Turndown is the ratio between the widest span a transmitter can measure, its upper range limit, and the narrower span it is actually calibrated to. Setting a small calibrated span on a large sensor gives a high turndown, and while it lets one transmitter cover many ranges, it comes at a cost: accuracy usually gets worse as turndown climbs. Understanding how a turndown accuracy spec is written is the difference between sizing a transmitter that meets its stated accuracy and one that quietly does not.
Turndown / Rangeability in one line: Pressure transmitter turndown, also called rangeability, is the ratio of the sensor's upper range limit to the calibrated span you set. A high turndown lets one transmitter cover a small range, but accuracy degrades as turndown increases because a fixed error becomes a larger percentage of the shrinking span.
A transmitter's sensor has a maximum measurable span, its upper range limit, or URL. The calibrated span is the actual range you assign, say 0 to 50 psi on a cell whose URL is 500 psi, which is a 10:1 turndown. Part of a transmitter's error is fixed by the physics of the sensor and does not shrink when you narrow the span. As the calibrated span gets smaller relative to the URL, that fixed error becomes a larger and larger fraction of the span you actually care about.
That is why the same transmitter can be excellent at low turndown and mediocre at high turndown. Ranged near its URL, the fixed error is a tiny percentage of a big span and accuracy is superb. Ranged to a small fraction of its URL, the identical fixed error now dominates a tiny span, and the percent-of-span accuracy gets noticeably worse. Nothing about the hardware changed; the arithmetic did.
The practical rule that falls out of this is to choose a sensor whose URL is not wildly larger than the span you need. A cell sized so the required span sits at a modest turndown, often somewhere around a few-to-one rather than tens-to-one, keeps the accuracy penalty small. Oversizing a transmitter just so it can cover every conceivable range is a common way to end up with poor real-world accuracy.
Because accuracy depends on turndown, vendors do not publish a single accuracy number; they publish a formula. A typical spec has two parts: a reference accuracy that applies up to a stated base turndown, plus an additional term that grows with turndown beyond that point. Below the base turndown you get the headline reference accuracy; above it, you add the turndown-dependent term to find the real accuracy at your span.
The turndown-dependent term is often written as a coefficient multiplied by the turndown ratio, and it is expressed as a percent of the calibrated span. So to find the accuracy you will actually get, you compute your turndown, plug it into the formula, and add the result to the reference accuracy. A datasheet that quotes, for example, an excellent reference accuracy but only up to a 5:1 or 10:1 turndown is telling you that beyond that ratio the accuracy degrades on a known curve.
This is exactly why comparing two transmitters on their headline accuracy alone is misleading. One may hold a great reference accuracy to a high base turndown while another degrades quickly, and the right choice depends entirely on the turndown your application demands. Always evaluate the spec at your intended calibrated span, not at the vendor's most flattering reference condition.
Turndown is fundamentally a sizing and selection decision, distinct from simply re-ranging a transmitter to fit an operating window. Two people can range the same transmitter to the same span, but if one selected a sensor with a modest URL and the other selected an oversized one, they will get very different accuracy from an identical calibration. Getting the turndown right belongs to the specification stage, before the instrument is even ordered.
The consequences surface later in the trended data. A level or pressure transmitter running at excessive turndown produces a signal that is noisier and less repeatable near the ends of its span, and small real changes can be buried in the accuracy band. On a cloud SCADA platform like Merobix, that shows up as a reading that looks jittery or that disagrees with a known reference more than it should, and the root cause is often a poorly sized sensor rather than a fault.
For remote and custody-relevant measurements, this matters. Where the number feeds allocation, billing, or a tight control target, an over-turned-down transmitter can silently miss the accuracy the site assumed it had. Sizing the sensor so the required span sits at a sensible turndown is the cheapest way to make sure the numbers on the dashboard are as good as the datasheet promised.
Turndown, or rangeability, is the sensor's upper range limit divided by the calibrated span you set. For example, calibrating a cell with a 500 psi upper range limit to a 0 to 50 psi span gives a 10:1 turndown. A larger ratio means you are using a smaller slice of the sensor's capability, which is where the accuracy penalty comes from.
Generally yes, beyond the transmitter's base turndown. Part of a transmitter's error is fixed and does not shrink as you narrow the span, so at high turndown that fixed error becomes a larger percentage of the calibrated span. Most datasheets quote a reference accuracy up to a base turndown, then add a turndown-dependent term above it, so accuracy degrades on a defined curve.
Ranging is the act of setting the lower and upper range values on a given transmitter to fit the operating window. Turndown is a selection decision about how large a sensor you pick relative to the span you need, which determines the accuracy penalty you will live with. You can range two transmitters identically yet get very different accuracy if one was sized at a modest turndown and the other was badly oversized.
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