a bearing 60 a washer 62 and nut 64 these elements form a seal with the housing 54 to minimize the prior art shortcoming of leakage of toxic gases from the orifice restriction area 52 attention now returns to the orifice restriction area 52 as depicted in fig4 the orifice 40 is defined by a toroid 66 which is concentric with the exhaust line 32 the toroid 66 has a rounded inner surface 67 fig4 simply depicts the concentric toroidal member 66 in cross section in contrast to the rollers of the prior art this toroid member 66 is relatively easy to manufacture referring now to the left hand portion of the cross section of the concentric toroid member 66 a spring 68 is depicted as being positioned over the rounded inner surface 67 the spring is secured at its ends to shafts 58 which ride on bearings 60 which are attached to the housing 54 as previously described preferably the top and bottom shafts 58 are aligned so that spring 68 is in alignment with the central longitudinal axis 34 of the exhaust line 32 in one example the dimensions of toroid 66 and spring 68 were as follows the thickness of the toroidal member 66 was approximately 1 250 inches and the rounded surface 67 had a radius of approximately 0 625 inches.