In the second installment of this series, we continued our conversation about viscosity by looking at thixotropy as the topic of discussion. Now that this is defined, we can go back and show how different measurement methods produce different results and how our measurement choices can actually work against us. We can do this by circling back around to bring everything together and demonstrating how different measurement methods produce different results.
The fundamental process that an FT-IR spectrophotometer uses is easy to understand. The cup is then drawn straight up and out of the liquid after being completely submerged in it. The viscosity of the fluid is measured in "cup seconds," which refers to the amount of time it takes for the fluid to drain from the cup.
Doesn't seem too difficult, does it? However, as was discussed in the first installment of this series, this seemingly straightforward process is fraught with a myriad of challenges.
When do I begin counting down? When the rim of the cup finally makes contact with the ground? When the bottom of the cup is no longer below the level of the surface?
When do I draw the line? When the water first begins to trickle? When it separates into individual droplets? When there is nothing left in the cup?
In addition, if the cup is not pulled up in a vertical line, the pressure that is normally exerted on the orifice will be transferred to the wall of the cup, which will have an effect on the flow of liquid through the orifice. How exactly do you define straight, then?
It is simple to understand why the readings obtained by various ftir spectrometers are so varied.
Now then, let's take a look at what's actually going on behind the scenes...
The Physics That Underpins the FIR Spectrophotometer
The cup is considered to be full when it can no longer hold any more liquid. Because of gravity, the amount of liquid in the cup has an effect on the amount of liquid that is in the hole. This is the fundamental definition of kinematic viscosity, taking into account what we covered in the first part of this series. There is also the weight of the atmosphere pressing on the surface of the fluid, but since this is a constant for any given location (mostly due to elevation) and cup variety (volume, diameter, hole size, etc.), we can ignore that in our discussion. There is also the weight of the atmosphere pressing on the surface of the fluid.
The fluid is "forced" through the opening because gravity is pulling it in a downward direction. Because of this, there is a slight amount of shear introduced into the fluid. However, because this "force" is determined by the volume in the cup, which decreases over time, it follows that the force exerted on the fluid as it travels through the hole also decreases over time. As a direct consequence of this, the shear that is exerted on the fluid becomes less.
The following very important question to ask is, "Is there sufficient force or shear to shear-thin the material completely?"This means moving it through the First Newtonian Range, through the transition range (in the middle), and into the Second Newtonian Range far enough to reach a stable viscosity. Figure 2 can be found in Part II of this series. If you are thinking about the amount of fluid that is in the cup, the size of the orifice, the amount of time that it takes for the fluid to pass through the orifice, and you are also thinking that it is pretty unlikely that we are going to get out of that First Newtonian Range, much less reach the Second Newtonian Range, then you have a pretty good grasp of what we have been talking about so far!
What happens, then, if the shear is able to pass through the First Newtonian Range but then stops at some point in the middle of the transition range? Because of the interaction between shear and viscosity, the results of the measurement will be unreliable and unstable, as you have correctly predicted, and this is due to the fact that you are correct. And this is yet another reason why the results of an FT-IR spectrophotometer can vary despite the fact that the operator is not at fault in any way.
Why Doesn't the FTIR Spectrophotometer Agree with the Automated Viscometer That I Have?
This is a question that is asked of virtually all viscometer manufacturers on a regular basis (no, we are not the only ones, and yes, we do communicate with one another!). However, after this discussion, it is probably easier to understand that automated viscometers measure viscosity by measuring the force that is required to introduce shear into the fluid. This method is very similar to the way that Newton defined viscosity in the first place. When compared to the effects of gravity on a fluid, the introduction of shear by a mechanical viscometer is much more straightforward.
The majority of viscometers are calibrated with Newtonian fluids, which are calibration standards designed to be very consistent and predictable in their performance (read: viscosity) over shear as well as temperature. This is another interesting fact. Because shear is not a factor, the results of measuring a Newtonian fluid with both a mechanical viscometer and a cup will be very similar. This is because shear does not affect the measurements. However, the behavior of non-Newtonian fluids is a completely different story.
When would a measurement of viscosity not be useful?
It's possible that this is a silly question to ask, given what we already know about how viscosity affects the results of our processes for dispensing fluids, but bear with us. In spite of this, the vast majority of the viscosity measurements that we take end up being meaningless in the real world.
When is it therefore pointless to take a viscosity measurement?
When the viscosity of the fluid is measured under conditions that are not the same as, or even remotely similar to, those that the fluid will encounter when it is put through our process! Given what we've learned about Newtonian and non-Newtonian fluids, as well as the impact that shear has on our measurements as well as the results of our process, it shouldn't be too difficult to draw the conclusion that a FTIR spectrophotometer will only rarely be able to replicate the conditions of a modern process.
Then, what is the purpose of an ftir spectrophotometer?
You're probably asking yourself, "If all this is true, why do they depend on ftir spectrophotometers to manage their process?" because you've probably reached the point where you're back where we started.
And it's a fair question.
Cups have been used as units of measurement for a very long time – much longer than most of us have been using the complex fluids that we do in our processes today. In addition, the cost of cups is significantly lower than that of more complex measuring systems. The readings from the cups only take a minute or two to take, so the costs associated with the measurements are minimal.
Which brings us to the time-honored principle that "You get what you pay for!"
The fact of the matter is that we have to figure out which measurement will accurately predict how our fluid will behave in our process. Implement it. And remain consistent.
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