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High Resistance / Low Conductance
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High Resistance / Low Conductance
This meter can measure conductivity of a resistor that is as high as 1015 Ohms, so it is effectively a 1000 TeraOhmmeter. The meter can also read resistance directly up to 2000 GigOhms (2 TeraOhms, or 2 x 1012 Ohms), and conductance down to a resolution of one Femto Siemen (10-15 Siemen) which corresponds to 1015 Ohms or 1000 TeraOhms (resistance in Ohms is the inverse of conductance in Siemens, so you can calculate the resistance if you know the conductance).

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There is always a problem with ultra high resistance measurements: only a very small amount of current is flowing through the component that is being tested. Therefore certain precautions must be taken to prevent interference from external sources. A grounded conductive box with a copper mesh viewing screen allows testing of small components without interference from stray static electricity (usually caused by the operator’s movements). This box, which attaches to the meter, is included. In addition, the "sensitive" terminal (the terminal which is sensitive to static electric fields) can be connected through a shielded test cable (included) to measure any assembly that can’t fit in the conductive box.

The meter’s six resistance ranges are 19.999/ 199.99/1999.9 MegOhms and 19.999/199.99/1999.9 GigOhms. The highest direct resistance that can be read is 1999.9 GigOhms, and the minimum resolution is .001 MegOhm (1 kiloOhm), when the meter is set for reading 19.999 MegOhms. Overall accuracy is +/-1% of the reading, +/- one count.

In addition, there are two conductance (inverse of resistance) ranges. They are 19.999 NanoSiemens and 19.999 PicoSiemens. A NanoSiemen is 1 divided by a GigOhm. It’s the same as one NanoAmp per Volt. A PicoSiemen is one PicoAmp per Volt, or the inverse of one TeraOhm. Therefore, a 1 GigOhm resistor will have a conductance of 1 NanoSiemen, and a 1 TeraOhm will have 1 PicoSiemen. Note that because one is the inverse of the other, then 2 TeraOhms corresponds to 0.5 PicoSiemen. The two conductance ranges therefore cover the following ranges of equivalent resistance: The first conductance range goes from .001 x 10-9 Siemens (=1012 Ohms or 1 TeraOhm) to 19.999 x 10-9 Siemens (=5 x 106 Ohms or 5 MegOhms), and the second conductance range goes from .001 x 10-12 Siemens (=1015 Ohms or 1000 TeraOhms) to 19.999 x 10-12 Siemens (=5 x 109 Ohms or 5 GigOhms). Accuracy is +/-1% of the reading, +/- one count, but external interference may cause additional inaccuracy.

It’s easier to measure very high resistances by using the conductance method than by using the standard resistance measurement method. With a standard resistance measurement, a certain pre-set amount of current is passed through the resistor that is being measured. For example, when the meter is set for 19.999 MegOhms, the internal circuitry causes exactly 100 NanoAmps (10-7 Amps) to flow through the resistor. A 10 MegOhm resistor would then have a voltage of 1 Volt (=107 Ohms x 10-7 Amps) across it, and the display will read "10.000" as a result. When the meter is set to any of the 6 resistance ranges, the display reads proportional to whatever the voltage is across the test resistor. One volt always produces a half-full-scale reading, and 1.999 Volts always produces a full-scale reading of 19999, but the decimal point position depends on which scale is being used. There are 6 different pre-set amounts of current used: Starting with the 19.999 MegOhm and ending with the 1999.9 GigOhm range, the pre-set currents are 100,10, and 1 NanoAmp; and 100,10, and 1 PicoAmp. In each case, if the resistor voltage happens to be exactly 1.9999 Volts (when the appropriate amount of current is passing through it) the meter will show exactly full scale. When measuring conductance, a different technique is used: A voltage difference is applied across the resistor and then the current flowing through the resistor is measured. For purpose of illustration, think of the voltage as 1 Volt. Then the conductance (in Siemens) is the same number as the current (measured in Amps). For example, if a 10 GigOhm (1010 Ohms) resistor has 1 Volt applied across it, a current of 0.1 NanoAmp (10-10 Amp) will flow through the resistor, and this means its conductance is 10-10 Siemens or 0.1 NanoSiemen.

There are two major differences between resistance and conductance measurements:

1) A resistance measurement may take a very long time to settle to its final value if there is very much capacitance connected across the resistor; and 2) By the nature of an Ohm reading vs. 1/Ohms, if a very high resistance is being measured (a resistance which measures near full-scale on the meter), and the resistance is fluctuating by a factor of 10% from one second to the next (it may be noisy because a high resistance has a very "weak" signal), then the display will fluctuate 10% of 19999 counts, or about 2000 counts. This is very difficult to read. Oddly, a low resistance (which is usually much less noisy) would show a very low number on an Ohm setting, so it would be much more stable. This means that when doing resistance measurements, small resistances are extremely stable, while large resistances are disproportionately unstable. However, if the meter is switched to conductance measurement, a high resistance will read a low number on conductance, and a 10% fluctuation may not even be visible as a one-count fluctuation.

Here’s an example of how capacitance can slow down the resistance reading: when measuring a 1 TeraOhm resistor, if less than 1 PicoFarad of capacitance is present, the resistance measurement will settle to within 1% of the final value in 4 seconds, and conductance measurement will settle in 3 seconds. However, if 10 PicoFarads is connected in parallel with the 1 TeraOhm, then resistance measurement requires 40 seconds to settle, while conductance still requires only 3 seconds. (Please note that these settling times apply to air-gap capacitance in the circuit. Solid-gap capacitors often require a longer time to come to equilibrium, because they polarize slowly.) The slow settling of resistance measurements is because the tiny amount of current passed through the capacitor-resistor combination requires a long time to charge the capacitor up to its final voltage.

There are additional control knobs on the meter. The RESPONSE SPEED (signal averaging time) can be set to FAST (recommended) or SLOW (if the reading on FAST fluctuates too much). The previous quoted settling times were on FAST. There are two offset controls: 1) a zero-ohms offset (accessed through a small hole on the right side of the meter), which is adjusted to make the display read zero when the two test terminals are shorted. This zero can be done on any of the 6 resistance ranges and it does not need readjustment when switching to another resistance range. Readjustment is only needed if the temperature changes more than 30 degrees F. 2) A zero-conductance offset (larger knob on right edge). This is adjusted to read zero when the terminals are not connected, and only when the RANGE knob is set on {CONDUCTANCE}. It must be readjusted when switching the toggle switch between {NanoSiemens} and {PicoSiemens}. This subtracts out the tiny currents (usually a few FemtoAmps) from the amplifier’s input, and from other mechanical effects that produce weak currents. This offset should be adjusted only after the other (small knob) offset is done, and should be adjusted more frequently than the small knob offset is adjusted.

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