The perfect regulator for backmounted twins doesn't exist (anymore)
Scubapro makes great regulators, but has a conspicuous gap in its regulator lineup: a compact, lightweight, high-performance first stage optimized for backmounted doubles. Ironically, the company once manufactured an almost ideal solution: the MK21 with its optional fifth-port cap. An updated MK21 EVO or EVO 2, equipped with a factory-installed fifth port, would fill a genuine deficiency in today's technical diving market. The original MK21 combined Scubapro's proven balanced flow-through piston architecture with a remarkably compact body, two opposing high-pressure ports, and four angled low-pressure ports. Its optional fifth-port cap provided an additional axial high-flow outlet, creating an exceptionally versatile configuration without a swiveling turret.
The confined space behind a twinset manifold rewards short, compact first stages. Excessive length and poorly positioned ports complicate hose routing and introduce unnecessary strain. The MK21/5 addressed these problems elegantly. Its central axial port allowed the inflator and secondary regulator hoses to run naturally along the upper back without any strain at all, while four strategically angled peripheral ports accommodated the remaining connections. The result was a clean, streamlined arrangement without unnecessary bulk. A swiveling turret of the Mk25 & Co. contributes remarkably little to this configuration. Unlike stage or sidemount regulators, backmounted first stages remain stationary relative to the diver. Properly routed hoses have no meaningful need to change orientation during the dive. The turret adds length, weight, mechanical complexity, and another dynamic sealing interface without delivering a corresponding operational benefit.
Weight is important. On backmounted doubles, the first stages occupy the extreme headward end of the equipment assembly, immediately behind the diver's head. Two DIN MK21 first stages weigh approximately 1.4 pounds less than two Dive Rite XT1s. That is a substantial reduction in a part of the equipment configuration where weight distribution is particularly consequential. For technical divers already struggling with head-heavy trim, placing another pound and a half at the top of the cylinders is precisely the wrong approach.
The excellent Apeks DST 5-port, Scubapro MK25 EVO (and its Halcyon brethren), and others offer excellent breathing performance and flexible hose routing, but their swiveling turrets represent unnecessary bulk and length in this application. The Atomic Aquatics Z2/Z3 is arguably the closest conceptual alternative, with its balanced piston and fixed-port arrangement. Unfortunately, its elongated architecture and crowded port layout are exactly opposite to the compactness and distributed port architecture that makes the MK21/5 so attractive.
Other fixed-body regulators, including Scubapro's MK11 and MK17 EVO, Apeks DS4, and many others lack the axial fifth port that makes clean twinset routing so straightforward.
There are plenty of regulators that work on doubles. There are remarkably few designed around what backmounted doubles actually need.
A MK21 EVO should preserve the original's balanced piston design, compact body, opposing HP ports, and intelligently angled LP outlets, with a fifth axial port installed at the factory. The design priorities should be straightforward: minimize the weight and axial length, optimize port geometry for conventional twinset hose routing, maintain excellent breathing performance, and avoid unnecessary moving components. Scubapro does not need another swiveling turret regulator in their lineup. It needs a purpose-built first stage for the technical diver who values simplicity, compactness, and proper weight distribution.
The MK21/5 was an unusually intelligent, mature design. Scubapro should bring it back.