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