalled by a flashing heart symbol. if no chest strap is used, a "P" appears. The heart rate of "90" flashes. The +/- buttons can be used to set the maximum
value can be set using the +/- buttons."
Hammer offers the Polar chest strap heart rate monitor T34.

I used the Polar P1