The most applicable temperature sensor for a pH electrode is a Pt1000
temperature detector. The Pt1000 sensor is a so-called Resistance
temperature detector (RTD). RTDs operate on the principle of changes in
electrical resistance of pure metals and are characterized by a linear
positive change in resistance with temperature.
By far the most common metal is platinum (Pt), because of its wide temperature range, accuracy, stability and repeatability. A RTD sensor element can be made very simple and compact, and that makes it particularly suitable for a pH electrode.
The basic idea is that
the "temperature meter" (pH meter) places a voltage across the RTD and
this will cause a current to flow in the RTD for which the resistance is
being measured, see the figure above. When you have the resistance
value it's only a multiplication of the RTD constant to get the actual
temperature of the RTD.
RTDs are commonly categorized by their nominal resistance at 0°C, and
a Pt1000 temperature sensor has a resistance of 1000 ohms at 0°C. The relationship
between a Pt1000 sensor's resistance and temperature are very nearly
linear. A 1°C temperature change will cause an approximate 4 ohm change
in resistance. A 4 ohm change per 1°C means the Pt1000 linear equation
has a larger slope and makes the resistance measurement relatively
straight forward. You don't need to worry too much about errors
introduced by the sensor cable.
You will also find Pt100 temperature sensors for pH electrodes.
Pt100 sensors have been around for a long time and are still used a lot,
especially in industry and process applications.
A Pt100 sensor has a resistance of 100 ohms at 0°C. A 1°C temperature change will cause an approximate 0.4 ohm change in resistance, so even a small error in measurement of the resistance can cause a large error in the measurement of the temperature. For example, the resistance of the wires leading to the sensor can be a significant source of error.
For precision work, a Pt100 temperature sensor has four wires, two to carry the sense current, and two to measure the voltage across the sensor element. This mean you measure the voltage (U) directly across the resistance sensor and not include the resistance of the current cables. It is also possible to obtain three-wire sensors, although these operate on the assumption that the resistance of each of the three wires is the same.
Because of the low signal levels, it is important to keep any wires away from electric cables, motors and other devices that may emit electrical noise. Compare to a Pt1000 temperature sensor this of course increase the cost and complexity of the sensor cables.
RTDs are also characterized by a relative slow response time and
because they require current excitation, they can be prone to
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