Connect-Tek Transa PRO 17 Manual de Instruções descarregar pdf (Página 88)

Proportional control mode: When process temperature approaches

set point and enters the proportional band, the output is switched on and

off at the established cycle time. The change in power to the load pro-

vides a throttling action which results in less temperature overshoot.

Ramp: A programmed rise or fail in temperature at a constant rate.

Range: An area between two limits in which a measurement or control

action takes place. Typically expressed in upper and lower limits.

Rankine (°R): An absolute temperature scale based upon the Fahrenheit

scale with 180° between the ice point and boiling point of water.

459.69°R=0°F.

Rate (derivative): A control function that measures the rate of increase

or decrease of the system temperature and brings the control into an

accelerated proportioning action. This mode prevents an overshoot con-

dition at initial heat-up and with system disturbances.

Rate time: The interval over which the system temperature is sampled

for the derivative function.

Span: The difference between the upper and lower limits of a range

expressed in the same units as the range.

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 tem-

perature of a particle 1 degree to the thermal energy required to raise an

equal mass of water 1 degree.

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

Stability: The ability of an instrument or sensor to maintain a constant

output when a constant input is applied.

Surge current: A current of short duration occurring when power is

initially applied to capacitive or resistive loads, usually lasting no more

than several cycles.

Temperature gradient: The range of temperature variations at various

physical locations throughout a thermal system.

Tera: The prefix for one trillion (T).

Thermal expansion: An increase in size due to an increase in temper-

ature expressed in units of an increase in length or increase in size per

degree, i.e., inches/inch/degree C.

Thermal lag: The time delay in the distribution of heat throughout a

thermal system.

Thermal system: A regulated environment consisting of a heat source,

heat transfer medium, sensing device and a process variable control

instrument.

Thermistor: A temperature sensing probe made of a mixture of metal

oxides and encapsulated in epoxy or glass. A large change in resistance is

exhibited proportional to a change in temperature. The resistance usually

decreases as temperature rises.

Thermocouple: A temperature sensing probe consisting of the junction

of two dissimilar metals which has a millivolt output proportional to the

difference in temperature between the “hot” junction and the lead wires

(cold junction).

Thermocouple break protection: Fail-safe operation that assures out-

put shutdown upon an open thermocouple condition.

Thermowell: A closed-end tube designed to protect temperature sen-

sors 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.

Transducer: A device which converts the parameter being measured

into another form which is its output. For example, a thermocouple trans-

forms heat into a millivolt output.

Transmitter (two-wire): A device which is used to transmit tempera-

ture data from either a thermocouple or RTD via a two-wire current loop.

The loop has an external power supply and the transmitter acts as a vari-

able resistor with respect to its input signal.

Triac: A solid state switching device used to switch alternating current

wave forms.

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 at .01°C.

TTL: Transistor-to-transistor logic. A form of solid state logic which uses

only transistors to form the logic gates.

UL: Underwriters Laboratories, Inc. An independent laboratory that

establishes standards for commercial and industrial products.

Ultraviolet: That portion of the electromagnetic spectrum below blue

light (380 nanometers).

Undershoot: Excursion of temperature below set point.

Ungrounded junction: A thermocouple junction fully insulated from

the sheath.

Viscosity: The inherent resistance of a substance to flow

Volt: The (electrical) potential difference between two points in a circuit.

The fundamental unit is derived as work per unit charge —(V=W/Q).

One volt is the potential difference required to move one coulomb of

charge between two points in a circuit while using one joule of energy.

Voltage: An electrical potential which is measured in volts.

VPFW SSR: A type of Solid State Relay with Variable Period and Full

Wave switching characteristics.

Wattage: A measurement of electrical power. In a resistive circuit,

W=VI (see Ohms Law formulas).

Working standard: A standard of unit measurement calibrated from

either a primary or secondary standard which is used to calibrate other

devices or make comparison measurements.

Zero voltage switching: The making or breaking of a circuit timed

such that the transition occurs when the voltage waveform crosses zero

voltage; typically it is only found in solid state switching devices.

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