HAV you got Enough Measurements?

How many Hand-Arm Vibration (HAV) measurements should you take to find a ‘good’ tool vibration level?

Hand Arm Vibration is undoubtedly a serious issue and can lead to severe disability in the hands if exposure is allowed to go unchecked. Assessing the risk of injury according to the Control of Vibration at Work Regulations 2005 is the best way to ensure that you can prevent employees being overexposed.

To create such a risk assessment requires you to know the vibration level of each tool being used by a worker along with the time they are exposed for. The vibration level can be found by looking at existing databases to find similar tools in similar situations, but there is nothing quite like knowing the real levels in your workplace by taking measurements. This will also be of huge benefit if you ever face a personal injury claim from one of your employees.

Taking measurements with a modern vibration meter is quite straightforward, but there remains the question, ‘How many measurements should you take for each tool?’.

The HSE guidance document, L140 states, ‘A single measurement for a machine, an operator and a task provides limited information on vibration risk.’ This is because of the variability you can expect to see with almost any task. When measuring, the guide recommends that you should measure several operators, each working across a range of common operating conditions. In other words, you should try to account for as much of the variation as possible.

Achieving the above can be challenging, especially if you are trying to measure a lot of tools in one session to tag those tools with a vibration level. It might also be that you do not have several operators available or the time to take so many measurements. In this case, so long as you are aware of the potential variability, and you can capture some of this by working with one operator, then you should be able to get good data. Taking at least 3 or 4 measurements in this situation will help to highlight any inherent variation in the task. Don’t forget that you should measure anywhere a tool can be held and not just the trigger, so you can select the biggest one.

Once you have a set of measurements for the task, the HSE suggests that you should take those data points and calculate the mean plus one standard deviation (you might need a spreadsheet for that!), or the upper quartile (also known as the 75^th percentile). Handily, if you have taken FIVE measurements and you put them in order of size, the 75^th percentile will be the second largest one – after all, if you have a number, then why mess about with it. One caveat here is that there’s more than one way to calculate an upper quartile and you will get different results – I will leave you to Google that one if you’re minded to!

This is the Excel formula for the mean plus one standard deviation:

• AVERAGE(B1:B10)+STDEV.P(B1:B10)

This is the Excel formula for the 75^th percentile:

• PERCENTILE.INC(B1:B10,0.75)

In both cases, B1:B10 is where you put your measurements

There is room for common sense here, such that if your measurements are all within a few points of each other, then just take the largest one. Conversely, if you have a serious outlier, or if the measurements are all miles apart, then you might be well advised to re-investigate that tool and probe further into why there is so much variability.

In summary, you should always take more than one measurement and the more, the better (we would suggest at least 3 or 4). Once you have the numbers, look at them with a critical eye and try to work out what is going on. What number you take to put onto your tool must represent what is likely to be happening in the real world, so take that into account whichever method you use.