Pressure Gauge Accuracy Compared: What 2.5% vs 0.025% Means in Real Numbers

Pressure gauge accuracy is expressed as a percentage of the gauge’s full range, so the same “2.5%” or “0.25%” on a datasheet means very different real-world errors depending on the range. A class 2.5 gauge on a 0 to 400 bar scale can be wrong by 10 bar, while a 0.25% digital gauge on the same range is within 1 bar. This guide turns the percentages into real numbers, compares the actual gauges we stock, and shows when the extra accuracy is worth paying for and when it is wasted money.

How pressure gauge accuracy is stated

For traditional Bourdon tube gauges, the European standard EN 837-1 defines accuracy classes. The commonly listed classes are 0.1, 0.25, 0.6, 1.0, 1.6, 2.5 and 4.0. Each number is the permitted error as a percentage of the gauge’s span (its full range). WIKA’s technical information sheet on accuracy classes lists these same values, and it notes an important detail: for gauges with a pointer stop at zero, the stated class applies over 10% to 100% of the scale, and for gauges with a free zero point it applies over 0% to 100% (WIKA data sheet IN 00.02). In practice that means you should not expect the stated accuracy in the bottom 10% of the dial on a typical gauge.

Digital gauges usually quote accuracy as a percentage of full scale (% FS) directly. The idea is the same: the error is a fixed slice of the range, not a slice of the reading.

The key formula

Maximum error = accuracy percentage × full range.

A 0 to 100 bar gauge at 1.0% can be out by 1 bar anywhere on the scale. That 1 bar is a fixed amount, so it matters far more at low readings. At 100 bar it is 1% of the reading, but at 10 bar it is 10% of the reading.

Accuracy in real numbers: the comparison table

The table shows the largest error you should expect, in bar, for each accuracy level on three common ranges.

The gauge accuracies are those stated on our product datasheets. The error columns are simple arithmetic on the full range. Check each datasheet for the exact range options and whether the stated accuracy includes temperature effects.

Example 1: the workshop air compressor

A workshop runs its compressor at 8 bar and uses a 0 to 16 bar gauge.

• A class 2.5 liquid filled gauge may be out by 2.5% of 16 bar, which is 0.4 bar. At an 8 bar reading that is 5% of the value.
• A PDK PDR100 digital gauge at 0.5% FS on the same range may be out by 0.08 bar, which is 1% of an 8 bar reading.

Verdict: for watching the pressure on a compressor, the 2.5% gauge is fine. Its 0.4 bar uncertainty does not change any decision. For setting a regulator on a spray painting line that needs a precise 6 bar, the digital gauge is the better tool. See pressure gauges for air compressors.

Example 2: the hydraulic relief valve

A machine handbook says the relief valve should open at 210 bar, with an allowed tolerance of plus or minus 5 bar. You want a gauge on a 0 to 400 bar range to check it.

• A class 2.5 gauge can be wrong by 10 bar. That is twice the tolerance you are trying to verify, so it cannot prove the valve is correct.
• A class 1.0 gauge can be wrong by 4 bar. That uses up most of the 5 bar tolerance, leaving almost no margin.
• A PDK PDR250 at 0.25% FS is within 1 bar, which is one fifth of the tolerance.

A widely used rule in calibration is that the test instrument should be at least four times more accurate than the tolerance you are checking. With a 5 bar tolerance, you want a gauge accurate to about 1.25 bar or better, so the 0.25% digital gauge qualifies and the others do not. See digital hydraulic pressure gauges.

Example 3: the calibration bench

A workshop wants to check its working gauges, which are class 1.0, using a reference gauge. Following the four-to-one rule, the reference must be four times better than 1.0%, so 0.25% or better. For checking class 0.25 instruments you would need 0.05% or better, which is territory for a high-end digital reference. The PDR1000 at 0.025% FS is designed for this level of work. See pressure gauge calibration.

Accuracy is not the only quality factor

Two gauges with the same accuracy number can differ a lot in how they behave over time. When comparing gauges, look at all of these.

Is higher accuracy worth the money?

Use this simple test.

1. What decision does the reading drive? If it is just “is the pump running?”, a 2.5% gauge is plenty.
2. What error would change that decision? If a 5% error could cause a safety problem or scrap, tighten the accuracy.
3. Is the reading used to set or verify something? Then choose an instrument at least four times better than the tolerance.
4. Is the range sensible? A better gauge on a badly chosen range wastes its accuracy. See the range selection guide.

A better class costs more, and the extra cost is justified only when the reading matters. For general indication, spending more does not buy you anything useful.

Common accuracy mistakes

• Confusing resolution with accuracy. A digital gauge that shows four decimal places is not necessarily accurate to four decimal places.
• Reading near the bottom of the scale. The error is the same in absolute terms, so it is a larger share of a small reading.
• Using an uncalibrated gauge as a reference. Accuracy on a datasheet only applies if the gauge is in calibration.
• Ignoring temperature. A gauge calibrated at 20 degrees Celsius may read differently on a hot pipe.
• Believing a new gauge is automatically accurate. Ask for a calibration certificate if the job needs one.

What digital accuracy figures usually include

A digital gauge datasheet may quote accuracy in different ways. Some figures include non-linearity, hysteresis and repeatability together, some add a “plus or minus one digit” allowance for the display, and some apply only within a stated temperature band. Before comparing two digital gauges, check that the accuracy figures are stated on the same basis and over a similar temperature range. When two datasheets are not directly comparable, ask the supplier to explain the difference in writing.

A quick rule for choosing accuracy

Write down the smallest pressure error that would change a decision, then divide it by four. That number is the accuracy you need, as a share of your range. If a 5 bar error would matter on a 0 to 400 bar range, you need about 1.25 bar, which is roughly 0.3% of the range. Choose the next better class and you have a comfortable margin. This one calculation avoids both under-buying and over-buying.

Remember to write the result into your maintenance procedure, so that a replacement gauge is chosen on the same basis in future, not on whichever happens to be in stock.

Frequently asked questions

Is a class 1.0 gauge twice as accurate as class 2.5?

Yes in terms of the error limit: 1.0% versus 2.5% of the range, so the permitted error is 2.5 times smaller.

Why does the datasheet say 0.25% FS but my reading is off by more?

The gauge may be out of calibration, outside its temperature range, used in the lowest part of the scale, or damaged by overpressure.

What accuracy do I need to check a hydraulic relief valve?

Work out the tolerance on the valve setting and choose a gauge four times better. For a 5 bar tolerance, that means about 1.25 bar or better.

Is a digital gauge always more accurate than an analogue gauge?

No. Compare the numbers. A 0.5% digital gauge is more accurate than a class 2.5 analogue gauge, but a class 0.25 analogue test gauge can be more accurate than a low-cost digital one.

How often should I recheck accuracy?

Twelve months is a common starting interval. Shorten it for critical or harsh use.

Compare and choose

For a side-by-side comparison of specific models, read analogue vs digital pressure gauges compared, then browse the pressure gauges and digital pressure gauges. To check a supplier’s claims, use our supplier checklist, or ask us to help you match accuracy to the job.