Question: Why are Continuity or Loop tests reading too low or even 0.00
Answer: Often caused by an artificially high value in the zero memory.
The Loop test (Zl) and continuity test (Rlo) have independent zero memories which both stores the resistance value of your used test leads. When the instrument has a value in its memory for those functions there is a “Ø” symbol on the display when that function is selected…
The zero function will measure the test leads and show the reading on the display, this is stored in the zero memory for the function. The value displayed should be consistent, some small variation is normal. For Continuity, a zero can be easily verified by simply pressing Test after the Zero, however this is not possible on Loop.
When measurements are showing unexpected results it can be useful to clear the zero memory and take a measurement that includes the lead resistance. Perform the “Zero” with the leads open circuit, the display will show “ > 3.00” and the “Ø” will disappear from the display, indicating that the zero memory for that function is empty.
Low batteries can cause an issue with the Zero function, press F3 when in the voltage mode to display the off-load battery voltage. The 165x and 166x series use 6 x AA cells in series providing 9V, it is recommended that this battery test should show at least 7V.
The battery monitor circuit is only active when no testing takes place, during a test it is switched out. The Zero function and continuity test draw most current from the batteries (EN61557 requires a 200mA short circuit current). If the batteries are low, but not low enough to trigger the monitor, the 200mA current draw can make the battery voltage collapse below an operational level.
If the test leads have an intermittent fault where their resistance changes, this can cause problems with the lead zero too.
To check test lead stability, select the continuity function (Rlo) and clear the zero store (as above). Remove the probe or crocodile clip from a single test lead and use that lead to connect between the two input terminals selected for the measurement. Push and hold the test button to make a measurement of the lead, during this measurement move the lead and apply gentle pressure to the terminals. Ensure the reading does not change.
The expected measurement result from the Loop or continuity function can be relatively low compared to the measurement range of the instrument, eg a 0.20 Ω result on a 20.00 Ω range is only 1 % of range.
The accuracy specification is ±(1.5% of reading + 3 digits), for 0.2 Ω this calculates to ±0.03 Ω (between 0.17 Ω and 0.23 Ω).
Although the display is digital the measurement is an analogue process, consider a deflection of 1% on an analogue display (moving coil meter - with a needle).
Similar to the resistance of the test leads and to bad contact resistance, eg new leads are typically between 0.12 Ω & 0.18 Ω.
It is good practice for test probe contact during the zero to match the connection during the measurement. Instead of simply joining the croc clips together (small points of contact), place them close together on an end of the cable to be measured (gripping onto a circular profile as they will be during the test)
Last comment: When you change your test lead configuration, e.g. using a long reel for Rlo measurements, then re-initiate the zero function again to have the correct compensation value in the memory!