There’s a trick journalists use to get you to read their stuff. They set up a false but interesting premise. ‘The Beatles’ songs are underrated’ for example. In fact nobody underrates the Fab Four’s music, but the article can now be framed as an argument rather than an account. This gives it a bit of energy it wouldn’t otherwise have had. So I had better confess now, penetrometry isn’t so much underrated as hardly heard of. You can’t assume it is something a cosmetic formulator will know about.
This isn’t too surprising as it is really a technique that is used in the food industry, being originally devised for measuring the viscosity of butter. So it isn’t an obvious tool for a cosmetic laboratory.
What is Penetrometry?
The idea is simple. A cone is placed on the surface of the sample, a weight is placed on top of it and it is allowed to drop for a specified length of time. Once the time has elapsed the depth it has penetrated is measured. The stronger the texture of the sample the lower the number.
This simple technique has some advantages over more sophisticated instruments. For a start you can see exactly what is going on and you don’t need to analyse numbers whose meaning isn’t obvious. It can sometimes pick up changes that are not easy to notice with the naked eye. It is very repeatable between operators and between labs which makes it a good choice for a specification parameter.
Sadly this rather splendid instrument is not seen as much as it used to be, but it still has its uses. If you have a product that is difficult to measure by other techniques the penetrometer might work for you. Balms and lipsticks for example, don’t really work very well on rotary viscometers like Brookfields, or cone and plate rheometers.
At the very least, with a wide enough choice of weight you ought to be able to get a reading from just about any sample. This is not something you can say about every rheological technique.
How to do it
A penetration method needs to define the geometry to be used and the time it will be allowed to fall for, along with any other important conditions. So allow some time to play around getting the settings right.
You need to have a very good handle on just how repeatable your particular test is for your particular sample, so multiple determinations are a prerequisite.
So for example if you were investigating a balm the proposal might be that 5 determinations should be made at points approximately half way from the edge of the container to the centre spread out evenly across the jar. This will allow both the standard deviation of a set of samples and those from a particular sample to be calculated.
Importance of Variability
There are many sources of variation in penetrometer readings –
Temperature and operator variability will be familiar issues with any rheological reading, and most labs will have developed techniques for dealing with them.
Penetrometry is very dependent on how the sample is prepared. You need a surface wide enough for the cone to penetrate, and deep enough that it doesn’t hit the bottom. And you need a level surface. The starting point is where the cone meets the sample, so it has to be clear where the sample starts.
The generally used measure of the variability of a set of readings is the standard deviation. This can easily be found with a pocket calculator or as a function on a spreadsheet. Its precise statistical properties don’t need to be evaluated here, it is sufficient to note that the higher the value of the standard deviation the more variable the data. This is useful for two purposes. Monitoring the standard deviation can be a way of ensuring the testing is being carried out in an optimal way.
It also allows the inherent variability of a particular sample to be assessed. Ideally successive readings taken from a single jar would be consistent in the numbers they reveal, demonstrating that the product is even from top to bottom and from side to side. This being quantifiable via the standard deviation.
The value of all this diligent measurement is that it tells you quite a lot about the crystalline structure that is formed in the balm. All other things being equal, a balm with an even crystal structure will have a lower standard deviation than one which has crystals of all different sizes distributed in a non homogeneous manner.
Should You Get A Penetrometer?
Penetrometers can be bought very cheaply, but the cheapest ones are hard to use and the money you save in the purchasing might be lost in the extra time you use generating data. But having said that, if you have plenty of time and not much money, you can generate some pretty useful data with the most basic of equipment. But moderately priced automated machines are worth considering for busy labs. If your current rheological toolkit is adequate for your needs I don’t think a penetrometer is going to add anything to it. But if you’ve got products in need of control that don’t work with more conventional meters you might well find a penetrometer is just the ticket.
If you have a problem that you think penetrometry might help solve, get in touch. We may be able to help you.