Glossary of Terms
°C (Centigrade; Celsius)
0° = Freezing and 100° = Boiling point of water at sea level.
The resistance versus temperature relationship of a platinum wire element over the temperature range of -183°C to 630°C. Normal values have been calculated into tables for R vs. T.
To determine the indication or output of a device with respect to a standard.
The amount of heat required to raise the temperature of one gram of water by 1°C. A mean (or average) calorie is 1/100 of heat required to raise the temperature of one gram of water from 0°C to 100°, at one atmosphere pressure.
Energy storage device.
Special coil to block high frequency AC currentflow.
Stabilized An amplification technique which converts a low level DC signal to a square wave DC and amplifies this signal through AC couple amplifier stages. The signal is then converted back to DC. This method eliminates the amplifier DCs drift from being amplified while preserving the original amplitude of the signal.
CMR (Common-Mode Rejection)
The ability of a panel meter to eliminate the effect of AC or DC noise between signal and ground. Normally expressed in dB at DC to 60 Hz. One type of CMR is specified between SIG LO and PWR GND. In differential meters, a second type of CMR is specified between SIG LO and ANA GND
CMV (Common-Mode Voltage)
The AC or DC voltage which is tolerable between signal and ground. One type of CMV is specified between SIG LO and PWR GND. In differential meters, a second type of CMV is specified between SIG HI or LO and ANA GND (METER GND).
Cold Junction Compensation
If two wires of different metals are connected end to end it has two junctions. Current will flow in the circuit if one junction is at a different temperature than the other (Seebeck Coefficient). This effect is the same for either junction except for polarity. Call one of the junctions the measuring junction. Separate the other junction and connect the two leads to a current meter. When making temperature measurements, if the current produced by this other junction (cold junction) is ignored, an unknown error is created. If the temperature of the cold junction is noted at the same time a measurement is made, then the error from the cold junction can be added to subtracted from the reading to give a correct answer. Automatic cold junction compensation is accomplished by sensing the terminal temperature (cold junction) with an RTD. The RTD is in circuit which produces a current equal but opposite to that produced by the cold junction. Thus the current change from the cold junction plus that from the compensation circuit cancel one another. Once this is accomplished then it can be assumed that the current reading on the voltmeter represents the current produced by the measuring junction only.
An extra pair of wires going to the tip of an RTD but not connected to the element, a novel way of lead wire resistance compensation.
The ability of a substance to transmit heat by conduction.
For thermocouples and RTDs, the difference between the actual reading and the temperature shown in published tables for a specific voltage input.
The link between the Process (conduit) and the Sensor, containing a terminal block for lead connections.
A special polarized disconnect device whose current-carrying parts are of thermocouple alloy material.
A device capable of receiving a signal from a temperature sensing probe within a process and regulating an input to that process in order to maintain a selected temperature (control point).
Non-proportional control in which the controlled process input is either fully ON or fully OFF depending on whether the temperature is above or below the control point deadband.
Simplest type of proportioning control in which the controlled process input is regulated to a value proportional to the difference between the control setpoint and the measured temperature. contintued - (i.e.- control within the proportional band.) Process inputs may be throttled mechanically or electrically or may operate On-Off on a time-proportioned basis.
Operates on same basic proportional band principle as one-mode, but introduces an additional reset control action which depends upon the accumulated error x time product (integral). The sum of the error and reset signals continuously act to make the actual error signal smaller. (i.e. controls to the selected control point.) Three-Mode Control Similar to two-mode control, but with an additional rate sensing (derivative) action which reduces tendency to over-shoot a control setpoint by anticipating the approach of a zero-value error signal and initiating a control response reversal before the sensed temperature actually gets there.
The number of analog-to-digital conversions performed per second by a DPM.
A method of interpolation for a platinum RTD. This method is based on the measurement of two or three fixed points and their correlation to an NIST calibrated reference thermometer, essentially fitting the slope of the reference thermometer to the calibrated thermometer. This technique is used from approximately -200°C to 0°C.
The number of time intervals counted by the dual-slope A/D converter and displayed as the reading of the decimal point.
Commercially pure, usually considered to be not more than 1 part in 10,000 impurities (i.e.; 99.99%).
Central Processing Unit in digital computing systems. See MICROPROCESSOR.
The ratio of the maximum (crest) value of a periodic function (AC voltage or current) to its RMS value.
Very low temperatures; used in reference to liquefied gases such as LOX (-297°F) and LN2 (-320°F). Requires use of high resistance Platinum RTDs (500-5,000 ohms), gold-chromed thermocouples or low temperature thermistors.
An instrument accessory which detects current flow without breaking the circuit under test. An AC transformer, usually step-down; typical ratio listing would be 1000:1. This would indicate 1000A on the primary and 1A on the secondary. Also called Toroidal coil for process applications. Typical output is 5 AAC when full load current is applied to the system.