Glossary of Terms [Direct Readout Meters (1966)]


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Accuracy of an indicated or recorded value (percent of reading). This is expressed by the ratio of the error of the indicated value to the true value. It is usually expressed in percent.

Accuracy rating of an instrument. The limits which errors will not exceed when the instrument is used under prescribed conditions.

Balance. See Null balance.

Balance detector. In digital instruments, an electronic amplifier which compares input and feedback voltages and issues commands to the logic circuits to change the feedback voltage as needed to attain a balance.

Balancing time. In a d-c dvm ratiometer or ohmmeter, the elapsed time from when the instrument is commanded to make a measurement to when the computation has been completed and is ready for visual readout or automatic recording. In an a-c/ d-c converter it is the time elapsed between application of a step input voltage of a stated amplitude and the time when the a-c/ d-c converter d-c output has stabilized to within some specified amount of its final value. Balancing time may be expressed as a maximum value or average value for a single range and single polarity or for all range and polarity conditions.

Conversion, analog servo. The operating network in which the feedback-voltage generator consists of a motor-driven variable resistor, servo-controlled by an error detector.

Conversion, prescribed sequence. (Successive approximation, whole number, or discrete conversion). The operating principle in which the feedback-voltage generator provides a signal of regulated voltage corresponding in value to the input voltage. The voltage corresponding to the most significant digit is first compared with the scaled input voltage. If smaller, it is left in the circuit. The comparison continues with the voltage sources left in the circuit at the end of the process representing the input voltage.

Conversion, ramp. (Voltage-to-time conversion). The operating principle in which the feedback-voltage generator periodically produces a voltage which changes linearly with time (ramp voltage) and is compared with the scaled input voltage by an error detector.

Conversion, voltage-to-frequency. The operating principle in which a converter generates pulses of frequency directly proportional to the input voltage.

Current-summing instrument. One in which the sum of the input current and the feedback current approaches zero at null balance.

Dead band. The total range through which the input voltage can be varied without initiating a change in reading. It is usually expressed in digits.

Decade. See Digit position designation.

Digit. One of the symbols of the decimal number system. The term is also applied to the position of symbols in the decimal system. For example, in the number 7851 the term "second digit" identifies the position occupied by the 8.

Digit position designation. (Display position). The designation of positions of digits in the display or record is generally in accordance with the following table: Number of Digits 3 4 5 Designation (starting at left)

First, second, last First, second, third, last First, second, third, fourth, last Digital instrument. One in which the indicated or recorded value is expressed directly in terms of the decimal number system.

Dynamic response. Behavior of the instrument output as a function of the measured quantity when both output and the measured quantity are varying with time.

Error detector. See Balance detector.

Extreme operating conditions. The limits of specified variables or conditions within which the instrument may be operated. Under these conditions, performance ratings do not necessarily apply.

Feedback voltage or current. An internally-generated voltage or current used for comparison with the scaled input voltage or current by means of an error detector.

Input voltage. (Measured quantity). The voltage to be measured.

(See also Scaled input voltage.)

Interference. Any physical phenomenon tending to adversely affect operation of the instrument.

Least increment. The value corresponding to the least digit displayed. It is expressed in volts in a voltmeter, in "per unit" in a ratiometer.

Logic. The sequence in which feedback-voltage range-divider operations and polarity-switching operations occur in attaining a balance.

Logic, double scan (or "double-duty no-needless-nines" or "tracking scan"). A later improvement of no-needless-nines logic in which the logic does not have to check polarity switch, range switch, and some or all decade switches before changing position of a given decade switch which requires change.

Logic, no-needless-nines. A form of scan logic which eliminates needless cycling to 9's and 0's position in each decade of stepping switch type digital voltmeters.

Logic sequence is: set polarity switch, set range switch, set decade switches starting with leftmost and proceeding to the right, changing only those switches whose numerical positions (values) differ from the final numerical reading.

Logic, scan. Logic in which each decade of the feedback voltage divider is changed in value only one per measurement and in which these changes progress from any decade (often, but not always, from the leftmost decade ) toward the right. See also Conversion, prescribed sequence.

Logic, successive trials. See Conversion, prescribed sequence.

Logic, tracking. Logic in which an increase or decrease in any decade requires that all decades to the right of it be switched to 9 or 0, respectively, and in which decade changes start with some decade and progress leftward (one measurement often requires several cycles of skipping again to the rightmost decade and progressing leftward). In this logic, an input-voltage change immediately results in a change in value of the decade which was last changed (in some cases the change will occur in the decade to the right or left of the decade last changed if the decade last changed is at a 9 or 0).

Longitudinal interference. Interference which appears between measuring circuit terminals and ground.

Monotonicity. The characteristic of a digital instrument such that the readings of the total count appear successively, in proper order, when the input voltage is continuously varied throughout a range.

Null balance. The condition which exists in the circuits of an instrument when the absolute value of the error voltage or current is less than that required to initiate changes in the instrument indications.

Potentiometric instrument. One in which a feedback voltage is generated by means of a voltage divider.

Range. A continuous band of voltage values within which the instrument is capable of making readings to stated accuracy. Digital voltmeters normally provide more than one range by shifting the decimal point.

Range, basic. The range in which the input voltage is compared directly with the feedback voltage.

Range divider. The circuit element that converts the input voltage to the scaled input voltage.

Reference voltage, external. The external voltage source with which the input voltage is compared in digital ratiometers.

Reference voltage, internal. The voltage source for the feedback generator in digital voltmeters.

Resolution. See Dead band.

Scaled input voltage. A voltage proportional to the input voltage for comparison with the feedback voltage. The output of the range divider.

Sensitivity. See Dead band.

Total count. The total number of least increments available in a range. For example, in the case of a four-digit instrument, the total count is normally 10,000. Transverse interference. Interference which appears as a voltage between measuring circuit terminals.

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