A measuring head has a base and extending spaced apart vertical sidewalls that form a slot into which a tube through which a fluid flows is placed and at least one sensor element mounted in at least one of the sidewalls to detect a characteristic of the flowing fluid. At least one through passage is formed in each of the sidewalls that are aligned opposing each other to form a pair of passages through which a strap is passed above the tube in the head slot and a fastener holds the strap bound to the sidewalls to retain the tube in the slot. One embodiment has a pair of the sidewall slots on each side of a sensor element mounted in a sidewall. A passage also can be provided in the head base through which a strap can pass and then be passed through a pair of the sidewall passages.