Glossary S

Following is a definition of terms frequently used in the instrumentation, industrial automation and test & measurement communities.

Salt Bridge: The salt bridge of a reference electrode is that part of the electrode which contains the filling solution to establish the electrolytic connection between reference internal cell and the test solution. Auxiliary Salt Bridge: A glass tube open at oneend to receive intermediate electrolyte filling solution, and the reference electrode tip and a junction at the other end to make contact with the sample.

Salt Effect (fx): The effect on the activity coefficient due to salts in the solution.

SAMA: Scientific Apparatus Makers Association. An association that has issued standards covering platinum, nickel, and copper resistance elements (RTDs).

SCE: Saturated calomel electrode.

SCR: Ssilicone controlled rectifier.

Scroll: To move all or part of the screen material up to down, left or right, to allow new information to appear.

Secondary Device: A part of the flowmeter which receives a signal proportional to the flowrate, from the primary device, and displays, records and/or transmits the signal.

Secondary Standard: pH buffer solutions which do not meet the requirements of primary standard solutions but provide coverage of the pH range not covered by primary standards. Used when the pH value of the primary standard is not close to the sample pH value.

Seebeck Coefficient: The derivative (rate of change) of thermal EMF with respect to temperature normally expressed as millivolts per degree.

Seebeck Effect: When a circuit is formed by a junction of two dissimilar metals and the junctions are held at different temperatures, a current will flow in the circuit caused by the difference in temperature between the two junctions.

Seebeck EMF: The open circuit voltage caused by the difference in temperature between the hot and cold junctions of a circuit made from two dissimilar metals.

Self Heating: Internal heating of a transducer as a result of power dissipation.

Sensing Element: That part of the transducer which reacts directly in response to the input.

Sensitivity Shift: A change in slope of the calibration curve due to a change in sensitivity.

Sensitivity: The minimum change in input signal to which an instrument can respond.

Sequential Access: An access mode in which records are retrieved in the same order in which they were written. Each successive access to the file refers to the next record in the file.

Serial Transmission: Sending one bit at a time on a single transmission line. Compare with parallel transmission.

Set Point: The temperature at which a controller is set to control a system.

Settling Time: The time taken for the display to settle within one digit final value when a step is applied to the meter input.

Shear Modulus: The ratio of the shear stress and the angular shear distortion.

Shear Stress: Where normal stress is perpendicular to the designated plane, shear stress is parallel to the plane.

Shearing Strain: A measure of angular distortion also directly measurable, but not as easily as axial strain.

Sheath Thermocouple: A thermocouple made out of mineral-insulated thermocouple cable which has an outer metal sheath.

SI: System Internationale. The name given to the standard metric system of units.

Signal Conditioner: A circuit module which offsets, attenuates, amplifies, linearizes and/or filters the signal for input to the A/D converter. The typical output signal conditioner is +2 V dc.

Signal Conditioning: To process the form or mode of a signal so as to make it intelligible to, or compatible with, a given device, including such manipulation as pulse shaping, pulse clipping, compensating, digitizing, and linearizing.

Signal: An electrical transmittance (either input or output) that conveys information.

Single Precision: The degree of numeric accuracy that requires the use of one computer word. In single precision, seven digits are stored, and up to seven digits are printed. Contrast with double precision.

Single-ended Input: A signal-input circuit where SIG LO (or sometimes SIG HI) is tied to METER GND. Ground loops are normally not a problem in AC-powered meters, since METER GND is transformer-isolated from AC GND.

Single-Plane (Static) Balancing Machine: A single plane balancing machine is a gravitational or centrifugal balancing machine that provides information for accomplishing single plane balancing.

Slope (Electrode Sensitivity, Span): See Nernst factor.

Smallest Bending Radius: The smallest radius that a strain gage can withstand in one direction, without special treatment, without suffering visible damage.

Software: Generally, programs loaded into a computer from external mass storage but also extended to include operating systems and documentation.

Solvation: Ions in solution are normally combined with at least one molecule of solvent. This phenomenon is termed solvation.

Source Code: A non-executable program written in a high-level language. A compiler or assembler must translate the source code into object code (machine language) that the computer can understand and process.

Span Adjustment: The ability to adjust the gain of a process or strain meter so that a specified display span in engineering units corresponds to a specified signal span. For instance, a display span of 200°F may correspond to the 16 mA span of a 4-20 mA transmitter signal.

Span: The difference between the upper and lower limits of a range expressed in the same units as the range.

Spare: A connector point reserved for options, specials, or other configurations. The point is identified by an (E#) for location on the electrical schematic.

Specific Gravity: The ratio of mass of any material to the mass of the same volume of pure water at 4°C.

Specific Heat: The ratio of thermal energy required to raise the temperature of a body 1° to the thermal energy required to raise an equal mass of water 1°.

Spectral Filter: A filter which allows only a specific band width of the electromagnetic spectrum to pass, i.e., 4 to 8 micron infrared radiation.

Spectrum Analysis: Utilizing frequency components of a vibration signal to determine the source and cause of vibration.

Spectrum: The resolving of overall vibration into amplitude components as a function of frequency.

Spot Size: The diameter of the circle formed by the cross section of the field of view of an optical instrument at a given distance.

Spurious Error: Random or erratic malfunction.

SSR: Solid state relay (see relay, solid state).

Stability: The quality of an instrument or sensor to maintain a consistent output when a constant input is applied.

Stagnation Pressure: The sum of the static and dynamic pressure.

Standard Electrode Potential (E0): The standard potential E0 of an electrode is the reversible emf between the normal hydrogen electrode and the electrode with all components at unit activity.

Standardization: A process of equalizing electrode potentials in one standardizing solution (buffer) so that potentials developed in unknown solutions can be converted to pH values.

Static Calibration: A calibration recording pressure versus output at fixed points at room temperature.

Static Error Band: The error band applicable at room temperature.

Static Pressure: Pressure of a fluid whether in motion or at rest. It can be sensed in a small hole drilled perpendicular to and flush with the flow boundaries so as not to disturb the fluid in any way.

Static Unbalance: Static unbalance is that condition of unbalance for which the central principal axis is displayed only parallel to the shaft axis

Steady Flow: A flow rate in the measuring section of a flow line that does not vary significantly with time.

Steady State Vibration: That condition of vibration induced by an unchanging continuing periodic force.

Stiffness: The ratio of the force required to create a certain deflection or movement of a part expressed as (Force/deflection) lbs/in or grams/cm.

Stop Bit: A signal following a character or block that prepares the receiving device to receive the next character or block.

Strain Gage: A measuring element for converting force, pressure, tension, etc., into an electrical signal.

Strain: The ratio of the change in length to the initial unstressed reference length.

String: A sequence of characters.

Strouhal Number: A nondimensional parameter important in vortex meter design defined as: s = Fh/V where f = frequency, V = velocity, and h = a reference length

Super Cooling: The cooling of a liquid below its freezing temperature without the formation of the solid phase.

Super Heating: (1) The heating of a liquid above its boiling temperature without the formation of the gaseous phase. (2) The heating of the gaseous phase considerably above the boiling-point temperature to improve the thermodynamic efficiency of a system.

Surge Current: A current of short duration that occurs when power is first applied to capacitive loads or temperature dependent resistive loads such as tungsten or molybdenum heaters-usually lasting no more than several cycles.

Suspension Effect: The source of error due to varied reference liquid junction potential depending upon whether the electrodes are immersed in the supernatant fluid or deeper in the sediment. Normally encountered with solutions containing resins or charged colloids.

Syntax: The rules governing the structure of a language.