How to Read a Sensor Datasheet

Sensor datasheets are written to be accurate and are read to be compared, and the gap between those two purposes is where most selection mistakes live. A part that looks better on the front page is frequently worse in the application, because the front page quotes the best number the part can produce and the application is not the conditions under which it produced it.

Start with the conditions clause, which is usually in small type under the specification table and is the most important sentence on the page. Every number above it is valid only under those conditions: a stated supply voltage, a temperature, sometimes a specific measurement range, an averaging period, or a calibration performed immediately before the measurement. A temperature sensor quoted at ±0.1 °C is often quoted at 25 °C, and its accuracy at the edge of its operating range may be five times worse. Neither figure is dishonest. Only one of them is about your application.

Then separate three words the marketing copy tends to blur. Resolution is the smallest change the output can represent — a property of the converter, not the sensor, and the easiest number to make impressive. Precision, or repeatability, is how much the reading varies when nothing else does; it sets the smallest change you can actually detect. Accuracy is how close the reading is to the truth. A 24-bit converter gives you resolution and tells you nothing about whether the reading is right. Most industrial applications need repeatability far more than absolute accuracy, because they are looking for a change over time rather than a true value: a bearing that is warming, a tank that is emptying, a filter that is loading. If that is your case, a sensor with a large but stable offset is fine and a sensor that wanders is not, regardless of which one has the better accuracy line.

Which brings up drift, the specification that decides how the deployment ages and the one most often skipped during selection. Long-term stability is usually quoted as something per year — ±0.5% of reading per year, say — and over a five-year service life that dwarfs an initial accuracy figure. It also determines the calibration interval, which is a recurring operational cost rather than a one-off purchase decision. Two sensors with identical accuracy and different drift are not comparable products; one of them has a maintenance contract attached.

Temperature coefficients deserve the same attention. A pressure sensor's zero and span both move with temperature, and the numbers are given per degree, which makes them look small until multiplied by the range an outdoor cabinet actually sees. Forty degrees of swing between a winter night and a summer afternoon turns a modest coefficient into a substantial error, and in an application measuring a slow trend that error looks exactly like the trend.

Response time is worth checking against how it was defined. Time constants are usually specified as the time to reach 63% of a step change, and the figure depends heavily on the medium and flow conditions — a thermocouple in stirred water and the same thermocouple in still air are different instruments. If you need to catch a short excursion, work in multiples of the time constant and check the assumed conditions, because a specification measured in flowing liquid is not a promise about your still-air enclosure.

Then read for what is absent, which takes practice. If a datasheet quotes accuracy but not long-term stability, the stability is probably not a selling point. If it quotes a wide operating temperature range but a narrower compensated range, the difference between them is where the part still works and no longer measures well. If there is no statement about the effect of supply variation, humidity, or exposure to the substance being measured, those effects have not been characterised for you, which is not the same as them being absent.

Two practical habits close the gap. First, read the errata and the application notes, not just the datasheet — the honest description of what a part struggles with is frequently in a note about layout, decoupling or thermal isolation rather than in the specification table. Second, test the finished assembly rather than the sensor. A well-specified part mounted next to a warm regulator, on a board that flexes when the enclosure is screwed shut, in a housing that traps air, will not deliver its datasheet figures, and the difference is a property of your design rather than of the component.

The summary that survives most projects: the accuracy number chooses the sensor, the drift number decides whether you were right, and the conditions clause tells you whether either applies to you.