My Gauge is Better than Your Gauge

Q: “A new digital vacuum gauge I’m thinking of buying has an accuracy of ±1%, but the more expensive Capsule Gauges have only ±2% accuracy. Why should I buy a capsule gauge?

A: Because a capsule gauge is more accurate, of course! No, really, it is. OK, it's another one of those "it depends" answers, which means I may be opening a can of worms. But it's a good question, because gauge accuracy is stated in many ways and has many variables.

Generally, the majority of gauges measuring “gauge” (or, relative) pressure are rated with percent-of-full-scale, according to an ASME standard that ranks gauge accuracy by grades. Grade D is the least accurate and Grade 4A is the best. To further complicate the grading process, some grades have different accuracies depending on where you are on the scale. 

Accuracy depends on a number of factors including resolution, readability (size and scale length), repeatability, friction, range, parallax, and hysteresis (variation difference from ac­tual pressure between rising and falling pressures after tapping to eliminate friction errors), and whether the error is percent of reading (%R) or full scale (%FS).

To begin with, %R is always better than %FS for two gauges with the same percentage of error. A %R gauge can even be better than a %FS gauge that has a smaller (better) percent­age of error, depending on where the pressure is read.

I know that sounds confusing, but it’s because with a %FS gauge, the total percentage of the error at any indicated pressure gets worse as you approach the zero point. Let’s assume we have a ±1%FS 0-30 in.Hg relative vacuum gauge; one percent of the full scale num­ber (say 30, for 30 in.HgV) is 0.3% at close to the 30 in.HgV point on the scale. At 1 in.HgV (nearly atmospheric), the error of 0.3 in.Hg is 33 percent! Add to that, the fact that some gauges have different accuracies at the middle versus both ends—the high and low quar­ters—of the scale, and it gets worse. For example, one brand’s 2-inch relative vacuum gauge has an accuracy of ±2/1/2% of span (which is another way of saying full scale). That’s not two-and-one-half; rather, it is two/one/two, and it means that the quarters clos­est to the two ends of the scale have a ±2% accuracy, but in the middle half of the scale (7.5 to 22.5 in.HgV) the accuracy is ±1%. If you are operating at, say, 25 in.HgV, then the actual pressure can be anywhere between 24.4 to 25.6 in.HgV.

A ±1%R gauge, however, has the same 0.3 in.Hg error at near the 30 in.HgV reading, but at 1 in.HgV, the error is only 0.01 in.Hg, or one percent. That’s a big difference from 33 percent. The percentage of the error did not change, but the possible actual pressure variation did.

Consider four different gauge styles, each with a different accuracy.  A 2½” liquid-filled gauge has an accuracy of ±2/1/2%FS, while a 4” liquid-filled gauge has ±1%FS accuracy. The capsule gauges have ±2%R accuracy, and the digital gauges are ±1%FS accurate. From reading this, one would expect that the digital gauge would be best. But being digital doesn’t ensure accuracy, only readability. Since it has only one pressure reading and often has large digits, its readability is unparalleled. Any error resulting from trying to interpolate between the printed numbers on the dial face by counting the lines indicating the increments between those numbers, is eliminated.

Digital gauges usually “sample”, or take a pressure reading every second or so; therefore, you get readings that look like they are jumping all over the place since instantaneous pressure is always changing (pressure fluctuations are less noticeable if the system volume is large). An analog gauge reacts to minor pressure fluctuations much more slowly, and therefore gives more stable readings. Speaking of stability, a liquid-filled gauge is not nec­essarily more accurate. The liquid mostly smooths out pressure fluctuations and vibrations.

Now, let’s look at the capsule gauge. The capsule gauges can be ordered with different scales, so you can select a range that best meets your operating conditions. Most im­portant, the capsule gauge measures absolute pressure. That means it is unaffected by effects of altitude, but gauge accuracy is also affected. The zero point is at the opposite end of the scale from a scale on a relative pressure gauge. That means that the gauge accuracy is best at the deepest vacuum level; add to that the fact that we can improve readability by choosing a gauge with a smaller portion of the total pressure range—say 20 to 0 torr—and the accuracy of a 2%R gauge can be ±0.2 torr at 20 torr, or ±0.02 torr at the 2 torr reading. That’s several orders of magnitude better than a ±1%FS relative vacuum gauge at about 29 in.HgV, and a lot better than the ±7.5 torr variation of the ±1%FS digital gauge. These capsule gauges are made using jeweled bearings, which eliminates most friction and adds to their accuracy, repeatability … and cost.

Of course, you should always purchase the gauge that best suits your application. If you are a thermoformer working at, say 22 in.HgV, then a bourdon tube type gauge should be sufficient. If you are working with evaporation of water or solvents, then an absolute pressure gauge like the capsule gauge would be your choice.

One final note:  Please, USE gauges. In my experience of nearly 45 years, very few users even have a gauge present on their vacuum systems or processes. In trying to trouble-shoot application  questions, when I ask what vacuum level users have in their processes, most don’t know. In these cases, any gauge would have been helpful—regardless of accuracy.