Not very.

I have worked my entire adult life in and around measurements. There are instruments to measure web thickness, length and width; instruments to measure web tension and nip pressure; instruments to measure wound-roll hardness and roller-cover hardness; instruments to measure this, that and the other thing.

When I check them out, however, I almost always find serious problems. Some instruments are bent and broken. Others are uncalibrated or even impossible to calibrate. Still others are operator dependent. Readings don't agree with other measurements or observations. The customer still complains even when everything is in spec.

At home you will have the clock that is a few minutes off or even stopped. Thermometers usually read close, though the thermometer at the local bank reads high when the sun shines on it. None of this is of great concern. It is usually close enough, easy to tell when it is not—and seldom a big deal. The problem, however, is that the plant is nowhere near so trivial nor so trustworthy.

I have written about the difficulties of gage-profile measurement (July 2004) and length measurement (April 2005). Elsewhere I have written about wound roll hardness and moisture. I teach about tension-, nip- and temperature-measurement in my web seminars. All have a consistent theme: watch out! Things are not as they seem, nor as reliable as you might expect.

Home thermometers are simple, reliable and easy to check. Not so with plant equipment such as load cells. Every single load cell needs to be field calibrated because every position has a different roller weight, mounting, web width, ingoing wrap angle and outgoing wrap angle. Calibrating a load cell is mostly straightforward; hang a weight on the end of a thin strap routed the same as the web. Calibrating a nip is much more difficult, especially if there are geometry changes.

In another winder model, no one in the world, including the builder, could tell you what tension was being pulled by a centerwind assist motor. Compare this to your home thermometer. You don't have to calibrate your thermometer differently whether you hang it on the tree or on the fence, or whether it is two feet or four feet off the ground.

Proper units are another area of confusion. Temperature is a no-brainer: degrees F or C. What are the proper tension units? Candidates include: total force, force per unit width (tension) or force per unit width per unit thickness (stress). Yet we see many other tension-like units—such as dancer pressure, percent, motor amps and others that are nothing less than baffling.

A number of our instruments, such as wound-roll or cover hardness, have no means of independent calibration. These unique inventions have no simple first principles definition. The best we can do is to provide a test sample for calibration. However, this sample is for only one hardness, so the rest of the range is not possible to check. Also, how do you know this test sample was manufactured the same way over the years and that it remained that same hardness (rubber hardens with age)? A good share of our test lab instrumentation is like this.

What is a person to do? First, don't trust important decisions unless you check the measurement first. Check its conclusion with other measurements and observations for consistency. For example, a rejectable spec ought to align with customer returns else it is suspect for its primary use. The wound roll ought to look uniform if the scanner or test lab says it is. Second, regularly check and calibrate things that can be calibrated. Statistics are also very helpful to define confidence. Finally, avoid instrumentation when the supplier has not given independent calibration procedures, preferably in fundamental units.

920/725-7671, DRroisum@aol.com, www.roisum.com