AEMC Instrument’s Micro-Ohmmeter Model 6240 is a rugged low resistance tester designed
for both indoor and field use. Typical applications for this instrument include measuring resistance
for
Metallic coatings, especially in aeronautics Ground connections and continuity
Motors and small transformers Breakers and switches
Electrical cables Wire to terminal connections
…and rail bonds, among others.
The Model 6240 measures resistance from 5 micro Ohms to 400 Ohms. The instrument provides
selectable test currents of 10A, 1A, 100mA, and 10mA;
and includes a polarity reversal function.
Up to 99 measurements can be stored in memory.
The Model 6240 employs the Kelvin testing method to measure very low resistances to within
within a quarter of a percent accuracy. Kelvin testing uses two separate pairs of leads. One pair,
called the “source” leads, provide a known quantity of constant current. The second pair,
called the “sense” leads, measures voltage. There is virtually no current flowing through
the sense leads; nearly all the current flows through the source leads. This allows for
a high level of measurement sensitivity and accuracy when the load under test is of low
resistance.
This video is the first of a two-part series describing how to use the Model 6240. In this
video we explain how to set up and run a test on the instrument. In a separate video, we
will show how to download the test results to your computer using AEMC’s DataView software.
For our demonstration, we will connect the instrument to a 166.6 micro-Ohm shunt and run
a one minute test with a current of 10 amps. To make the connection, we will use Hippo
type Kelvin Clips. The Model 6240 also accepts alligator and spring tip probes.
Be sure all leads are correctly attached to the instrument’s terminals, as explained in a separate AEMC
video. And when using hippo or alligator probes, ensure that both jaws are in contact with
the resistor under test. One jaw functions as a source lead, and the other as a sense
lead. Therefore both must be firmly in contact with the resistor surface to provide an accurate measurement
Before we begin, let’s take a moment to get acquainted with the Model 6240 user interface.
Below the LCD are five function buttons.
The Memory button, labeled MEM, stores the test in the instrument’s memory where it
can be subsequently viewed on the LCD and downloaded to a computer running DataView.
This button also initiates Auto mode, which we explain later in this video.
Below the Memory button is the Clear button, labeled CLR. This removes the selected test
from the instrument’s memory.
To the right of the Memory button is the Memory Recall button, labeled MR. This displays records
stored in memory.
The next button to the right is the Polarity button, labeled ± 1. This reverses the polarity
of the test current, and displays the average of the two polarity measurements. This is
useful for minimizing the effects of stray DC currents when measuring very low resistances.
This button also serves as the Up arrow in user-configurable screens, such as those displayed
in Set-Up mode.
Display toggles between displaying current measurements and voltage measurements. This
button also functions as a Right arrow in user-configurable screens.
To the right of the LCD is the instrument’s selection dial. This dial turns the instrument
on and off, places the instrument in Set-Up mode, and selects the test measurement resistance
and current range.
Below the dial is the Start/Stop button for starting and pausing test measurement sessions.
The Model 6240 interface is described in detail in the user manual that accompanies the instrument.
We begin our simple demonstration by turning the dial on the Model 6240 to Set-Up. After
a few seconds the first setup screen appears. Set-Up screens allow you to set several configuration
parameters for the instrument. Use the Right button to cycle through the setup screens
to ensure the settings are appropriately configured for the test you want to run. For example,
to check the instrument’s time, press the Right button once to display the current time
setting. If this is incorrect, press and hold down the Right button until the setting blinks.
Then use the Up button to change the setting. Use the Right and Up buttons to make any other
required configuration modifications, as explained in the Model 6240 user manual.
When the configuration is set, you are ready to take a resistance measurement. If the resistance
you are testing is unknown, we suggest starting with the instrument’s highest resistance
range and then lowering the range until you obtain meaningful results. In our demonstration
we are using a shunt of known resistance, so we will turn the dial to the 10A 4000 μOhm
setting.
To begin the test, press the Start/Stop button once. After a moment the resistance and current
measurements appear in the LCD. As you can see, current is slightly over 10A, and resistance
is approximately 167 μOhm. Note that you can also display the test voltage by pressing
the Display button. In our example, test voltage is approximately 1.7 mV.
To end the test, press Start/Stop a second time. The word HOLD appears on the screen,
indicating the test has been halted. To resume the test, press Start/Stop.
To save the stopped test in the instrument’s memory, press the Memory button. This displays
the default Object Number and Test Number under which this test will be saved. An Object
can contain multiple tests, similar to a folder containing multiple files. You can accept
the displayed Object and Test numbers, or change them by pressing the Up button, and
then using the Up and Right buttons to make changes. To save the test to memory, press
the Memory button and hold it down until the word Done appears.
Stored tests can be viewed in the Model 6240’s LCD. To do this, press the Memory Recall button.
This displays the last test viewed. Press the Up button to display any other tests in
this Object. To view tests stored in another object, press the Right button and use the
Up button to select the object number.
Note that the Model 6240 also provides Auto mode, initiated by the Auto button. In Auto
mode, the instrument begins recording a new test each time you connect the probes, and
ends the test when you disconnect. Tests are automatically incremented and saved with each
connection and disconnection.
This concludes Part 1 of our Model 6240 demonstration. In Part 2, we show how to connect the instrument
to a PC to download, display, and analyze the record we have created, using AEMC DataView
software. And be sure to check our YouTube channel for additional instructional videos
about other topics in electronics, including the many products offered by AEMC.