A simple sensitivity analysis can prevent a lot of quality issues

A basic sensitivity analysis can save considerable time. The other day I was working with another engineer who was working on a water filtration system that relied on passing household water through a sand filter. The aim was to have the water take as along as possible to move through the filter, to increase the filtration effectiveness, while still ensuring that the time taken to filter the maximum anticipated amount of water would not exceed statutory requirements. The simplest way to achieve this was to tune the flow rate by having it pass through an orifice at the bottom of the filter tank. The design would require no moving parts and the orifice could be moulded into the main body of the filter housing, further reducing the number of parts. This all seemed good. The orifice size was found by using a simple fluids equation that models the time taken to empty a tank through an orifice at the bottom and solver in Excel. As usual the answer in solver had around 14 significant figures so the other engineer took only the first two to specify the design. Just to be on the safe side I put the rounded diameter into the Excel spreadsheet to see what effect the rounding would have. To the surprise of both of us the effects of rounding made the design unsuitable. In fact, we found that simply changing the diameter by an amount representative of moulding tolerances produced an excessive amount of variability in the time taken for the water to empty. The design option was unviable. If we hadn’t performed this check, it might not have been until production that this problem would have been found. This simple example shows two things that I think are worth keeping in mind. First, it is always worth performing some kind of analysis at the start of a design to evaluate the effects of random variability. Second, the analysis does not need to be a full scale Monte Carlo, FORM/SORM or DOE type of analysis. Simply being mindful of the fact the random variability can be a problem and performing some kind of simple analysis during the early stages can save you a lot of quality grief later on. What’s more, it is within the capabilities of most of us so there is really no reason not to.