= Measuring transducer
A \b[measuring transducer] is a device whose output quantity has a specified relation to its input quantity and is used in measurement. VIM examples include a <thermocouple>, a <strain gauge> and a <Bourdon tube>.<International vocabulary of metrology by JCGM>[VIM3entry3.7, definition and examples]{ref} A <measuring transducer> provides a link in a <measuring chain>; it need not itself display a finished <measurement result>.
For an original pressure readout, choose a linear model
$$
V=Sp+V_0,
$$
with <pressure> $p$, output <electric potential difference> $V$, <transducer sensitivity> $S=2\,\mathrm{mV\,kPa^{-1}}$, and offset $V_0=1\,\mathrm{mV}$. An output $V=201\,\mathrm{mV}$ then corresponds to $p=100\,\mathrm{kPa}$. An <amplifier>, <analog-to-digital converter> and display can follow the transducer, each contributing to the whole <measuring system>.
The inverse calculation $p=(V-V_0)/S$ exposes distinct uncertainty contributions. Holding the other inputs fixed, a small output change $\delta V$ changes inferred pressure by $\delta V/S$; a small offset error changes it by $-\delta V_0/S$. An output increment $0.2\,\mathrm{mV}$ corresponds to $0.1\,\mathrm{kPa}$ for the stipulated sensitivity, but does not establish a total <measurement uncertainty> of that size.
Characterize the relevant range, temperature conditions, response time and possible <instrumental drift> during <measurement calibration>. The relation can be nonlinear, in which case local <transducer sensitivity> is the appropriate <derivative> rather than one gain valid everywhere. The definition also allows input and output quantities of the same kind; it does not require changing from a mechanical quantity to an electrical one. A <sensor> denotes the part directly affected by the phenomenon being sensed, whereas a complete <measuring chain> can contain further <measuring transducers>.
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