The measurements of highest precision in a standards lab are the 1:1 comparisons of similar impedance standards, particularly the comparison between the standards calibrated at the National Institute of Standards and Technology (NIST) and similar reference standards that remain in a lab. This application requires measurement resolution and repeatability to detect parts-per-million (ppm) differences but does not require extreme, direct reading accuracy. Two standards of very nearly equal value are compared using "direct substitution" they are measured sequentially and only the difference between them is determined. When reading directly in value, the resolution of the QuadTech 1689 is between 10 and 100 ppm. However it has a ∆ % mode where the difference between an entered nominal value and the measured value is displayed in percent with resolution of 0,0001 % which is 1 ppm. Single measurements made in this ∆ % mode at a one/second rate have a standard deviation of about 2 ppm at 1 kHz. The use of the instruments AVERAGE mode reduces this by  where N is the number of measurements averaged. Thus, an average of 5 measurements or more reduces this deviation to under 1 ppm. Using averaging, it is possible to get the difference between two impedance’s to 2 ppm or better. It should be noted that although averaging many measurements takes time, an automatic bridge like the 1689 can take a lot of measurements in the time it takes to balance a high resolution, manual bridge.
A favorite technique is to record 5 averaged measurements and take the median of these, which is very easy to do. This gives a record of the spread as well as increasing the precision, and is independent of a large error caused by a line spike, lightning or other non Gaussian noise source. It should be noted that the 1689 has a MEDIAN mode, that takes the middle value of three measurements, these median values can be averaged automatically to give one final result.
The 1 ppm resolution of the 1689's ∆ % mode is not limited to values near full scale as it is on six digit, manual bridge readouts. For these, the resolution of a six digit reading of 111111 is 9 ppm. The 1689 does not discriminate against such values; it has the same 1 ppm resolution at all values. It also has 1 ppm resolution in D and Q (tangent of phase angle), a useful capability but more important for dielectric measurements than for RLC calibrations.
Scaling a calibration of one impedance value to another value is another precision measurement required in the standards lab. There are many techniques used for this process which differ for different types of impedance standards; resistance, capacitance or inductance. One method common to all is to simply measure each value with a bridge, assume the bridge ratio is perfect and apply the ppm correction of the known standard to the unit being measured. This method gives high accuracy when using a transformer ratio arm bridge such as the QuadTech 1615 or 1616 Capacitance Bridges, but such bridges aren't available for inductance or high capacitance. The 0,02 % accuracy of the 1689 compares favorably with available manual bridges that make these measurements. Its ∆ % mode can be used to give ppm resolution for both measurements by entering a different NOMINAL VALUE for each measurement. This improves the instrument accuracy as well because it is not limited by resolution of the display. Moreover, ratio measurements on the same range are independent of a calibration error or drift in the internal standard. Measurements on different ranges can be improved by a recalibration using special standards, such as the calibration kit available for use with the 1689.
Another important feature not usually available in precision lab bridges is the multi-connection capability which allows both 4 terminal, Kelvin connections and 3 terminal, guarded measurements. This adds up to a 5 terminal capability (not 7) which is rarely found in lab bridges and which is particularly important if measurements must be made on both very high value and very low value impedance’s. The automatic open  and short circuit zero correction capability of the 1689 augments the multi-terminal capability by subtracting out the effects of unguarded stray capacitance and mutual inductance between connecting wires.
Obviously, automatic instruments such as the QuadTech 1689 have the advantage of speed because a balancing procedure is not required, but this is not a particularly important advantage when only a few calibrations need to be made. However, there are situations where balancing an ac bridge can be tiresome, especially when low Q components are being measured as they require an annoying series of alternate balances of the two adjustments needed to null an ac bridge. Speed itself can be important when checking multi dial decade boxes. For this task, a suggested trick is to use the MEDIAN mode which will reject the erroneous measurements made while the dial settings are being changed.
A final advantage of an automatic instrument with IEEE 488 bus interface capability is the opportunity of having the result printed out, thus avoiding the opportunity of making a mistake in recording the result, as well as making the record more legible. Moreover, with a computer in the system, any required correction calculations can be made without the chance of more errors, especially the all to common problems with + and - signs. Of course a computer, suitably programmed, also can lead the measurement technician through a complicated calibration process with prompts and procedures that ensure proper measurement techniques and precise data manipulation.