When a pressure transmitter datasheet quotes “±0.075% accuracy”, that single number hides several different error sources — and a statistical assumption that determines how many units off the production line will actually meet that figure. This article breaks down what goes into a pressure transmitter’s accuracy specification and how to read it correctly.
The 3σ Rule: What ±0.075% Really Means
Pressure transmitter errors, measured across a production batch, follow a normal (Gaussian) distribution. The three-sigma rule of the normal distribution states:
- 68.2% of measurements fall within μ ± σ
- 95.4% fall within μ ± 2σ
- 99.7% fall within μ ± 3σ
When a vendor specifies “±0.075% accuracy at 3σ”, that is a commitment that 99.7% of transmitters from the production line will meet the ±0.075% figure. The remaining 0.3% may exceed it. A spec quoted at 1σ would only commit to 68.2% — a much weaker promise for the same headline number.
Not every datasheet states the confidence level. When comparing two transmitters with the same accuracy figure, ask which sigma level the spec is referenced to. If the answer is not in the datasheet, the comparison is not apples to apples.
The Five Components of Accuracy
Under reference conditions — the standard ambient temperature, supply voltage, and process media defined for testing — what a datasheet calls “accuracy” is actually a combination of five separate metrics. Each is defined and measured differently.
1. Inaccuracy (Reference Accuracy)
For any single test cycle, with the input increasing and then decreasing across the full range, inaccuracy is the maximum positive or negative deviation between the measured value and the nominal value at any test point. Put plainly: the worst-case error on either the upscale or downscale traverse.
2. Maximum Mean Error
First, the upscale and downscale errors at each test point are averaged. Then the test point with the largest positive or negative deviation from nominal is selected. This removes hysteresis from the calculation and isolates the systematic bias of the transmitter.
3. Terminal-Based Nonlinearity
A straight reference line is drawn through the lower range value (LRV) and upper range value (URV) of the average calibration curve. Nonlinearity is the maximum deviation between the actual curve and this line.
Pressure transmitters are calibrated in the factory and then re-zeroed and re-spanned in the field at the range endpoints. Terminal-based nonlinearity is the form that matches this real-world commissioning practice. Best-fit straight-line nonlinearity, while common in lab specs, is less practically relevant once a transmitter has been adjusted on site.
4. Hysteresis
At any given test point, hysteresis is the difference between the upscale and downscale outputs within the same test cycle. It comes from mechanical effects in the sensor diaphragm and fill fluid — small amounts of elastic memory in the materials.
The dead band is determined at 50% of the input span by increasing or decreasing the input until the output begins to change. Hysteresis error is then calculated as total hysteresis minus dead band.
5. Non-Repeatability
The algebraic difference between the limit values obtained when the same input is applied from the same direction, multiple times in short succession, under unchanged operating conditions. Non-repeatability is normally expressed as a percentage of the ideal output span and is reported separately from hysteresis.
How Microcyber Combines These Into a Single Spec
Microcyber’s monosilicon pressure transmitter pairs a high-resolution ADC with a multi-point software compensation algorithm. The standard accuracy class is ±0.075% of full span (FS), with ±0.05% FS available as an upgrade. Both figures are committed at 3σ, and both already include nonlinearity, hysteresis, and non-repeatability — these are not separate adders to be combined by the user at the spec sheet.
The transmitters carry both flameproof (Ex d) and intrinsically safe (Ex ia) hazardous-area certifications. The intrinsic safety rating reaches the highest level — “ia” — so the transmitters are usable in Zone 0 hazardous locations as well as in Zone 1 and Zone 2.
What to Check on Any Pressure Transmitter Datasheet
Three questions are worth asking when comparing pressure transmitter datasheets:
- Is the accuracy quoted at 1σ, 2σ, or 3σ? A 3σ commitment is a much stronger statistical guarantee than a 1σ commitment for the same percentage figure.
- Does the figure include linearity, hysteresis, and repeatability — or are they listed separately as adders? If they are separate, the user has to combine them to get the real installed accuracy.
- Is nonlinearity measured terminal-based or best-fit? Terminal-based is more conservative and matches the behaviour the device will show after field adjustment.
A datasheet that answers all three explicitly is a sign that the manufacturer has tested honestly. A datasheet that quotes a single accuracy number with no further detail is a signal to ask follow-up questions.
