‰  NOW 40 WPM  ‰  
MEASURING CURRENT AND MEASURING VOLTAGE OR RESISTANCE, EXCEEDING THE
MAXIMUM INPUT OF THE METER, PLACING THE FUNCTION SWITCH IN THE WRONG
FUNCTION FOR ATTEMPTED MEASUREMENTS, OR ASSUMING THE RIGHT MOST DIGIT IS
EXACTLY CORRECT.  PAUL DANZER, N1II, STARTED HIS CAREER WITH RADIO AND
ELECTRONICS WHEN HE TOOK A FAMILY RADIO APART AT 9 YEARS OLD AN ACTIVITY
FOR WHICH HIS MOTHER NEVER FORGAVE HIM.  HE WAS FRST LICENSED AS KN2DGR IN
1953, AND CREDITS HIS HAM RADIO EXPERIEN

ACCURACY AND READING THE SCALE THE GOOD NEWS ABOUT A DMM IS THAT IT IS
BASED ON AN A/D CONVERTER.  OF COURSE, THE BAD NEWS IS THE SAME A/D
CONVERTER MAKES IT VERY DIFFICULT TO STATE THE METER ACCURACY IN JUST A FEW
WORDS.  THE ACCURACY, USUALLY EXPRESSED AS A PERCENTAGE, DEPENDS ON THE
BASIC ACCURACY OF THE REFERENCE USED IN THE A/D CONVERTER AS WELL AS THE
NUMBER OF BITS IN THE CONVERTER.  ON TOP OF THAT, THE DESIGN OF THE INPUT
CIRCUITS PLAYS A MAJOR ROLE IN THE FINAL NUMBER.  GENERALLY, THE ACTUAL
ACCURACY IS SPECIFIED FOR EACH SCALE OF THE METER.  AS YOU CAN SEE IN
FIGURE 10, THE WORD AUTO INDICATES THAT THE METER IS DOING AUTOMATIC
RANGING.  MOST, IF NOT ALL, DMMS HAVE AUTOMATIC RANGING.  THEY SET THE
SCALING TO PRODUCE THE MAXIMUM NUMBER OF DIGITS FOR THE VALUE BEING
MEASURED.  IF THE VALUE IS OVER THE ALLOWABLE RANGE, BUT BELOW THE POINT
WHERE THE METER IS DAMAGED, THE METER WILL READ OL, FOR OVERLOAD.  YOU CAN
SEE THIS IF YOU TURN THE METER TO THE OHMS BUT DO NOT CONNECT THE PROBES TO
ANYTHING.  THE METER SEES A RESISTANCE OVER THE MAXIMUM VALUE IT CAN
MEASURE, SO IT DISPLAYS OL.  ONE NICE FEATURE IS THE AUTOMATIC SENSING OF
POLARITY FOR VOLTAGE AND CURRENT.  FOR EXAMPLE, IF YOU HAD A 9 V BATTERY
AND CONNECTED THE COMMON OR BLACK TERMINAL TO THE BATTERY POSITIVE AND THE
RED TERMINAL TO THE BATTERY NEGATIVE, A MINUS SIGN WOULD APPEAR IN FRONT OF

THE READING, BUT THE NUMBERS WOULD BE CORRECT.  THIS IS FAR DIFFERENT FROM
AN ANALOG METER, WHERE CONNECTING THE LEADS BACKWARDS MIGHT DAMAGE THE
METER WHEN THE MOVING NEEDLE HITS THE RETAINING PIN ON THE METER FACE.
THERE IS ONE MORE PROBLEM WITH THE METER ACCURACY, AS WITH ANY DIGITAL
DEVICE DISPLAYING NUMBERS.  THE METER IN FIGURE 10 IS DISPLAYING 9R21 V.
THE LAST DIGIT IS ROUNDED OFF, JUST AS YOU MIGHT DO WHEN CALCULATING WITH
PENCIL AND PAPER.  THE ACTUAL VALUE MIGHT BE 9R210 OR PERHAPS 9R213, OR
EVEN R208, BUT IS ROUNDED TO 9R21 V.  WHILE THIS CAN SIMPLY BE AN ANNOYING
PROBLEM, IT CAN ALSO BE A VERY SERIOUS ERROR IF THE METER SELECTS THE
LOWEST POSSIBLE RANGE AND ONLY ONE SIGNIFICANT DECIMAL DIGIT IS SHOWING FOR
EXAMPLE, 0R03 V.  THEN YOU ARE NOT SURE IF THE TRUE NUMBER IS 0R02, 0R03,
OR 0R04 V.  OTHER FUNCTIONS  THE MODEL SHOWN HERE HAS A BUILT IN BATTERY
TESTER.  SELECTING ONE OF THE FOUR BATTERY SCALES ALLOWS YOU TO PLACE THE
LEADS ACROSS THE BATTERY UNDER ITS INSTALLED AND WORKING CONDITION, AND GET
AN INDICATION OF THE BATTERYS CONDITION BY LOOKING AT THE THREE LEDS.
GREEN MEANS OKAY, YELLOW MEANS QUESTIONABLE, AND RED MEANS FULLY
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ