The unit I believe you flew was the backpack unit, and yes, it was strapped to the back of the pilots seat. It was designed to be strapped to a grunt's back but that didn't work at all with the way wind currents behave on the ground in the jungle... A point man carrying a back pack detector was more likely to pick up his own squad and completely miss enemy troops right in front of him... and if anything happened, he was stuck with a then worthless hunk of metal on his back to serve as a target and jungle tangler... the pickup rod was actually intended to be mounted on the front of a rifle if you can believe that!
Yes, I remember that setup... Actually, I was trained with that setup when I first started. The new boxes were in, but Chuck and I did not have one in Dak To because the first one that the 4th ID had was flown out of Division Chemical in Camp Enari.. I only flew with the backpack a couple of times maybe. I recall my first solo with the equipment, ie the first time I operated it myself and called out the marks was with the old model (Chuck was still tutoring me) and it happened to also be my baptism by fire (that unforgettable sound of an AK-47!). I remember thinking at the time "you can stop firing now, we found you." which was pretty ridiculous if you think about it, but I was excited about sniffing my first sniff and figured the mission was over.
I think we got the second modern version soon after and quickly learned that it was much more effective than the old. It was much larger, weighing maybe 80 pounds and no strapping was needed. It just sat on the floor. That's what I was operating when we were shot down. I was concerned about leaving it in the ship because it was classified, but it was retrieved a couple days later and we put it back into action.
The backpack unit took one sample per second and routed the air through a single light defraction detection chamber filled with a super-saturated air sample that had passed through a "corona point" setup exposed to HCL (Hydro-chloric acid). The corona point acted as a catalyst that caused creation of Ammonium Chloride (NH4CL) from Ammonium (NH4) (a common human effluent) and Chlorine (CL)... that's probably more than you want to know, but it sets the stage for describing the technical improvements that got us into some serious sniffing... first the air sample frequency was increased to 5 samples per second (well, first the detection unit was taken off the back of a grunt and placed into a helicopter picking up air samples at the tops of trees after they had finished their swirling through the jungle, but you already know that)... then the number of chambers was doubled with one including the HCL and corona point and the other excluding that chemical reaction. This resulted in two different readings and the operator (me) was then able to make a subjective decision based not only on the actual readings, but the difference between the two... one was detecting NH4CL, which consists of condensation nuclei, and the other detected other condensation nuclei from other types of natural (e.g. decay, smoke,...) or unnatural (muzzel, rocket or engine exhaust, camp fires, marijuana smoke,... well, maybe that's natural, but you get the, uh, drift.).
The bottom line is that an experienced operator could "smell" something was going on. I was confident that I could ID as few as one or maybe a couple of bad guys if they had been in the area awhile (e.g. urea contains NH4, but also that ol' wacky tobacky let off some nice particles)... but the real hits were major troop movements preparing an offensive... they could fill the air with odiferous, tell tail signs... Interestingly, we more often took fire from onesey-twosey groups than large ones (unless there was a visual and we started it first which happened more frequently after we started going back in with Loaches blowing away the foilage with our prop wash - the S.C.O.R.P.I.O.N. modus operandi) because large groups typically tried to avoid detection. But I believe we sniffed out several large groups quite successfully... I'd bet that if all of the confirmed kills were totalled for the AO's I sniffed, the number would be in the thousands. Our job was to find them and then call in whatever we could call in and go onto the next one. The ground troops would go in, engage, and count the bodies after the smoke cleared... There were other means of detection to be sure, but snoopy could definitely find them where they didn't want to be found... and drawing fire was, well as they say, always a "smoking gun".
By the way... I think Warren was operating the old type model, perhaps even my "hand-me-down", or more accurately I guess Chuck Whalen's "hand-me-down" and my training wheels. He was solo, but had not flown many missions at the time he was downed. I think he started sometime in September or October after we picked up the new box. I sometimes feel bad about perhaps encouraging him because Snoopy operators were exempt from pulling duty and he hated KP! One time, back at base camp, I recall him getting off of KP and then listening to me tell war stories and pointing out to him that that's what I did while he was cleaning grease traps. I went back to Dak To with the new, more effective, box and he signed up for Snoopy duty.