But before I put the strimmer away, I thought I’d look in more detail about how to make the measurements that feature in the handbook which, as a reminder, stated that the typical weighted vibration (a_h) for this, rather budget model was 2.5 m/s² with an uncertainty of 1.5 m/s².  Of course, that data would have been provided by a notified body; a test laboratory ISO17025 ^[3] accredited to make such measurements and indeed they get a name check in the CE/UKCA declaration, but I thought how hard can it be to make a measurement? ‘Not very’ as I subsequently found out.  

Try before you buy!

A quick phonecall to Casella resulted in the receipt of a demonstration version of their HAVex vibration meter and they also pointed me to an independent review ^[4] which I found very helpful. A quick look on the back of the instrument confirmed it was to the correct standard, BS EN ISO 8041 ^[5] which means that it has the correct frequency weighting (Wh) according to the widely accepted measurement standard BS EN ISO 5439 [6].  You can’t just use any old vibration meter! I noticed that the calibration certificate was way out of date but that’s OK for a piece of demo kit but for a ‘proper measurement’ it should of course be in-date and ideally the instrument should be calibrated annually for compliance purposes.

As a reminder, the vibration magnitude (a_h) is ascertained using the formula a_h = √ (a²_hwx + a²_hwy+ a²_hwz), where a_hwx a_hwy and a_hwz are the root-mean-square acceleration magnitude measured in the three orthogonal directions x, y and z at the vibrating surface in contact with the hand and frequency-weighted using the weighting W_h.  

So, I’d already heeded my own advice from the last article ^[2] and purchased the HSE’s Guidance on the Regulations ^[7] and referring to appendix 2, I quickly set about fixing the sensor or as the previously mentioned review put it, “the magic happens in the Hand Arm Accelerometer–a small metal box that connects to the main body with a roughly 5-foot cable. That’s the piece you need to fasten to your equipment”. 

It is preferable to measure all three orthogonal axes x, y & z at the same time and fortunately, the HAVex has such a tri-axial accelerometer, screwed to a v-block fitting which is ideally suited to curved surfaces such as tool handles.  

Figure 1 shows the axes in detail (on another tool) while you can see in figure 2, that fitting was achieved on my strimmer’s handle using some heavy-duty cable ties although jubilee (pipe) clips would achieve an even more rigid fit and as such is a better practice.

Viewing Vibration

Once you’ve attached the accelerometer cable to the body of the instrument using the Lemo connector and given the HAVex time to ‘warm up’, it immediately displays current vibration on each axis although it’s advisable to give the meter 30 minutes to settle into the environmental conditions for the most accurate readings.

It’s powered by a rechargeable NiMH battery and the 2100 mAh battery lasts up to 10 hours of continuous use.

Depending on the level of vibration you expect, you need to select an appropriate measurement scale. Low reads from 0.050–200.0 m/s² and high reads 0.500–2000.0 m/s² with both under and over range indicators. When you’re ready to start recording, simply hit the “OK” button. Hit it again when you’re finished. You can store up to 992 measurements in the flash memory.  

To get the best results, record for at least 30 seconds if you are doing a static test but longer measurement runs can be achieved to get a more representative exposure of actual use. Pressing the scroll button (button top centre) lets you cycle through several modes.

• A_rms: W_h weighted, root mean square acceleration
• A_eq: Time-averaged and W_h weighted acceleration
• A_max: Maximum A_rms level
• Peak: Highest W_h weighted instantaneous level
• Vector: Vector Sum (tri-axial) calculation from each of the orthogonal A_eq values
• Exposure: Point system feedback for UK guidelines

References

1.  What is HAVS, and why is it important? (casellasolutions.com)

2. How do you measure HAV? (casellasolutions.com)

3. ISO/IEC 17025:2017 General requirements for the competence of testing and calibration laboratories

4.  Casella HAVex Vibration Meter M104006 Review - Pro Tool Reviews

5. BS EN ISO 8041-1:2017 Human response to vibration — Measuring instrumentation — Part 1: General purpose vibration meters

6. BS EN ISO 5349-1:2001 Mechanical vibration. Measurement and evaluation of human exposure to hand-transmitted vibration General requirements

7.  The Control of Vibration at Work Regulations 2005:  Guidance on Regulations.  HSE publication L140, second edition 2019