exhaust sampling

[AlexNeo]

hi!!

i am trying to measure PM emssions from a DI diesel engine. my experimental set-up consits of a vacuum pump which and a flow controller to draw in exhaust into the sample chamber of the PM emssions instrument. the set value for the flow rate of exhaust is 5 SLM. but the actual exhaust flow rate seems to be exceeding the set flow rate. a lot of exhaust is flowing through the sampling tube. i guess the pressure of the exhaust is too high. also the exhaust flow rate in the sampling tube varies with engine speed. how do i sample the exhaust so that the flow rate stays at 5 slm and the sampling method does not affect teh PM in the exhaust sample? i was thinking of using a flow meter before the sample chamber. but the flow rate will still vary with engine speed and i am not sure if the floe meter will affect the PM. is there a cheap and easy way to fix this problem?

thanks!
ales

 

[GregLocock]

It sounds as though the back pressure of the exhaust downstream of your sampling point is too great. You could put a small orific plate or a throttle flap into your sampling tube, between the flow limiter and the exhaust, or you could move your sampling point further down the exhaust, or fit a free flowing exhaust. Cheers

Greg Locock