Math Feature - HP 3455A Operating And Service Manual

Model 3455A
3-19. Math Feature.
3-20. The math feature of th 3455A allows the measure-
ment value to be offset and/or scaled by known values
or to be expressed in percent of a reference value.
3-21. Scale Mode, The scale mode of the math feature is
described by the formula: result = X-2 where x is the
y
measurement value, z is the offset value, and y is the
scale factor. This mode allows the measurement value to
be modified by the addition, subtraction, multiplication
or division of a known value. Addition and subtraction
are performed by entering the number to be added or
subtracted in ''z'' and entering | in ''y''. The scale for-
mula then becomes: result = X- (+ 2) = x - (+ 2).
1
Division is performed by entering @ in ''z'' and the
divisor value in ''y.'' The scale formula then
becomes: result = X-9 =_X, Multiplication is perform-
y y
performed by dividing the measurement value by the in-
verse of the multiplier value; that is, multiplication is
performed by dividing by a fraction. The scale formula
becomes: result = X - 8 = xy. As an example: to
l/y
multiply by 10, divide by the inverse of 10 which is 1/10
or .1. Various examples using the scale mode are as"
follows:
a. Current Measurement: Accurate current
measurements can be made by using a low value resistor
shunting the 3455A's input terminals. The value of the
resistor is then entered in the ''y" register (see
Paragraph 3-22), and zero is entered in the ''z'' register
With the resistor connected at the input terminal and the
instrument set in the voltage mode, current
measurements can now be made. You can do this by
connecting the input across the resistor and measuring
the voltage drop across the resistor. This voltage drop, is
proportional to the current through the resistor. By
switching the 3455A to the scale mode, the reading
becomes an accurate current reading in milliamps. Since
the resistor value is in kilo ohms (R) and stored in ''y'',
and since zero is stored in ''z'', the scale equation
becomes:
Vv - ays
ST = R= ® = current in milliamps
where R = Resistor across the input terminals
V = Voltage drop across the resistor
b. Temperature Measurement: A temperature mea-
surement can be made by using a line or resistive
temperature sensor.
Assume that the sensor has a resistance of 1 kilohm at
25°C and changes 5900 ppm/°C. At @°C the sensor
would have a resistance of 852.5 ohm (1 kilohm - [5.9
ohms] 25). This number is divided by 1000 since the
Section II
3455A measurement results are expressed in kilohm and
is entered in the ''z'' register to remove the offset at
0°C. The measurement result of the 3455A is scaled to
read directly in degrees centigrade by solving the equa-
tion for the value of ''y''. This is done where the results
of the equation are equal to 25°C since the sensor
resistance is specified at that temperature. The scale
equation becomes: -
x-z _ LK~.8525K _ 1475 K
yo y a ey
solving for y:y = 1475 K _ .0059 with this number
25
entered in the ''y'' register, the 3455A measurement
result will be presented directly in °C.
c. Accurate 2 Wire Ohm Measurement: When trying
to make an accurate 2 wire ohm measurement, the input
lead resistance and the internal resistance of the 3455A
should be subtracted out from the reading. This is done
by setting the instrument to the desired range and short
the input leads at the measuring point. Store a 1 in ''y'"'
and store the input lead resistance reading in ''z''. Open
the input leads and connect the unknown resistor to the
leads. With the 3455A set in the Scale mode, the value
of the unknown resistor is displayed without the input
lead resistance. Since a | is stored in ''y'' and the lead
resistance (R) is stored in '"'z'', the scale equation
becomes:
X-Z x-R 7 '
a Sy = unknown resistance in ohms
where x = total measured resistance including R
R = lead resistance
d. Limit Testing: The Scale mode of the 3455A can
also be used to do Limit Testing. This can be ac-
complished since the largest number which can be
displayed is + 200,000 and the smallest number is
-200.00. If the magnitude of the display exceeds
200,000, either a ''+LL'' or a ''-LL'' is displayed.
Therefore, the 'ty' and ''z'' constants must be chosen
so that when ''x'' (the reading) is equal to the upper
limit, the display is + 200,000 and when ''x'' is equal to
the lower limit, the display is -200,000. This can be ac-
complished as follows:
When x = the Lower Limit, the DISPLAY should =
-200,000
When x = the Upper Limit, the DISPLAY should =
+ 200,000
therefore, -200,000 = Lower Limit - z
y:
and + 200,000 = Upper Limit - z
y
RevA 3-3