Water temperature is a physical property of water indicating its relative hotness or coldness. More specifically, it is the measure of the thermal energy produced by the vibrations of water molecules. Inputs such as solar radiation increase the kinetic energy of the molecular vibrations, thereby increasing the temperature.
The Celsius scale with units of degrees Celsius (°C) is most commonly used to express temperature in water-related research applications. The Fahrenheit scale (°F) is also used in some applications and regions.
Why Measure Temperature?
Water temperature is significant due to its direct impact on aquatic organisms and influences on other important water quality parameters. For example, temperature plays a pivotal role in aquatic ecosystems as it determines the suitability of habitats for different species. Each aquatic organism thrives within a specific temperature range, and deviations beyond this range can lead to migration or die-offs.
Even within their tolerable temperature limits, fluctuations can significantly impact the metabolic rates and other biological processes of these organisms, thereby influencing the overall productivity and balance of the ecosystem. This makes temperature a key factor in the diversity and health of species in natural water bodies such as lakes, rivers, streams, and marine environments.
Besides direct impacts on aquatic ecosystems, temperature affects water chemistry and other physical properties and processes. Other measurements influenced by temperature include:
- Conductivity: Electrical conductivity is caused by dissolved ions in water. Higher temperatures increase the solubility of salts and other substances that contribute ions while simultaneously increasing ionic mobility, which allows water to conduct electricity. Temperature can, therefore, influence both ionic concentrations and the physical ability to conduct at any given concentration.
The electrical conductivity of water is temperature-dependent
- Dissolved oxygen: Oxygen, another essential component of aquatic ecosystems, dissolves in water with an inverse relationship to temperature. As a result, colder water can hold more dissolved oxygen (DO) than warmer water, all other factors being equal.
The saturation point of dissolved oxygen generally increases as water cools and decreases as it warms.
- pH: Pure water with equal hydrogen and hydroxide ions is neutral with a pH of 7.00 at 25°C. However, while remaining neutral, the pH will vary inversely with temperature changes (higher pH below 25°C and lower pH above). Temperature and pH have a more complex relationship in non-pure waters, but because temperature influences chemical reaction rates, shifts the equilibrium point of reactions that produce or consume hydrogen ions, and affects the solubility of carbon dioxide, it can influence pH in different ways.
Pure water at 25 degrees Celsius has a pH of 7.0, but the neutral pH varies with temperature.
- Oxidation-reduction potential: Like pH, temperature has a variable effect on oxidation-reduction potential (ORP). In the case of ORP, it depends on the chemical species present in the water sample.
- Water level: Water density changes with temperature. Density will generally decrease with increases in temperature. However, the maximum density of pure water is achieved at 4°C (this is why ice floats on liquid water). Density contributes to thermal stratification in some water bodies as cooler, denser water sinks to the bottom, impacting water level.
Pure water has its maximum density at 4°C.
In summary, water temperature is a key driver of various physical, chemical, and biological processes in aquatic environments, and it plays a substantial role in determining overall water quality. The interplay of temperature with other water quality parameters is a fundamental aspect of aquatic ecology and water resource management.
How Is Temperature Measured?
Temperature is most commonly measured with a thermistor in water quality applications. A thermistor is constructed of a semiconductor whose resistance changes predictably and consistently with changes in temperature. The word thermistor comes from a combination of “thermal” and “resistor”.
The electrical resistance of a thermistor decreases as temperature increases.
How to Select a Temperature Sensor?
Thermistors are preferred for temperature measurement in natural waters (lakes, rivers, streams, coastal, and ocean) because they offer a high level of precision and stability. Important factors to consider include specified accuracy and response time. Long-term sensor drift, although usually minimal, can be important for extended applications as well.
Many water quality sensors and multi-parameter platforms that incorporate a thermistor achieve accuracies ranging from 0.01°C to 0.2°C. This is sufficient for many applications, but for demanding research applications where accuracy is paramount, some sensors utilize advanced calibration, characterization, and quality assurance procedures to achieve accuracy as great as 0.001°C.
Response time is particularly important for sampling or other applications where the sensor will quickly be moved between locations, or in cases where rapid temperature changes are expected, such as thermal pollution.
What to Consider When Preparing a Temperature Sensor?
Since thermistors are generally stable and do not require calibration, there is relatively little preparation required compared to other sensor types. Fouling should be considered, as this can affect sensor response time. Measures such as a wiper or anti-fouling coating may help, but regular cleaning may be required.
If the measurements will be used to temperature-compensate other sensor readings, the placement of the sensor should also be considered to ensure that it is recording representative values.
How to Deploy a Temperature Measurement System?
Temperature measurement is often included with other sensor types or multi-parameter instrument designs. In such cases, care should be taken to ensure that the instrument is well protected from any physical obstructions, such as debris loads, and regularly cleaned to remove excessive fouling and inspect for corrosion or damage.
For dedicated temperature sensors, ensure secure mounting with direct contact of the thermistor with the water. Avoid contact with anything other than the water that could affect heat transfer to the thermistor, such as mounting components (hose clamps, cable ties, tape, etc.).
Thermistor strings with multiple nodes should be adequately supported by a mooring line.
Temperature profiling applications require mounting the nodes and data cables on a mooring line to ensure that they maintain their position and avoid cable strain. Secure the string at evenly spaced increments to a mooring line with a small amount of cable slack. If using a modular system, connect successive sensors together using cables of appropriate length, taking care that any connector O-rings are present and properly lubricated.
Data buoys offer a convenient method for the deployment of temperature profiling strings in surface waters. The string is suspended in the water column from an eye nut on the buoy hull such that the sensors are referenced at depth from the surface. The data cable connects to a data logger with wireless communications inside the buoy hull, which is powered continuously by a solar-charged battery. A multi-point mooring system is often preferred to accommodate water level changes without the risk of tangling or twisting the sensor string.
A two-point buoy mooring can accommodate a suspended thermistor string with minimal stress on its mooring line.
Conclusion
Temperature is a measurement of how hot or cold water is, which is a representation of the molecular kinetic (motion) energy. It is one of the fundamental measurements performed in natural waters due to its direct impacts on water quality and aquatic ecosystems, as well as its effects on other measurements and physical and chemical processes.
Thermistors are used to measure water temperature by correlating changes in resistance to temperature. These can be deployed individually for single-point measurement, combined with other sensors in a multi-parameter instrument, or in string configurations for multi-depth profiling.
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