Hi guys. Seems to me that there's a lot of ways of measuring aspects of the performance of a dust system - and that the choice depends a lot on what the objective in taking the measurement is.
Adequate CFM delivered at the point of collection in a hood that does a decent job of capturing flying dust is as David's method targets clearly the ultimate objective/the single factor that determines dust collection performance - all of the other stuff about duct and fan sizing and equipment pressure drops are only variables that play a role in determining this.
If you're selling/providing a system one major complication has got to be the likelihood that depending on which branch of the system and which machine is connected the resistance/pressure drops may be very different. e.g. it may be hooked up to a very long and bendy line of ducting, or the machine at the end has highly restrictive hooding - or not. Which means that by a CFM measure the system may end up looking good or bad depending on which line/machine it's hooked up to.
You can measure flow/CFM wherever you like in the system (below roughly 24in WG) and get reasonably accurate numbers, but the above is always going to kick in to mean that the figures may or may not be comparable with e.g. system specs.
Measuring pressure drop (across them) is the classic way of measuring how dirty your filters are. You probably can get a measure even when they exhaust to a room by reading the (positive pressure) at the filter inlet versus the zero in the space it exhausts into, but unless you can ensure a known air flow/CFM every time the numbers are not going to mean much. Meaning that the above question of which line and machine etc is connected is again a potential spoiler.
Motor amps (effectively the power drawn) can be an indication of the CFM and can hence can infer whether or not e.g. a filter is blocking. The problem again though is that unless the measurement is always made in the same situation (same line, no blocking - absolutely nothing changes except the state of the filter = same restriction/pressure drop except for the filter) it's not going to mean much. The related issue is that since there are three variables (amps/HP, pressure drop and CFM) involved you need one of the three plus the fan curve to extract an absolute value/measurement for the third. (calculation requires two of the three) Any of these be useful indicators if compared to each other - see below.
All this is a long winded way of saying that there is a reason for the the use stock test set ups for fan testing - specifically a given length of inlet duct of a given diameter with a given configuration of (variable) inlet. These deliver a known system curve and other consistencies, and hence comparable test results - normally fan curves that are reasonably good predictions of fan performance (CFM and pressure capability) when matched to system curves when designing systems.
It doesn't take a lot of thought to see that a fan maker putting out unreliable/inaccurate fan curves isn't going to be in business for very long - at least in industry anyway where engineering people use well established system design methods which use this information. Where hot air claims are quickly going to be quickly seen for what they are, leading rapidly to a court and claims for consequential losses.
Maintenance tends to be a different matter. Presuming no changes to a system, maintenance procedures normally entail taking pressure and amp readings in a known configuration (e.g. connected to a particular machine, and required that there have been no mods/leaks etc to the ductwork or machine hoods etc), and checking to ensure that they have not changed except within permitted limits. The point here of course is that these numbers are relative only to an original target set calculated or taken during commissioning - they are not taken to mean anything particular in terms of system capability.
Which brings us back to your issue Chris. Hooking a Clear Vue fan and cyclone up to a given line on somebody else's system should show up whether or not it's doing better (delivering more CFM) than the previous equipment, but the numbers are not in isolation typically going to be very comparable to specification data unless you have a good curve for their system as well to use in interpreting them.
If the requirement is tracking the state of filters then the route is probably to select a given line and machine hood, and establish a pressure drop (and maybe fan amps) figure for clean filters. Set the user up so that they know the figures will no longer work if they change hoods or ducting, and arrange that they repeat the reading at intervals in exactly the same way. (best to permanently install a Magnahelic meter or a U tube or similar if possible) That they change the filter if the numbers exceeds 'X'. (which number should be available from the manufacturer of the filters, but if not you it can be determined over time with reference to other measurements)
David's anemometer has to be a pretty good option for this purpose too, bearing in mind that there will be a bit of technique involved in using it.
Just don't expect the readings to be in any way very directly comparable with the fan specification unless as above you have a fan curve and good system information....
On woodworkers and not wanting to be bothered with the details. It's dead simple really I think. They either put a little time in to understand the basics - which actually are not complicated. If they can't be bothered the result is inevitably that they have to as above depend on others to tell them what to do. Which is (unfortunately) why the manufacturer's of hobby/DIY dust systems run riot with misleading performance claims and the like, and partly why even commercial woodworking has a pretty chequered history so far as the topic is concerned..
It's in the end an issue of motivation, and many people given the chance on issues that don't deliver immediate consequences will kick the can down the road rather than bite the bullet and solve the problem.....
ian