**Absolute Pressure Transducer:** A transducer which measures pressure in relation to zero pressure (a vacuum on one side of the diaphragm).

**Absolute Pressure:** Gage pressure plus atmospheric pressure.

**Absolute Zero:** Temperature at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be -273.15°C or -459.67°F.

**Acceleration:** A change in the velocity of a body or particle with respect to time. The parameter that an accelerometer measures (dv/dt). Units expressed in "g".

**Accelerometer:** A device which converts the effects of mechanical motion into an electrical signal that is proportional to the acceleration value of the motion. A sensor. A transducer.

**Accuracy:** The closeness of an indication or reading of a measurement device to the actual value of the quantity being measured. Usually expressed as ± percent of full scale output or reading.

**Acoustics:** The degree of sound. The nature, cause, and phenomena of the vibrations of elastic bodies; which vibrations create compressional waves or wave fronts which are transmitted through various media, such as air, water, wood, steel, etc.

**Alumel:** An aluminum nickel alloy used in the negative leg of a Type K thermocouple (Trade name of Hoskins Manufacturing Company).

**Ambient Compensation:** The design of an instrument such that changes in ambient temperature do not affect the readings of the instrument.

**Ambient Conditions:** The conditions around the transducer (pressure, temperature, etc.).

**Ambient Pressure:** Pressure of the air surrounding a transducer.

**Ambient Temperature:** The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test.

**Bearing:** A part which supports a journal and in which a journal revolves.

**BTU:** British thermal units. The quantity of thermal energy required to raise one pound of water at its maximum density, 1 degree F. One BTU is equivalent to .293 watt hours, or 252 calories. One kilowatt hour is equivalent to 3412 BTU.

**Calorie:** The quantity of thermal energy required to raise one gram of water 1°C at 15°C.

**Cation:** A positively charged ion (Na+, H+).

**Cavitation:** The boiling of a liquid caused by a decrease in pressure rather than an increase in temperature.

**Celsius (centrigrade):** A temperature scale defined by 0°C at the ice point and 100°C at boiling point of water at sea level.

**Center of Gravity (Mass Center):** The center of gravity of a body is that point in the body through which passes the resultant of weights of its component particles for all orientations of the body with respect to a uniform gravitational field.

**Centripetal Force:** A force exerted on an object moving in a circular path which is exerted inward toward the center of rotation.

**Ceramic Insulation:** High-temperature compositions of metal oxides used to insulate a pair of thermocouple wires The most common are Alumina (Al2O3), Beryllia (BeO), and Magnesia (MgO). Their application depends upon temperature and type of thermocouple. High-purity alumina is required for platinum alloy thermocouples. Ceramic insulators are available as single and multihole tubes or as beads.

**Ceramic:** Polycrystalline ferroelectric materials which are used as the sensing units in piezoelectric accelerometers. There are many different grades, all of which can be made in various configurations to satisfy different design requirements.

**CFM:** The volumetric flow rate of a liquid or gas in cubic feet per minute.

**Conduction:** The conveying of electrical energy or heat through or by means of a conductor.

**Confidence Level:** The range (with a specified value of uncertainty, usually expressed in percent) within which the true value of a measured quantity exists.

**Conformity Error:** For thermocouples and RTDs, the difference between the actual reading and the temperature shown in published tables for a specific voltage input.

**Connection Head:** An enclosure attached to the end of a thermocouple which can be cast iron, aluminum or plastic within which the electrical connections are made.

**Constantan:** A copper-nickel alloy used as the negative lead in Type E, Type J, and Type T thermocouples.

**Critical Speed:** The rotational speed of the rotor or rotating element at which resonance occurs in the system. The shaft speed at which at least one of the "critical" or natural frequencies of a shaft is excited.

**Cryogenics:** Measurement of temperature at extremely low values, i.e., below -200°C.

**dB (Decibel):** 20 times the log to the base 10 of the ratio of two voltages. Every 20 dBs correspond to a voltage ratio of 10, every 10 dBs to a voltage ratio of 3.162. For instance, a CMR of 120 dB provides voltage noise rejection of 1,000,000/1. An NMR of 70 dB provides voltage noise rejection of 3,162/1.

**Degree:** An incremental value in the temperature scale, i.e., there are 100 degrees between the ice point and the boiling point of water in the Celsius scale and 180°F between the same two points in the Fahrenheit scale.

**Density:** Mass per unit of volume of a substance. I.E.: grams/cu.cm. or pounds/cu.ft.

**Derivative:** The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.

**Deviation:** The difference between the value of the controlled variable and the value at which it is being controlled.

**Diaphragm:** The sensing element consisting of a membrane which is deformed by the pressure differential applied across it.

**Dielectric Constant:** Related to the force of attraction between two opposite charges separated by a distance in a uniform medium.

**Differential Input:** A signal-input circuit where SIG LO and SIG HI are electrically floating with respect to ANALOG GND (METER GND, which is normally tied to DIG GND). This allows the measurement of the voltage difference between two signals tied to the same ground and provides superior common-mode noise rejection.

**Differential Pressure:** The difference in static pressure between two identical pressure taps at the same elevation located in two different locations in a primary device.

**Differential:** For an on/off controller, it refers to the temperature difference between the temperature at which the controller turns heat off and the temperature at which the heat is turned back on. It is expressed in degrees

**Displacement:** The measured distance traveled by a point from its position at rest. Peak to peak displacement is the total measured movement of a vibrating point between its positive and negative extremes. Measurement units expressed as inches or milli inches.

**Dynamic Calibration:** Calibration in which the input varies over a specific length of time and the output is recorded vs. time.

**Dynamic Pressure:** The difference in pressure levels from static pressure to stagnation pressure caused by an increase in velocity. Dynamic pressure increases by the square of the velocity.

**Dynamic Unbalance:** Dynamic unbalance is that condition in which the central principal axis is not coincident with the shaft axis.

**Echo:** To reflect received data to the sender. For example, keys depressed on a keyboard are usually echoed as characters displayed on the screen.

**Electrode:** See Isopotential point.

**Electrolyte:** Any substance which, when in solution will conduct an electric current. Acids, bases, and salts are common electrolytes.

**Emissivity:** The ratio of energy emitted by an object to the energy emitted by a blackbody at the same temperature. The emissivity of an object depends upon its material and surface texture; a polished metal surface can have an emissivity around 0.2 and a piece of wood can have an emissivity around 0.95.

**End Point (Potentiometric):** The apparent equivalence point of a titration at which a relatively large potential change is observed.

**End Points:** The end points of a full scale calibration curve.

**Endothermic:** Absorbs heat. A process is said to be endothermic when it absorbs heat.

**Enthalpy:** The sum of the internal energy of a body and the product of its volume multiplied by the pressure.

**Environmental Conditions:** All conditions in which a transducer may be exposed during shipping, storage, handling, and operation.

**Exothermic:** Gives off heat. A process is said to be exothermic when it releases heat

**Fahrenheit:** A temperature scale defined by 32° at the ice point and 212° at the boiling point of water at sea level.

**Ferrule:** A compressible tubular fitting that is compressed onto a probe inside a compression fitting to form a gas-tight seal.

**Field Balancing Equipment:** An assembly of measuring instruments for performing balancing operations on assembled machinery which is not mounted in a balancing machine.

**Flow Rate:** Actual speed or velocity of fluid movement .

**Flow:** Travel of liquids or gases in response to a force (i.e. pressure or gravity).

**Flow meter:** A device used for measuring the flow or quantity of a moving fluid.

**FPM:** Flow velocity in feet per minute.

**FPS:** Flow velocity in feet per second.

**Freezing Point:** The temperature at which the substance goes from the liquid phase to the solid phase

**Frequency of Vibration:** The number of cycles occurring in a given unit of time. RPM - revolutions per minute. CPM- cycles per minute.

**Frequency Output:** An output in the form of frequency which varies as a function of the applied input.

**Frequency, Natural:** The frequency of free (not forced) oscillations of the sensing element of a fully assembled transducer.

**Frequency:** The number of cycles over a specified time period over which an event occurs. The reciprocal is called the period.

**Gage Factor:** A measure of the ratio of the relative change of resistance to the relative change in length of a piezoresistive strain gage.

**Gage Length:** The distance between two points where the measurement of strain occurs.

**Gage Pressure Transducer:** A transducer which measures pressure in relation to the ambient pressure.

**Gage pressure:** Absolute pressure minus local atmospheric pressure

**GPH:** Volumetric flow rate in gallons per hour.

**GPM:** Volumetric flow rate in gallons per minute.

**Head Loss:** The loss of pressure in a flow system measured using a length parameter (i.e., inches of water, inches of mercury).

**Head Pressure:** Pressure in terms of the height of fluid, P = yrg, where r = fluid density and y = the fluid column heights. Expression of a pressure in terms of the height of fluid, r = yrg, where r is fluid density and y = the fluid column height. g = the acceleration of gravity.

**Heat Sink:** 1. Thermodynamic. A body which can absorb thermal energy. 2. Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it.

**Heat Transfer:** The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are: conduction; the two bodies contact. Convection; a form of conduction where the two bodies in contact are of different phases, i.e. solid and gas. Radiation: all bodies emit infrared radiation.

**Heat Treating:** A process for treating metals where heating to a specific temperature and cooling at a specific rate changes the properties of the metal.

**Heat:** Thermal energy. Heat is expressed in units of calories or BTU's.

**Hertz (Hz):** Units in which frequency is expressed. Synonymous with cycles per second.

**Hooke's Law:** Defines the basis for the measurement of mechanical stresses via the strain measurement. The gradient of Hooke's line is defined by the ratio of which is equivalent to the Modulus of Elasticity E (Young's Modulus).

**Isothermal:** A process or area that is a constant temperature.

**Joule:** The basic unit of thermal energy.

**Journal:** A journal is that part of a rotor that is in contact with or supported by a bearing in which it revolves.

**Junction:** The point in a thermocouple where the two dissimilar metals are joined.

**Kelvin:** Symbol K. The unit of absolute or thermodynamic temperature scale based upon the Celsius scale with 100 units between the ice point and boiling point of water. 0°C = 273.15K (there is no degree (°) symbol used with the Kelvin scale).

**Kilowatt (kw):** Equivalent to 1000 watts.

**Kilowatt Hour (kwh):** 1000 watt hours. Kilovolt amperes (kva): 1000 volt amps.

**Kinetic Energy:** Energy associated with mass in motion, i.e., 1/2 rV2 where r is the density of the moving mass and V is its velocity.

**KVA:** Kilovolt amperes (1000-volt amps).

**Laminar Flow:** Streamlined flow of a fluid where viscous forces are more significant than inertial forces, generally below a Reynolds number of 2000.

**Mass Flow Rate:** Volumetric flow rate times density, i.e. pounds per hour or kilograms per minute.

**Maximum Elongation:** The strain value where a deviation of more than ±5% occurs with respect to the mean characteristic (diagram of resistance change vs strain).

**Maximum Operating Temperature:** The maximum temperature at which an instrument or sensor can be safely operated.

**Maximum Power Rating:** The maximum power in watts that a device can safely handle.

**Mean Temperature:** The average of the maximum and minimum temperature of a processequilibrium.

**Measurand:** A physical quantity, property, or condition which is measured.

**Measuring Junction:** The thermocouple junction referred to as the hot junction that is used to measure an unknown temperature.

**Mechanical Hysteresis:** The difference of the indication with increasing and decreasing strain loading, at identical strain values of the specimen.

**Melting Point:** The temperature at which a substance transforms from a solid phase to a liquid phase.

**Membrane:** The pH-sensitive glass bulb is the membrane across which the potential difference due to the formation of double layers with ion-exchange properties on the two swollen glass surfaces is developed. The membrane makes contact with and separates the internal element and filling solution from the sample solution.

**Mineral-insulated Thermocouple:** A type of thermocouple cable which has an outer metal sheath and mineral (magnesium oxide) insulation inside separating a pair of thermocouple wires from themselves and from the outer sheath. This cable is usually drawn down to compact the mineral insulation and is available in diameters from .375 to .010 inches. It is ideally suited for high-temperature and severe-duty applications

**Noise:** An unwanted electrical interference on the signal wires.

**Normal (axial) Stress:** The force per unit area on a given plane within a body a = F/A

**Normal Hydrogen Electrode:** A reversible hydrogen electrode (Pt) in contact with hydrogen gas at 1 atmosphere partial pressure and immersed in a solution containing hydrogen ions at unit activity.

**Normal-mode Rejection Ratio:** The ability of an instrument to reject interference usually of line frequency (50-60 Hz) across its input terminals.

**NPT:** National Pipe Thread.

**Null:** A condition, such as balance, which results in a minimum absolute value of output.

**O.D.:** Outside diameter.

**Operational pH:** The determination of sample pH by relating to pH measurements in a primary standard solution. This relationship assumes that electrode errors such as sensitivity and changes in asymmetry potential can be disregarded or compensated for, provided the liquid junction potential remains constant between standard and sample

**PID:** Proportional, integral, derivative. A three mode control action where the controller has time proportioning, integral (auto reset) and derivative rate action.

**Piezoelectric Accelerometer:** A transducer that produces an electrical charge in direct proportion to the vibratory acceleration.

**Piezoresistance:** Resistance that changes with stress.

**Poisson Ratio:** The ratio between the strain of expansion in the direction of force and the strain of contraction perpendicular to that force v = -Et/E1.

**Polarity:** In electricity, the quality of having two oppositely charged poles, one positive one negative.

**Polarization:** The inability of an electrode to reproduce a reading after a small electrical current has been passed through the membrane. Glass pH electrodes are especially prone to polarization errors caused by small currents flowing from the pH meter input circuit and from static electrical charges built up as the electrodes are removed from the sample solution, or when the electrodes are wiped.

**Positive Temperature Coefficient:** An increase in resistance due to an increase in temperature.

**Potential Energy:** Energy related to the position or height above a place to which fluid could possibly flow.

**Potentiometer:** 1. A variable resistor often used to control a circuit. 2. A balancing bridge used to measure voltage.

**Principal Axes:** The axes of maximum and minimum normal stress.

**Proof Pressure:** The specified pressure which may be applied to the sensing element of a transducer without causing a permanent change in the output characteristics.

**Proportioning Band:** A temperature band expressed in degrees within which a temperature controller's time proportioning function is active.

**Proportioning Control Mode:** A time proportioning controller where the amount of time that the relay is energized is dependent upon the system's temperature.

**Proportioning Control plus Derivative Function:** A time proportioning controller with derivative function. The derivative function senses the rate at which a system's temperature is either increasing or decreasing and adjusts the cycle time of the controller to minimize overshoot or undershoot.

**Proportioning Control plus Integral:** A two-mode controller with time proportioning and integral (auto reset) action. The integral function automatically adjusts the temperature at which a system has stabilized back to the setpoint temperature, thereby eliminating droop in the system.

**Proportioning Control with Integral and Derivative Functions:** Three mode PID controller. A time proportioning controller with integral and derivative functions. The integral function automatically adjusts the system temperature to the set point temperature to eliminate droop due to the time proportioning function. The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.

**PSIA:** Pounds per square inch absolute. Pressure referenced to a vacuum.

**PSID:** Pounds per square inch differential. Pressure difference between two points.

**PSIG:** Pound per square inch gage. Pressure referenced to ambient air pressure.

**PSIS:** Pounds per square inch standard. Pressure referenced to a standard atmosphere.

**Range:** Those values over which a transducer is intended to measure, specified by its upper and lower limits.

**Rangeability:** The ratio of the maximum flowrate to the minimum flowrate of a meter.

**Rankine (°R):** An absolute temperature scale based upon the Fahrenheit scale with 180° between the ice point and boiling point of water. 459.67°R = 0°F.

**Rate Action:** The derivative function of a temperature controller.

**Resistance:** The resistance to the flow of electric current measured in ohms (1/2) for a conductor. Resistance is function of diameter, resistivity (an intrinsic property of the material) and length

**RTD:** Resistance temperature detector.

**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.

**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

**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.

**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.

**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.

**Thermal Coefficient of Resistance:** The change in resistance of a semiconductor per unit change in temperature over a specific range of temperature.

**Thermal Conductivity:** The property of a material to conduct heat in the form of thermal energy.

**Thermal emf:** See Seebeck emf

**Thermal Expansion:** An increase in size due to an increase in temperature expressed in units of an increase in length or increase in size per degree, i.e. inches/inch/degree C.

**Thermal Gradient:** The distribution of a differential temperature through a body or across a surface.

**Thermal Sensitivity Shift:** The sensitivity shift due to changes of the ambient temperature from room temperature to the specified limits of the compensated temperature range.

**Thermal Zero Shift:** An error due to changes in ambient temperature in which the zero pressure output shifts. Thus, the entire calibration curve moves in a parallel displacement.

**Thermistor:** A temperature-sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients.

Thermocouple Type Material

(ANSI Symbol)

J Iron/Constantan

K CHROMEGA®/ALOMEGA®

T Copper/Constantan

E CHROMEGA®/Constantan

R Platinum/Platinum 13% Rhodium

S Platinum/Platinum 10% Rhodium

B Platinum 6% Rhodium/Platinum30% Rhodium

G* Tungsten/Tungsten 26% Rhenium

C* Tungsten 5% Rhenium/Tungsten 26% Rhenium

D* Tungsten 3% Rhenium/Tungsten 150 Rhenium

*Not ANSI symbols.

**Thermocouple:** The junction of two dissimilar metals which has a voltage output proportional to the difference in temperature between the hot junction and the lead wires (cold junction) (refer to Seebeck emf).

**Thermopile:** An arrangement of thermocouples in series such that alternate junctions are at the measuring temperature and the reference temperature. This arrangement amplifies the thermoelectric voltage. Thermopiles are usually used as infrared detectors in radiation pyrometry.

**Thermowell:** A closed-end tube designed to protect temperature sensors from harsh environments, high pressure, and flows. They can be installed into a system by pipe thread or welded flange and are usually made of corrosion-resistant metal or ceramic material depending upon the application.

**Thomson Effect:** When current flows through a conductor within a thermal gradient, a reversible absorption or evolution of heat will occur in the conductor at the gradient boundaries.

**Transducer Vibration:** Generally, any device which converts movement, either shock or steady state vibration, into an electrical signal proportional to the movement; a sensor.

**Transducer:** A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take physical phenomenon (pressure, temperature, humidity, flow, etc.) and convert it to an electrical signal.

**Transient Vibration:** A temporary vibration or movement of a mechanical system.

**Transitional Flow:** Flow between laminar and turbulent flow, usually between a pipe Reynolds number of 2000 and 4000.

**Transmitter (Two-Wire):** 1. A device which is used to transmit data from a sensor via a two-wire current loop. The loop has an external power supply and the transmitter acts as a variable resistor with respect to its input signal. 2. A device which translates the low level output of a sensor or transducer to a higher level signal suitable for transmission to a site where it can be further processed.

**Triac:** A solid state switching device used to switch alternating current wave forms.

**Triboelectric Noise:** The generation of electrical charges caused by layers of cable insulation. This is especially troublesome in high impedance accelerometers.

**Triple Point (Water):** The thermodynamic state where all three phases, solid, liquid, and gas may all be present in equilibrium. The triple point of water is .01°C.

**Triple Point:** The temperature and pressure at which solid, liquid, and gas phases of a given substance are all present simultaneously in varying amounts.

**Turbulent Flow:** When forces due to inertia are more significant than forces due to viscosity. This typically occurs with a Reynolds number in excess of 4000.

**Ultraviolet:** That portion of the electromagnetic spectrum below blue light (380 nanometers).

**Unbalance:** That condition which exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal forces.

**Vacuum:** Any pressure less than atmospheric pressure.

**Velocity:** The time rate of change of displacement; dx/dt.

**Viscosity:** The inherent resistance of a substance to flow.

**Volume Flow Rate:** Calculated using the area of the full closed conduit and the average fluid velocity in the form, Q = V x A, to arrive at the total volume quantity of flow. Q = volumetric flowrate, V = average fluid velocity, and A = cross sectional area of the pipe.

**Watt Density:** The watts emanating from each square inch of heated surface area of a heater. Expressed in units of watts per square inch.

**Young's Modulus:** Young's Modulus (the Modulus of Elasticity) is equivalent to the ratio of normal stress to strain.

**Zero Adjustment:** The ability to adjust the display of a process or strain meter so that zero on the display corresponds to a non-zero signal, such as 4 mA, 10 mA, or 1 V dc. The adjustment range is normally expressed in counts.

** **