(3) Thermal EMF in DC output system.
Nonsimilar conductors employed to
couple the DC OUTPUT binding posts to the readout instrument will introduce thermal
EMFs. It is recommended that conductors of the same material (copper) be employed.
(4) Settling time. When the ATS transfers
from the AC balancing state to
the DC output state in the AUTO mode, the internal
DC substitution voltage builds
up to balance the voltage stored in the digital
memory circuit.
The operator
should allow approximately 15 seconds to elapse
before reading the DC output
voltage.
(5) Stability.
Due to inherent temperature-dependent drift in the
thermoelement output (0.01 percent/minute after a 10-minute warmup), the ATS output
voltage should be measured as soon as the instrument has "settled" in the DC output
mode.
If a potentiometer system or differential voltmeter is employed as the
readout instrument, balance the readout instrument as closely as possible after the
ATS has transferred to the DC output state. Repeat the measurement cycle a second
time and then null the readout instrument.
Note that, for maximum DC output
stability, the ATS should be allowed to warmup for at least four hours before
measurements are made.
(6) Auto-zero. The 1600A Auto Balance AC/DC Transfer Standard incorporates
circuitry which automatically transfers from the signal input AC measuring mode to
the DC output read mode.
The ideal DC read time occurs some 15 seconds after
transfer since the same thermal conditions occur in the DC transfer system as
existed at the AC measuring point at which the transfer was initiated.
(a) Making a DC reading too early does not provide the DC transfer
voltage loop time to reach stable equilibrium required for repeatable 100 ppm
measurements.
Waiting too long (more than 20 seconds) causes changes in the
thermal equilibrium and thermal drifts which will cause additive errors to accrue
to the transfer DC reading.
(b) It is advisable to read the DC transfer voltage on the 15-second
basis.
Although the thermocouple is suitable "thermally lagged" and low
temperature coefficient metal film multiplier resistors are used, remember that the
thermal transfer standard is basically a "thermal" sensing instrument. The extreme
sensitivity of the 1600A can resolve ambient changes of as little as 20
millidegrees Celsius and the transfer DC output reading should be made within the
time constraints indicated above to assure specified transfer accuracy.
These
read-out timing conditions apply equally to the AUTO, MANUAL BALANCE, and MANUAL
TRANSFER operating modes.
(c) The 1600A is thermally lagged to provide precise, accurate, and
repeatable transfer measurements within the timing constraints indicated.
To
certify readings, it is recommended that three to four transfer readings are made.
Delete the first reading and average the remaining readings for best results. It
is recommended that the AUTO operating mode be used in recycle mode to make
continuous AC/DC/AC transfer measurements.
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