Sampling Shmampling

I get a lot of questions on calibration, but today, I want to talk a little bit about sampling, since it is absolutely key to take proper samples when you’re trying to build a calibration.

Why is sampling important?  It’s important because you are trying to get sense for what an instrument is telling you when it indicates something.  The only way that you can do that is to take physical samples of your process and compare the instrument’s responses with your manual evaluations of those samples.

There are two pieces to this.  Well, three actually.  One, of course, is the method someone uses to analyze a sample.  This is usually reasonably well documented, at least for the “Official” test.  TAPPI, for example, will publish an estimate for the repeatability of every lab test it recognizes  (the repeatability for the official consistency test, by the way, is 10%, which, to my way of thinking, ain’t so great. But hey, it is what it is).  Of course, if you’re not following the official procedure exactly, then your repeatability might not be as good as that.   I’ll talk more about this in another post).

The second aspect of this is just how representative the sample that’s being analyzed is of the process it was taken from in the first place.  If the sample you’re extracting from the process isn’t representative of the process, then you are basically analyzing something that really doesn’t mean much. Put another way, if your samples aren’t representative, then you are wasting your time with your calibrations.  You won’t get very far at all.

What do I mean by representative? 

Since it’s impossible to analyze absolutely all of your stock, you have to estimate what’s in your line by analyzing just a little tiny bit at a time – this is the sample that I’ve been talking about. If a sample is representative, then it means that you could have taken any number of samples in the same way and gotten roughly the same result. Of course, keep in mind that you won’t ever get absolutely the same result because the process isn’t homogenous, and no sampling method is absolutely perfect, but you can get reasonably close if you try.  Put another way, your samples will likely be close to the average of the stock in the line, and have a narrow two sigma.

If, however, the sample isn’t representative, then that means that you could get any number of widely different results each time you captured a sample.  You wouldn’t get samples close to the average, plus they would probably be biased one way or another, and your two sigma would be wide.

   

A good sampling regime is one in which a proper sampling valve is installed in a straight length of pipe of at least seven pipe diameters.  Valves are allowed to flow for a while to ensure the sampling line is flushed of any residual stock. 

A bad sampling regime would be something like a ball valve that’s just welded on the side of a pipe somewhere.  There is no thought given to the nature of the flow in the line at that point.  Is the stock flow stable, or is it turbulent?  Has the stock dewatered?  Was there some left over stock still in the sample line from yesterday or last week before you captured it?

And it’s not enough to ensure that your samples are merely statistically representative of the process.  You also have to ensure that both you and the instrument are actually looking at the same stock.

I really think that most people simply don’t pay enough attention to this last point. 

Why do I say that?  Here’s an example. 

I was once asked by a customer to help calibrate some of their equipment because they were having all sorts of problems and disagreements with their results.  They thought the problem was with the instrumentation.  As it turned out, it wasn’t the equipment at all, but with how they were sampling their stock.   

The equipment was installed in a stock line which then dumped into a chest.  The samples, however, weren’t taken from the same stock line as the instrumentation was in.  Instead, the samples were taken from the discharge of that chest.  The chest had a residence time of about 30 minutes, so whatever came out of the discharge was stock that had been mixed for thirty minutes.  There was no way that the lab could ever analyze the same stock that the instrument was exposed to. 

This situation was set up to fail.  It was guaranteed that the lab analysis and the instrument would always disagree because they were measuring two different things at different times.  Any effort expended under these conditions is a waste of time, because as the man from New England said,  “You just can’t get there from here”. 

When you install a sampling valve, you want to take care that it is close to the instrument that you are trying to calibrate so that you can be sure that both you and instrument are analyzing the same stock.

Let me also make the point that you shouldn’t balk at the cost of the sampling valve.  Yes, it’s more expensive than a ball valve, but it makes absolutely no sense at all to save a few hundred dollars on a sampling valve when you’re trying to calibrate a $50,000 instrument that will hopefully have a multimillion dollar impact on your process.  Saving those few hundred dollars may completely invalidate the whole thing.

So, here’s what you should shoot for when sampling your process for an instrument.

1)      Select a proper sampling point

a.       Site the sampling valve close to the instrument for which it is intended.

b.      Ensure that you will be sampling the same stock that the instrument is analyzing

2)      Ensure you are getting representative samples

a.       Use a proper sampling valve

b.      Install in a section of straight line at least seven pipe diameters long.

c.       Install in the side of the line, or according to the manufacturer’s recommended method.

d.      Open the valve fully when preparing to capture a sample.

e.      Allow the valve to flow to ensure that any residual stock is cleared from the line before capturing a bucket full

3)      Bracket the instrument analysis with Samples

a.       Capture samples of stock before during and after the instrument has completed it analysis.

                                                               i.      Capture a bucket of stock

                                                             ii.      Start the instrument analysis

                                                            iii.      Capture a second bucket of stock

                                                           iv.      Let the instrument complete its analysis

                                                             v.      Capture a third bucket of stock

Yes, it’s a lot of work, but the result is worth the effort.  You'll have captured meaningful samples that you can then analyze to build you calibration with.  

Drainac Performance Maintenance Program

I visit a lot of mills around the country and one thing that's common among just about all of them is that nobody has all of the maintenance people they'd like to have or need. 

That means that key equipment doesn't always get the attention it needs, when it needs it. In the best case, the equipment doesn't work as well as it should for as long as it should.  In the worst case, you get equipment that doesn't work at all and a hefty price tag to get it back into service.

 

We’d like to help.

We have just announced a new program for our valued Drainac customers, <drum roll> the Drainac Performance Maintenance Program.

Under this new program, we'll send one of our qualified Drainac technicians to your mill on a regular basis to assess your system and make sure it’s working as it should.  We’ll repair or replace any necessary parts, recalibrate the system if needed, provide training to your people and whatever else we need to do to help you get the most out of your Drainac freeness analyzer.

The program covers all necessary travel costs and service labor, plus you’re entitled to a 20% discount on any parts that your system needs.

We have three different levels of Drainac services available to you, or we can craft a custom one to meet your specific needs.  Contact me at your convenience for details.  I’ll be happy to put together a program that will work for you.

Darth Flow Rate

A customer  - I’ll call him Bob - recently asked me about his consistency transmitter. Bob said that they’ve noticed that their basis weight has been drifting, but that their consistency measurements were basically straight lines.  He was wondering if something might be up with his transmitters.

Good question, that.  The answer is probably both yes and no.

The answer is No because there is probably nothing wrong with Bob’s transmitter. 

The answer is Yes because Bob’s transmitter is likely responding to something other than variations in consistency in his process.

I can hear you asking, “What the ____?”.  That’s what Bob said, anyway.   Here’s something important to remember:

No consistency transmitter of any kind actually measures consistency directly. 

They all measure some other physical parameter which has been shown to have some kind of a relationship to consistency.  In the case of mechanical transmitters, they almost always measure force.  What kind of force?  Well, that depends on the physical design of the part that goes into the stock line.

If you’re talking about a blade style transmitter, the physical force of interest is Apparent Viscosity.  All the fancy talk aside, the apparent viscosity is a way of referring to the thickness of the slurry, and it’s easy to see how that “thickness” would relate to consistency.

The blade measures the force of the slurry as it passes the blade.  The higher the consistency, the more force which is transferred to the sensor.  There’s always a minimum amount of fiber that has to be in the line.  Less than that and you just won’t get enough force on the blade to measure.

That said, it’s just not the consistency that’s in play here.

In order for the blade to measure the apparent viscosity, it has to be moving.  If the slurry is at a standstill, the sensor will measure zero force.  It usually has to be moving at some minimum rate in order for the system to register the force, too.

Now here’s the key…

As the flow rate continues to increase, so too will the force measured by the blade, even if the consistency is constant.

Put another way, the blade doesn’t just respond to consistency.  It also responds to flow rate.  This is true for all blade style transmitters.  Ours.  Theirs.  Everybody’s.

That means that your consistency signal is really a consistency & flow rate signal.

What does flow rate look like in a consistency signal?  Well, mostly it looks like the stock is getting heavy or light as the flow rate goes up or down.  It gets worse as flow rate increases.  The higher the velocity, the larger the force component.   If you’re running really fast, you may have so much flow force that you will miss changes in consistency altogether.  This was Bob’s problem, by the way.  His flow rate was so high that the transmitter was pretty much only registering flow force.

So what do you do about that?

If your flow rates are relatively stable, you can just subtract out the average impact of flow rate when you first calibrate the transmitter.  If, on the other hand, you have variable flow rate (and you do, even if you don’t know it), then you’ll need to compensate for that variability. 

Compensating for flow rate is easy – it’s just an equation to subtract out the impact of flow rate on the consistency signal.  TECO’s C6000 Series transmitters have that function built-in.  Just land a flow signal on the transmitter and you’re good to go.

If you don’t have a TECO transmitter, you’’ need to program the equation into your DCS.  Personally, I think it’s easier to install a TECO C6000 transmitter.  Call me and we’ll talk about it.

I've often mentioned that Drainac is the easiest and simplest system to clean of any automatic online freeness analyzer in the world.  Well, here's yours truly on YouTube demonstrating the Official Cleaning Procedure for the Drainac.

As you'll see, it will almost certainly take you longer to walk to your system than it will to clean it.

And yes, my wife has already commented that I need to lose weight.

A Word on Managing Calibrations

I’ll often hear my customers say something along the lines of “I really wish we could get a more accurate consistency transmitter”.  When I dig a little deeper, I usually find that the problem isn’t necessarily so much an issue with the transmitter’s accuracy, but rather, with the way that the calibration for that transmitter is built and managed.

What do I mean?

When somebody installs a consistency transmitter, what they usually want is for that transmitter to generate a consistency number that agrees with whatever their lab says.  If the lab analyzes a stock sample and says its 3.4%, they want the transmitter to say 3.4%, too.

Of course, the reality is that the transmitter will never actually read 3.4%. It will always be different.  Always.

What most people don’t fully understand is that this behavior is absolutely normal.  The question isn’t if the transmitter and the lab disagree, because they will.  The real question is how much can the lab and transmitter disagree before you start to think that you’ve really got a problem.

And that’s a question that far too few people ask.  The consequences of not asking that question can be severe.  Take this scenario, for example.

The operators get a consistency reading, X, from the lab.  The value of X doesn’t agree with what the transmitter is saying, which is reading Y.  The operators call the E&I shop and tell them that they need to go and service the transmitter so that it reads X instead of Y. 

In a couple of hours, or maybe the next day, an E&I tech goes to the transmitter in question and adjusts the output so that it now reads X.  When the operators get the next lab analysis, they note that while the transmitter is now reading X, the lab now says it's actually Z.  So, again, they call up the E&I shop and tell them that transmitter has drifted again, and now they need to make it read Z instead of X. 

The E&I shop dutifully dispatches another tech, probably not the same one as before, who goes to the transmitter and readjusts it to now read Z.  It’s a good bet, by the way, that the first and second tech don’t use the same method to adjust the transmitter.  The next lab sample evaluation comes, and now, it’s W, instead of X, Y, or Z.  By this time, everybody starts calling the transmitter “that damn transmitter” and walks around cussing out the vendor for his piece of s**t instrument.

Does that sound familiar?

This is a situation that can be avoided if one knows the statistics behind a calibration.  When you build a calibration, there is something that you have accept as if it were the Gospel truth:  the instrument is better than your lab. 

What I mean by that is that your instrument will likely respond pretty much the same way given the same process conditions with very little variation.  Most undamaged and properly functioning instrumentation will respond with less than 1% of variation.

The same can’t be said of the lab.

This doesn’t mean that your lab is bad, or that your lab techs are lazy slobs.  It’s just the nature of the beast when it comes to manual analyses.

The TAPPI T-240 method for establishing consistency, for example, reports a repeatability standard of only 10%, and that presumes that your sampling techniques isn’t adding a whole lot more error.

In real terms, that means that you would expect that if the first lab analysis said 3.0%, then 95% of the time, you would expect a second analysis of the same stock to not deviate by more than 0.3%, or in other words, to come in somewhere between 2.7% and 3.3%.

That’s a pretty big range.  And you get that if you’re doing it properly. 

Now, what does this mean, exactly?

It doesn’t mean that your consistency readings are worthless.  Quite the contrary, it means that you now have a benchmark by which you can gauge the health of your consistency readings.

Remember that the instrument is probably responding to real changes in consistency with a repeatability variability of less than 1%.  If your calibration was built properly in the first place – a topic for another column, by the way – then if the lab typically reports X when the meter is reading Y, you shouldn’t get excited until the lab value exceeds X +/- 10% of X.

Remember, 95% of the time, you expect that lab to get within that 10% of X.  If it is within that 10%, then, everything is working OK and you shouldn’t mess with your transmitter or the calibration.

I wouldn’t even get excited if you occasionally get a reading in excess of 10% of X.  Remember, 95% of the time you expect it to be less, but that also means that 5% of the time, or about once every twenty times, it could be more than 10% and everything would still be OK.

So when should you get excited?

If your readings are consistently deviating more than that 10% of X, then it’s time to review the situation.

If your variation is always positive, or always negative, then it’s time to review the situation.  A valid calibration should see both positive and negative variation.

I stop here for now, but I’ll leave you with one more thought.  You can do a lot better than that 10%.  More on that in a future column.