This is the Inclined Liquid Column Manometer. It is for use for the treatment of air, air conditioning, dust elimination, heating and ventilation. The measurement range is 20-0-20, 25-0-25, 30-0-30, 40-0-40, 50-0-50 mmCE.
Of all the inclined manometers I've ever encountered (and trust me, I haven't half seen some inclined manometers!), I can honestly say, with God as my witness, that this is unarguably the best. It is a stout and sturdy unit, designed to withstand even the most perturbation-inducing industrial environments, not flinching in temperatures hotter than Hades and colder than the hearts of the unrepentant. Despite its awe-inspiring ruggedness, it still shimmers with flashes of French flair. Top notch.