10
example. Its high temperature is due to the absorption of high energy solar radiation
by resident gases but the sparsity of these same gases in the rarified atmosphere
results in its remarkably low heat content.
Prior to the advent of electronic circuits, the mercury/glass thermometer was the
reference for T measurement calibrated by way of two point standards of equilibrated ice/water and steam/water baths. For oceanographic applications where seawater pressure is a concern, elaborate reversing thermometers, developed by the
instrument manufacturing company of Negretti and Zambra in Victorian England
(accurate to 0.01 °C), were deployed in protected (glass sheathed) and unprotected
pairs. These thermometers provided readout by breaking the mercury column in
situ. A single loop of the glass capillary terminating in a constriction of the capillary
caused separation of the mercury column upon mechanical reversal of the thermometer assembly. The protected thermometer, not subject to local pressure due to the
glass sheath, provided in situ T readings. Readout discrepancies between the thermometer pairs caused by adiabatic (pressure-induced) warming of the mercury/
glass assembly in the unprotected unit allowed sample depth estimates independent
of the more uncertain estimates provided by the cosine calculation of wire paid out
and wire angle to arrive at true instrument depth (Sverdrup et al. 1942).
Resistance Thermometer Detectors
In practice today, temperature standards are established through the International
Practical Temperature Scale 1990 (ITS-90) developed by the SI Consultative Committee
for Thermometry. Preston-Thomas (1990) prescribes the following standard:
The Triple Point of Equilibrium Hydrogen (13.8033 K) to the Freezing Point of Silver
(961.78 °C)… In this range T 90 is defined by means of a platinum resistance thermometer
calibrated at specified sets of defining fixed points, and using specified reference and deviation functions for interpolation at intervening temperatures
The electronic platinum resistance detector (PRTD) calibrated to the standard
ITS-90 thus encompasses the full environmental range for liquid ocean water which
is roughly −2 to 400 °C (including deep-ocean hydrothermal vents). The PRTD, one
of various electronic resistance temperature detectors, was chosen as the SI standard
for its long-term stability, largely due to the chemical inertness of the metal
(Fig. 2.1). Platinum resistance temperature detectors provide accurate temperature
determination for many commercial, industrial, and research applications. For
oceanographic use, PRTDs are incorporated into laboratory bench salinometers (see
below), CTDs, and flow-through systems.
Thermistor Temperature Detector
Field applications are now shifting to reliance on thermistor temperature detectors,
temperature-sensitive ceramic or polymeric semiconductor materials that offer
faster instrumental response and greater resolution. While the PRTD resistance
response to temperature is close to linear, that of most thermistors is steeply curved
2 Electronic Sensors and Instruments for Coastal Ocean Observing
example. Its high temperature is due to the absorption of high energy solar radiation
by resident gases but the sparsity of these same gases in the rarified atmosphere
results in its remarkably low heat content.
Prior to the advent of electronic circuits, the mercury/glass thermometer was the
reference for T measurement calibrated by way of two point standards of equilibrated ice/water and steam/water baths. For oceanographic applications where seawater pressure is a concern, elaborate reversing thermometers, developed by the
instrument manufacturing company of Negretti and Zambra in Victorian England
(accurate to 0.01 °C), were deployed in protected (glass sheathed) and unprotected
pairs. These thermometers provided readout by breaking the mercury column in
situ. A single loop of the glass capillary terminating in a constriction of the capillary
caused separation of the mercury column upon mechanical reversal of the thermometer assembly. The protected thermometer, not subject to local pressure due to the
glass sheath, provided in situ T readings. Readout discrepancies between the thermometer pairs caused by adiabatic (pressure-induced) warming of the mercury/
glass assembly in the unprotected unit allowed sample depth estimates independent
of the more uncertain estimates provided by the cosine calculation of wire paid out
and wire angle to arrive at true instrument depth (Sverdrup et al. 1942).
Resistance Thermometer Detectors
In practice today, temperature standards are established through the International
Practical Temperature Scale 1990 (ITS-90) developed by the SI Consultative Committee
for Thermometry. Preston-Thomas (1990) prescribes the following standard:
The Triple Point of Equilibrium Hydrogen (13.8033 K) to the Freezing Point of Silver
(961.78 °C)… In this range T 90 is defined by means of a platinum resistance thermometer
calibrated at specified sets of defining fixed points, and using specified reference and deviation functions for interpolation at intervening temperatures
The electronic platinum resistance detector (PRTD) calibrated to the standard
ITS-90 thus encompasses the full environmental range for liquid ocean water which
is roughly −2 to 400 °C (including deep-ocean hydrothermal vents). The PRTD, one
of various electronic resistance temperature detectors, was chosen as the SI standard
for its long-term stability, largely due to the chemical inertness of the metal
(Fig. 2.1). Platinum resistance temperature detectors provide accurate temperature
determination for many commercial, industrial, and research applications. For
oceanographic use, PRTDs are incorporated into laboratory bench salinometers (see
below), CTDs, and flow-through systems.
Thermistor Temperature Detector
Field applications are now shifting to reliance on thermistor temperature detectors,
temperature-sensitive ceramic or polymeric semiconductor materials that offer
faster instrumental response and greater resolution. While the PRTD resistance
response to temperature is close to linear, that of most thermistors is steeply curved
2 Electronic Sensors and Instruments for Coastal Ocean Observing