Please disregard this post - wrong forum....oooops, sorry.

Under the IPC & IRC the maximum length of a fixture arm can be computed by dividing the diameter of the drain by the pitch. If an 1-1/2" line is installed with the code standard 1/4" per foot pitch the maximum lenght would be 1.5 / .25 = 6ft but in this instance the line has a 1/2" pitch so the maximum allowable length would be 1.5 / .5 = 3'. (The bottom of the drain line at the trap weir may not be higher than the top of the line at the vent opening). Both the configurations illustrated above would not pass code for a number of reasons. The simple solution would be to remove the drum trap & extend the length of the tub drain tailpiece. Code will allow up to 24" vertical drop from the tub drain opening to the water level in the trap. Attach a P-trap and run the line with a 1/4" per foot pitch. A wye can be installed on the drain line with the side input angling downstream. You can then put a street 1/8bend on the wye side opening input, which would turn the side opening vertical and attach an AAV where the base of the AAV is 4" above the drain line. REF-IPC 917.4 Location. The air admittance valve shall be located a minimum of 4 inches (102 mm) above the horizontal branch drain or fixture drain being vented The air admittance valve shall be located within the maximum developed length permitted for the vent. The air admittance valve shall be installed a minimum of 6 inches (152 mm) above insulation materials.

Thank you very much. The reason I was going to try two traps is the discharge tube that came with the disposal does an immediate 90 deg and discharges downward... which makes it very difficult to connect to the sink drain with enough height to fit the trap. I guess I should buy a straight discharge tube and then I think I can mate it up with the main sink drain using a sanitary tee, if that would be appropriate. I'm trying to improve upon are current design, which clogs frequently. We have the standard white thread-together "sink plumbing" and the disposal discharges horizontally and connects to the sink drain with a 90 deg fitting that has a separation plate in the middle. This is very restrictive. I would like to do it all in black ABS 1-1/2" piping with suitable couplers here and there so it can be disassembled. Regards, Tom

The lack of a vent is allowing the water to be siphoned out of the S trap when the sink is drained. If you can't install a proper vent line, I would use an 'air admittance valve' in the drain line to break the siphoning action. And, yes, that up-sloping drain line should be corrected.