We're going to need a bigger hashtag (iii) - separate session hashtags for big conferences

Big conference, lots of people tweeting in the first place (and people at home tweeting back on the hashtag) and people tweeting from different sessions too - harder to keep up.

How do other large conferences / attendees / watchers handle this? How can filtering be semi-automated (as it is with the main hashtag)?

I did a spot of research (Google) and haven't really found a satisfactory solution. Clearly the obvious way to do it is to have double hashtags for each session, so if there are 15 sessions throughout a two-day conference you could have

#mainhashtag #sesh1
#mainhashtag #sesh2
...
#mainhashtag #sesh15

and so on, although you may well want something a bit more informative than #sesh1 (however as long as everyone knows what it refers to and it doesn't take up too many letters it really doesn't matter that much).

In terms of filtering, it works perfectly - if you search for two hashtags it returns just those tweets so you can easily pull out all of the tweets from Session 12. (NB It wouldn't work to have one hashtag that covered both, eg #mainhashtagsesh1 #mainhashtagsesh2 as that wouldn't let you pick out all of the #mainhashtag tweets as well.)

How successful is this in real use though? Google pointed me to the BlogHer conference where they tried this last year, but the secondary hashtags didn't get that much use - the tweets were tagged with the primary hashtag of course but people didn't use the second one. Brian Kelly's blog referenced a situation where the conference organisears had suggested secondary hashtags but the ones in their app differed from those being recommended in the pocket guide, so a bit confusing.

Is this a cultural thing? Is it that people just need to know about using two hashtags or see them in use, and it probably helps if it's driven by the conference organisers? Or do people not see it as something to worry much about?

Reading the rest of Brian's post indicates that others have had this conversation and most people don't (or didn't at the time of Brian's first investigations into this in 2009) like the idea of using a second tag. Anyway I recommend reading his post on this, I'm a fan of more hashtags for clarity but I also note his final point "It would, I feel, be unfortunate if valuable Twitter discussions were fragmented across different session hashtags."

My previous ramblings on this topic
We're going to need a bigger hashtag(s) (28 August 2009)
We're going to need a bigger hashtag(s) II (5 September 2009)