C2) Do all machines give the same answer?
The simple answer is no. In dry conditions the differences tend to be relatively small, but significant differences can be found in wet readings.
The reason for this was discovered about 15 years ago by the UK’s Health & Safety Laboratory and the designer of SlipAlert, who were both working independently on the problem.
C3) Why do machines give different readings in the wet?
The simple answer is that the film of water that becomes trapped between the slider and the surface being measured produces an uplift on the slider and reduces the effective friction between the slider and the surface.
Understanding precisely how a film of water produces uplift and how it is going to affect the machine is much more difficult. This is because the uplift depends on several factors, all of which need to be taken into account when designing the machine. What the machine designer has to do is to ensure that the combination of those factors on his machine produces exactly the same proportional uplift as a pedestrian experiences when their heel slips across the same surface.
C4) Are there any machines that have been specifically designed to react correctly to the hydrodynamic film?
As far as we are aware, the only machine that has been designed to react to the hydrodynamic film in the same way as a pedestrian’s heel reacts on the same surface is SlipAlert.
The TRL Pendulum was not designed to do so, but by pure chance does react correctly. It is for this reason that measurements using the Pendulum have been found from many years experience to correlate with actual slipping accidents.
As one would logically expect, SlipAlert therefore correlates with the Pendulum (see graph).
C5) Could I convert the readings from another type of machine to give the same readings as a Pendulum or SlipAlert?
Unfortunately, because the matter is so complex, the answer is no. The problem is that two surfaces tested by machine A may seem identical, but the Pendulum and SlipAlert will indicate that they are quite different. Conversely, two surfaces that give very different readings on machine A may in fact have very similar slipping properties. There is simply no easy means of unravelling the complexity of the way that the uplift is produced and how it affects the final reading to make one machine correlate with another by use of a simple factor.
C6) How important is the slider?
The slider on the machine is designed to replicate the heel of the pedestrian. If you are investigating an accident, it is helpful to use a material that is similar to the heel on the shoe worn by the pedestrian who slipped. In simply monitoring a floor or testing it to see how it is likely to behave in use there are two schools of thought.
One school of thought suggests using a standardised rubber such as Four S. This means that your results can be compared directly with someone else’s. Whilst this can be very useful in terms of standardisation, there are three problems with Four S. The first is that it does not wear well and needs to be reprepared and replaced quite frequently. The second is that in the wet it does need to be very carefully prepared to ensure it is quite smooth, otherwise one gets an occasional significantly different reading. The third is that it represents a relatively good heel in terms of its performance. This means that one cannot tell whether the floor will be satisfactory against the whole range of heel materials that are considered to be acceptable in respect of their frictional qualities. Ideally, the standard heel should be at the lower end of the range rather than towards the top end; as yet no one has put forward such a ‘standard’ slider material.
The second school of thought is to test the floor using a range of heel materials. This can be quite illuminating, but because there are only two rubbers that have been standardised it can lead to disagreements.
C7) What about roughness measurements?
There are many types of roughness including 11 different types of micro roughness. Rz micro roughness alone is a poor measure of slip resistance, cannot detect contamination and cannot detect changes to slip resistance.
The overall roughness of the surface certainly has an effect on friction in both the wet and dry states. If one really understands the way that it does so, particularly in the partially lubricated wet situation, it will become apparent that measurements of roughness are not likely to lead directly to the slip resistance of the surface or be such that one could reliably make specific judgements about the surface using that data alone.
When the concept of the hydrodynamic film was first put forward, the measurement of roughness seemed a logical step forward that might assist in the measurement of slip resistance. However, if one goes back to first principles and starts manipulating the mathematical equation that relates to lubrication and at the same time considers how friction is itself developed, one begins to realise that although a roughness measurement of the surface might give a not unreasonable indication of the wet slip resistance, equally it might give a totally incorrect indication. Dr Malcolm Bailey has tested several hundred surfaces and found this to be true. The latest version of the UK Slip Resistance Group Guidelines gives the clear warning:
‘It should be noted that the micro roughness of one surface may have the same numerical value as measured as that of another surface but be quite different in profile, as illustrated by the difference in profiles shown in Figure 1.
For this reason, roughness readings should not be used in isolation but linked with other salient information, such as Pendulum readings from the particular material being tested or specified.’
‘It is imperative that roughness measurements should not be relied upon of themselves to judge the likely slip resistance of the floor. Some reliable means of directly measuring the slip resistance in wet conditions should always be used in conjunction with roughness measurements.’
Note: the 2011 version of the UKSRG Guidelines omit these warnings on roughness. You may wish to question members of the UKSRG as to why they lend tacit support to the use of roughness for monitoring slip risk. the UKSRG members are aware that rRz micro roughness does not measure slip risk. The UKSRG know that Rz micro roughness cannot be useful for monitoring slip risk and cannot detect contamination or change to slip risk.
C8) Does the ramp test give credible results?
The ramp, for those who have never seen one, is a platform on which a floor specimen is mounted and upon which a test subject walks in a downhill direction. The ramp angle is steadily increased until either the subject feels unsafe or slips. Whilst it may seem to be a very reasonable way of testing floors, it must be understood that the stepping method used (of necessity) does not correspond particularly well to normal walking with ‘heel strike’ and ‘push off’. Although the subject is facing downhill at all times he walks in short half steps, usually four half steps down, then four half steps up (walking backwards).
There are four ‘protocols’ currently used:
The German DIN protocol uses oil as the lubricant and the test subject wears boots with rubber heavily profiled soles.
The UK (HSL) protocol uses plain water as the lubricant and normal type shoes with Four S rubber soles and heels.
The UK (HSL) protocol no. 2 again uses plain water, but the subject is barefoot.
The fourth protocol is the CEN (European Standards) barefoot ramp test, which again uses bare feet, but a wetting agent is put in the water. Because a fixed amount of wetting agent is used, the results of this test vary from laboratory to laboratory depending on the hardness of the local water.
The German ramp test correlates with no known portable device and while it may be suitable for industrial situations, many consider it has limited value in other situations. The results are declared as ‘R’ numbers. These start at R9 and go up to R13 (there is no R1 to R8). R9 is the lowest classification and indicates a floor with minimal oil wet slip resistance.
The other protocols either declare a maximum ramp angle or convert it to the coefficient of friction using the tangent of the angle (tan α), i.e. 15 degrees would give µ as 0.27.
Limited tests have indicated a not unreasonable correlation between the HSL protocol and the TRL Pendulum (and thus SlipAlert) when both have been set up with Four S sliders. Also, in the barefoot HSL protocol, a correlation has been noted with the Pendulum fitted with a TRL (soft) rubber slider.