Thermistor Signal Conditioning in Subsea Data Logging Systems
Pulsed ratiometric excitation and ultra-low tempco references eliminate subsea thermistor self-heating and hydrostatic drift down to millikelvin accuracy.
Self-Heating Dissipation
Meaning
Electrical currents flowing through resistive sensing elements generate internal heat that can distort the accuracy of temperature measurements. The self-heating dissipation of a sensor represents the rate at which this thermal energy is transferred to the surrounding medium. It is an important factor in precision thermometry where the goal is to measure the environmental temperature without influence from the sensor itself.
Thermal Error
The temperature of the sensor rises above that of the surrounding water when the generated heat cannot escape quickly enough. This temperature offset leads to an overestimation of the ambient temperature. In stagnant water, this effect is more pronounced because the lack of fluid movement limits heat transfer away from the probe.
Mitigation Technique
Designers minimize this error by reducing the excitation current applied to the thermistor to the lowest practical level. Pulsed excitation methods are also used to apply current only during the measurement window. Increasing the surface area of the sensor sheath also enhances the dissipation rate, helping to stabilize the reading even in environments with low fluid velocity.
Calculation Method
The dissipation constant determines the magnitude of the self-heating error. This constant is determined experimentally by measuring the temperature rise in a controlled bath. It allows the correction of remaining thermal offsets during data post-processing.