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.
Kelvin Connection
Meaning
Four-wire electrical measurement configurations eliminate the effects of test lead resistance on low-resistance measurements. The kelvin connection separates the current-carrying leads from the voltage-sensing leads to measure resistance with high accuracy. Its application is restricted to precision sensing environments where even a fraction of an ohm of lead resistance would corrupt the measurement.
Measurement Principle
The test setup employs two pairs of leads attached to the resistor under test. One pair forces a known current through the resistor, while the other pair measures the voltage drop directly across the resistor’s terminals. Since the voltage-measuring circuit has very high input impedance, almost no current flows through the sensing leads, eliminating any voltage drop across them and ensuring the calculated resistance is highly accurate.
Lead Resistance
In conventional setups, the resistance of the test leads is added directly to the measurement. This addition introduces measurable errors when measuring low values. The four-wire configuration avoids this issue completely by ignoring lead-induced drops.
Practical Integration
Manufacturers of high-precision oceanographic sensors use this configuration to connect platinum resistance thermometers to the digitizing electronics. This ensures that changes in cable resistance caused by temperature and pressure variations do not affect the temperature reading. It is a standard design choice for all subsea instrumentation that demands millikelvin accuracy.