Since
HOT-24, samples of secondary lab-standard (substandard) seawater have
been measured periodically during the measurement run to monitor possible
drifts in Autosal electronics. The
use of substandard seawater saves the expense of numerous costly IAPSO
standards. A substandard seawater batch is a 50-liter
glass carboy filled with seawater and topped with two inches of mineral
oil, which acts as an evaporation barrier.
Substandard batches are made from 60 liters of seawater collected
from 1000 m at Station ALOHA during a HOT cruise.
The mean salinity of the substandard batch is determined by multiple
comparisons with IAPSO standards over the substandard batch lifetime.
The idea is to have a large, stable batch of seawater of known salinity
with which measurements using the autosal can be monitored. Table 7 below shows the substandard batches
used for each HOT cruise and the date each batch was made.
*Batch #7a was made as an emergency substandard for HOT-52 out of the remaining seawater used to make Batch #7. The salinity of this batch was higher due to evaporation in the plastic carboy over time (Figure 5a).
**Batches #11
and #13 had lower salinities than other batches because of mixed seawater
from depths other than 1000 m (see Section 3.2).