1.0. Introduction

The “National Field Manual for the Collection of Water-Quality Data” (NFM) is the official and citable protocol for the collection of water-quality data by the Water Mission Area (WMA) of the U.S. Geological Survey (USGS). The NFM provides guidelines and procedures for USGS personnel who collect data for water quality in surface water and groundwater, with detailed, comprehensive, and citable procedures.

National USGS program and project personnel, as well as those in USGS Water Science Centers collecting water-quality data, for projects supported by USGS Cooperative Matching Funds, are mandated to use protocols provided in the NFM (USGS Office of Water-Quality Technical Memorandum 2002.13—U.S. Geological Survey, 2002). Formal training, as provided in the USGS class “Field Water-Quality Methods for Groundwater and Surface Water,” and field apprenticeships supplement the information provided in the NFM and support the collection of unbiased, high-quality data.

Chapter A6.1 provides guidance and protocols for the measurement of temperature of air, of a surface-water body or in groundwater, which include the scientific basis of the measurement, selection and mainte­nance of equipment, calibration verification, troubleshooting, and procedures for measurement and reporting. Temperature is an important variable in aquatic systems, and accurate measurements of this property are the basis for describing other attributes of those systems. Temperature is routinely measured when water samples are collected, is often measured continually at USGS streamgages, and is regularly measured during laboratory and field experiments.

This chapter updates and supersedes USGS Techniques of Water-Resources Investigations, book 9, chapter A6.1, version 2.0 (Wilde, 2006). It covers the physical basis for measurement of temperature in aqueous solutions, as well as factors that affect the measurement, terminology, method protocols, and applications. The field method described in this chapter is applicable to air and most natural waters.

Before 2017, the USGS NFM chapters were released in the USGS Techniques of Water-Resources Investigations series. Effective in 2018, new and revised NFM chapters are being released in the USGS Techniques and Methods series; this series change does not affect the content and format of the NFM. More information is in the general introduction to the NFM (USGS Techniques and Methods, book 9, chapter A0—U.S. Geological Survey, 2018) at https://doi.org/10.3133/tm9A0. The authoritative current versions of NFM chapters are available in the USGS Publications Warehouse at https://pubs.usgs.gov. Comments, questions, and suggestions related to the NFM can be addressed to nfm@usgs.gov.

2.0. Equipment and Supplies

A temperature sensor is any device used to measure temperature. In its simplest form, a temperature sensor can contain a detector that creates a signal used to infer temperature according to a calibration. Standard procedures used in the past by the USGS to measure temperature involved the use of liquid-in-glass thermometers. Since the 1990s, digital thermometers and other equipment utilizing integrated thermistors and thermocouples, such as multiparameter instruments, have continued to undergo improvements in accuracy, reliability, and cost. Digital thermometers have many advantages over liquid-in-glass thermometers, including smaller uncertainties, faster response, ease of automation, and elimination of uncertainty related to lighting and viewing angle (Ripple and Strouse, 2005). Electronic temperature sensors vary voltage, resistance, current, or frequency outputs with temperature. Common sensors include thermistors, an electrical resistor with a resistance that varies sharply and predictably with temperature, and thermocouples, sensors containing two dissimilar metals bonded to form a connection with a dielectric potential that varies with temperature. To make a digital thermometer, a sensor—either thermistor or thermocouple—is connected to a signal conditioning device which linearizes the signal that communicates information to a display.

Digital thermometers and multiparameter instruments utilizing integrated sensors have replaced liquid-in-glass thermometers for nearly all USGS temperature measurements. Barring any project-specific requirements, using digital thermometers or equipment utilizing integrated sensors, such as multiparameter instruments, should be considered the standard field method for temperature measurement of water and air. Liquid-in-glass thermometers containing mercury must never be taken to the field (USGS Office of Water-Quality Technical Memorandum 1994.02—U.S. Geological Survey, 1993). Mercury is a hazardous substance, and its use should be avoided to prevent contamination and danger to personnel. Mercury-filled and liquid-in-glass thermometers in general should be phased out using digital thermometers and other equipment utilizing integrated sensors whenever possible.

Other types of sensors for temperature measurement include fiber-optic sensors and infrared (IR) thermometers. These systems are generally applied to situations where heat flow is occurring in a medium or temperature is used to trace mass flux. Fiber-optic temperature sensors contain a light source and detector. These sensors work by detecting changes in the wavelength of light caused by temperature. Fiber-optic sensors can be useful for measuring distributed temperatures across multiple locations along a fiber-optic strand and relative temperature differences. Infrared thermometers and temperature sensors work by focusing the temperature-dependent IR radiation emitted from an object onto a detector and converting the energy into a measurable electrical signal. The construction of the focusing and detecting elements of an IR thermometer determines its range and accuracy. Forward-looking infrared (FLIR) thermometers combine an IR temperature sensor with camera technology to produce an image or a movie of temperatures depicted by a scale of colors. Depending on the processor, focusing method, and detector used, FLIR thermometers can produce high-resolution temperature images and are powerful tools for determining relative temperature differences.

Infrared thermometers (including FLIR) detect surface temperature only and can be used for groundwater/surface-water interaction studies or other hydrologic applications where information about surface temperature variation is useful. Their accuracy is limited due to reflections of IR sources off the water surface and movement of the water itself, making them undesirable for routine water temperature measurement. Procedures for the use of fiber-optic, IR, and other non-contact methods of measuring temperature are beyond the scope of this NFM chapter. Quality assurance (QA) procedures for these and other sensor types is left up to the users and is determined by the specific project objectives.

Where applicable, temperature-measurement equipment and supplies (described in box 6.1-1) must be tested before temperature data are collected and cleaned soon after use. Each thermometer or sensor must have a logbook in which all calibrations and repairs are recorded, along with the manufacturer make and model and serial number. Logbooks may be in digital or paper form. A sensor should have its own logbook if calibration information is stored in the sensor or a digital thermometer may have a logbook if the thermometer electronics and sensor are calibrated as a unit as in the case of many digital thermometers.

2.1. Definitions

Within this chapter the following terms will be used and are defined as follows:

• Certified Temperature Sensor.— A sensor with a valid calibration certificate of NIST traceability from a laboratory certified by NIST's National Voluntary Laboratory Accreditation Program (NVLAP) (NIST, 2024). For use in the procedures outlined in this chapter, these sensors or digital thermometers are required to have a range of at least -5°C to 45°C, and a certified accuracy of less than or equal to ±0.05°C with a readout resolution of at least 0.001°C. The calibration certificate is valid for up to 2 years.

• Traceability.— Property of a result of a measurement or value of a standard whereby it can be related with a stated uncertainty, to stated references, usually national or international standards, through an unbroken chain of comparisons (NIST, 2023).

• Traceable Temperature Sensor.— A sensor that is compared to a certified sensor every 6 months in an unbroken chain of comparison measurements that lead back to NIST standards through a certified sensor. It has a range of at least -5°C to 45°C and a manufacturer’s stated accuracy of less than or equal to ±0.2°C. Section 2.2 Establishing and Maintaining a Traceable Sensor outlines the procedure required to establish the chain of comparisons, which are made with a certified sensor. Traceable sensors are used for discrete data collection and as a reference sensor to verify the accuracy of deployed sensors.

• Verified Temperature Sensor.— A sensor with documented data quality determined via an annual comparison to a traceable or certified temperature sensor, creating NIST traceability. Uncertainty in a sensor's measurements increases with each additional degree of separation from a certified standard. Therefore, the sensor is required to go through procedures described in section 3.0 Calibration Verification to define and verify the uncertainty of data collected with a sensor and establish traceability, regardless of the sensor's accuracy requirement. Verified sensors may be used for continuous temperature measurements and data collection for publication. An accuracy specification for a verified sensor is only required if the sensor is bundled onto a multiparameter instrument and is used to correct another water-quality parameter (such as dissolved oxygen or specific conductance). The accuracy of such a sensor is required to be less than or equal to ±0.2°C.

• Water Temperature Sensor.—Any integrated sensor or technology used to measure water temperature utilizing a thermistor or thermocouple. This can be a sensor combined with a readout device to form a digital thermometer or a multiparameter instrument with an integrated temperature sensor.

The relation of the sensor classifications defined above can be seen in figure 6.1-1. The procedures in this chapter should provide traceability of all temperature measurements to certified sensors.

3.0. Calibration Verification of Temperature Sensors

Temperature sensor and digital thermometer calibration differ from the process by which a pH or conductivity sensor is adjusted until the accuracy of its performance conforms to that of an accepted calibration standard. For temperature sensors, calibration refers to a verification of calibrated accuracy at a range of temperatures covering the expected or actual range of use or deployment. The verification comparison is done with a certified or traceable thermometer or sensor. A traceable classification for a temperature sensor can be accomplished by following the procedures in section 2.2 Establishing and Maintaining a Traceable Sensor. A new sensor is required to be checked against a certified or traceable sensor at points covering the range of expected temperature measurements using a water bath, before the sensor is used to collect environmental data. Calibration verification of sensors collecting data for USGS publication is required to be performed every 12 months through one of the following two methods. Both methods are acceptable and neither method is preferred.

Method 1.—The annual, required, verification is performed in a laboratory setting using a water bath by comparison with a certified or traceable sensor as described in section 3.2 Calibration Verification in a Laboratory Using a Water Bath. The water temperature range for verifications must bracket and include points that represent the range expected to be measured in the field. See section 3.1 Determination of Points for Sensor Calibration Verification to determine the range and number of verification points.

Method 2.— The annual verification requirement for deployed temperature sensors is met by performing a verification during each site visit throughout the year or field deployment period by comparison with a certified or traceable temperature sensor, in situ, and is an ongoing process for the annual temperature cycle. If this option is chosen, consult section 2.2 Establishing and Maintaining a Traceable Sensor and section 3.3 In Situ Calibration Verification of Sensors. Many traceable sensors may be established for use in calibration verifications. In situ verifications must be within the range of ambient temperatures for the site. See section 3.1 Determination of Points for Sensor Calibration Verification to determine the range and number of verification points. The methods and results of the sensor verification process must be documented and readily available.

In addition to the requirements for annual calibration verification, if a traceable or verified sensor is used to collect data, it must be checked against a certified sensor at one point, immediately before the field trip. This 1-point comparison can be done during calibrations of other sensors or as a stand-alone comparison.

Sensor verifications must be documented in the sensor logbook be it paper or electronic and may be duplicated with the data in a database or data release. Verification documentation is required to include the make, model, serial number, and verification water temperature measurements from the verified sensor and certified or traceable sensor.

4.0. Measurement

Air and water temperature should be measured and recorded whenever water-quality samples are collected. Water temperature must always be measured in situ or in a well-designed groundwater flow-through cell and in a manner that ensures that the measurement accurately represents the intended sample conditions. Procedures in this chapter for measuring air and water temperature are for attended, discrete measurements. Sensors classified as certified, traceable, or verified may be used so long as the accuracy and precision requirements of the project are met and the sensor comparisons and verifications described in sections 2.1 Definitions, 2.2 Establishing and Maintaining a Traceable Sensor, and 3.0 Calibration Verification of Temperature Sensors are met.

5.0. Troubleshooting

Contact the equipment manufacturer if the suggestions on table 6.1-1 fail to resolve the problem, or if additional information is needed. There are very few problems with temperature sensors and digital thermometers that can be solved in a field setting. The best practice is to have new, verified water temperature sensors available to replace malfunctioning field sensors. Be sure to follow manufacturer’s recommendations for cleaning and troubleshooting in general.

6.0. Reporting

USGS temperature measurements should be stored in the National Water Information System (NWIS) database (U.S. Geological Survey, undated). These data may be published as the verified negative or positive value measured.

At the time of writing (2024), NWIS (Aquarius Time Series and Aquarius Samples) is undergoing system changes that should allow the data to be stored with its associated uncertainty. When NWIS can store uncertainty, it is to be stored with the data in the form of metadata (like ±0.2°C) or the manufacturer’s stated accuracy that is verified through the procedures in section 3.0 Calibration Verification. Until NWIS can store the uncertainty of the data, only data that are verified as accurate to less than or equal to ±0.2°C should be stored in NWIS. Data not verified as accurate to less than or equal to ±0.2°C should be published as a USGS data release with its associated verified uncertainty. Temperature data may be stored in the database unrounded. Unrounded temperature data in the database will be rounded when retrieved for publication.