Temperature problems solved for you (VII) Fluid temperature monitoring

In previous articles, we have introduced the basic principles of temperature sensors. For many metering and industrial processes, it is necessary to directly measure the temperature of the fluid, or use temperature data for compensation to more accurately calculate the volume flow of the fluid. Fluid temperature monitoring applications require not only a small sensor size to reduce flow resistance, but also low power consumption in the case of flammable liquids. TI's low-power analog and digital solutions have comparable accuracy to Class AA resistance temperature detectors (RTDs) and consume only 6.3μW.

Introduction to Using Digital Temperature Sensors to Replace RTDs in Calorimeters

A calorimeter is a device that measures thermal energy by measuring the flow rate and temperature change between the inlet and outlet pipes of the system. These devices are commonly used in factories for boiler output measurement and in residential heating and cooling systems for heat transfer measurement.

Since the measurement of thermal energy requires flow rate and temperature, it is imperative that both parameters are accurately measured. Inaccurate measurements can result in incorrect measurements of energy delivery, leading to under- or over-billing.

A passive component called a resistance temperature detector (RTD), whose resistance changes with temperature, can avoid measurement errors. RTDs are made of metals such as platinum, copper, or nickel and support a wide temperature range (approximately -200°C to +850°C).

As shown in the table below, the accuracy of an RTD is defined by its class. The International Electrotechnical Commission (IEC)/European Standard (EN) 60751 defines four RTD classes: C, B, A, and AA, with Class C having the lowest accuracy and Class AA having the highest accuracy. The lower accuracy classes will have a wider temperature range. For example, a Class C thin-film RTD covers a temperature range of -50°C to +600°C, while a Class AA thin-film RTD covers a temperature range of 0°C to +150°C.

Temperature accuracy classes defined by IEC 60751

Applying a current source will produce a voltage difference across the RTD; this voltage is proportional to the resistance of the RTD and its excitation current. This voltage data is used to measure the difference between the two temperature sources from the inlet and outlet tubes.

Fluid temperature measurement in calorimeters

Solid-state heat meters are becoming increasingly popular for calculating heat energy bills for residential and industrial users. These meters measure flow at either the inlet or outlet pipe and have a pair of matched RTD probes at both the inlet and outlet pipes. The following figure shows a block diagram of a heat meter system using RTDs.

Fluid Temperature Measurement Using RTDs

RTDs offer low power and high accuracy, which are ideal because in most residential units, heat meters are stand-alone, battery-powered systems. The ability for the system to quickly wake up from power-down mode, sample the RTD temperature, and then return to power-down mode extends battery life and minimizes energy consumption.

However, these systems require well-matched RTDs to correctly read differential measurements and require careful consideration of system cost and complexity during design.

A typical heat meter uses a pair of PT100, PT500 or PT1000 sensors connected to a high precision analog front end. Specifications such as European EN 1434 define the requirements for these meters. The two main aspects of this specification are:

• Sensor accuracy and type. Chapter 5.4 of EN 60751 recommends the use of industrial platinum resistance thermometers with accuracy greater than Class B or equal to Class A or AA in a three-wire or four-wire configuration. EN 1434-2 states that other types of temperature sensors may be used, but if they are done, they must not be separated from the calorimeter or calorimeter device.

• Calibration. Chapter 4.1 of EN 1434-2 states that all pairs of temperature sensors must be pre-calibrated and paired before being built into a calorimeter. Alternatively, the sensors should be securely mounted on the calorimeter's printed circuit board and then calibrated to minimize the temperature offset between the two PT sensors.

Design Considerations for Digital Temperature Sensors for Calorimeters

High-precision digital temperature sensors integrate the sensor and analog-to-digital converter into a single device. No additional temperature conversion processing is required by the host microcontroller.

Let’s evaluate the design considerations based on the industry standard specifications listed above:

• Sensor accuracy and type. Digital temperature sensors such as the TMP117 support an accuracy of ±0.1°C over the -20°C to +50°C temperature range, with a maximum accuracy specification of ±0.3°C over the -55°C to +150°C temperature range. These specifications exceed the accuracy of Class AA RTDs over the same range and meet the requirements of the EN 60751 and EN 1434-2 specifications for accuracy and sensor type.

• Calibration. The ability of a sensor to reproduce readings when taking consecutive temperature measurements under the same conditions is called repeatability. The TMP117 has a repeatability of 7.8125m°C, so the sensor itself has very consistent and reliable performance. The TMP117 also includes a temperature offset register that can be used to store a temperature offset during initial calibration and then added to the temperature result after linearization. Calibrating the sensor pair at the inlet and outlet is a must for fluid temperature measurements using a calorimeter. Having good repeatability specifications and an offset register makes for a reliable, repeatable, and ultimately simplified design.

Using digital temperature sensors in your design eliminates the offset and gain calibration steps of the analog signal chain (which is required for traditional analog signal measurements in two-, three-, or four-wire RTD-based configurations). Compared to traditional analog RTD sensors, digital temperature sensors can store user-defined calibration parameters in an 8-byte electrically erasable programmable read-only memory (offset register), thus eliminating even the paperwork and calibration data handling. In addition, it is more energy-efficient to read out the nonvolatile memory contents at the beginning and apply an offset or any other adjustment to the result each time the temperature is measured.

Fluid Temperature Measurement Using TMP117

The four-wire digital interface using this architecture is electrically compatible with standard analog four-wire RTD sensors. RTD-based designs also require a high-precision reference resistor with an accuracy typically better than 0.1% and ±25ppm/°C and a matched resistor-capacitor filter. The use of an integrated digital design approach also eliminates the need for this high-precision reference resistor. The following table summarizes the design considerations for the RTD sensor and the TMP117 digital temperature sensor.

Design Considerations for RTD and Digital Temperature Sensors

Overall, the high-precision TMP117 temperature sensor eliminates multiple narrow-tolerance discrete components and integrated devices, saving PCB space, complexity, and cost in a calorimeter.

Click here to quickly locate the TI Analog column for more information on this topic, or for general advice on measuring temperature.

Temperature problems solved for you (I) Basic principles of temperature sensing

Temperature Problems Solved for You (II) System Temperature Monitoring

Temperature Problems Solved for You (III) High-Performance Processor Mold Temperature Monitoring

Temperature problems solved for you (IV) Ambient temperature monitoring

Temperature Problem Solved for You (V) Efficient Cold Chain Management through Scalable Temperature Sensors

Temperature Problems Solved for You (VI) Design Challenges of Wearable Temperature Sensing