Texas Instruments | RTD replacement in heat meters using digital temperature sensors | Application notes | Texas Instruments RTD replacement in heat meters using digital temperature sensors Application notes

Texas Instruments RTD replacement in heat meters using digital temperature sensors Application notes
RTD replacement in heat meters using digital temperature
sensors
A heat meter is a device which measures thermal
energy by measuring the flow rate and the change of
temperature between the inlet and outlet of the
system. These devices are very commonly found in
both industrial plants to measure boiler outputs and in
residential heating and cooling systems to measure
the heat delivery.
Since the measurement of thermal energy requires
both flow rate and temperature, it is imperative that
they be measured accurately. Accuracy is important
because an inaccurate measurement can result in
under billing or over billing.
To avoid measurement errors, a resistance
temperature detector (RTD) is commonly used. An
RTD is a passive component whose resistance
changes with temperature. RTDs are made using
metals—such as platinum, copper, or nickel—and
support a wide temperature range (approximately
–200°C to +850°C). The accuracy of a Platinum RTD
is defined by its class. The IEC/EN60751 standard
defines four RTD classes—Class C, B, A, and
AA—where Class C is the least accurate and Class
AA is the most accurate. The lower accuracy classes
will have a larger temperature range. For example, a
Class C thin-film RTD covers the temperature range of
–50°C to +600°C, while a Class AA thin-film RTD
covers the temperature range of 0°C to +150°C.
Most RTD applications use a current source to excite
the RTD element and create a voltage difference
across the RTD, as shown in Figure 1. This voltage is
proportional to the resistance of the RTD and the
excitation current. The voltage potential is amplified,
converted to a digital output by an ADC, and then fed
into an MCU where a lookup table is used to convert
the digital output to temperature.
RTDs in Heat Meters
Solid-state heat meters are gaining popularity in heat
energy billing for residential and industrial users.
These meters come with both flow measurement on
either the inlet or outlet pipe and a pair of matched
RTD temperature probes on both inlet and outlet
pipes. Figure 2 shows a block diagram of a heat meter
system using RTDs.
Figure 2. Fluid Temperature Measurement With
RTD
Low-power and high-accuracy RTDs in heat meters
are desired because heat meters are standalone,
battery-powered systems in most residential units. The
system’s ability to quickly wake from power-off mode,
sample the RTD temperature, and return to power-off
mode extends battery life and minimized energy
consumption.
However, these system require that the RTDs are wellmatched and have matched traces to read the
differential measurement correctly. At the same time,
the system cost and complexity requires careful design
consideration and costly calibration. High-accuracy
digital temperature sensors like the TMP117 can
provide a cost-optimized, yet equally accurate
replacement for RTDs.
Replacing RTD With TMP117 Digital Temp Sensors
The TMP117 is a digital temperature sensor designed
for low-power, high-accuracy applications. The device
provides a 16-bit temperature result with a resolution
of 0.0078°C, along with a typical factory-calibrated
accuracy of ±0.1°C across –20°C to +50°C with a
maximum accuracy specification of ±0.3°C over the
temperature range of –55°C to +150°C, which exceeds
the accuracy of a Class AA RTD in the same range.
Figure 1. Basic RTD Circuit
SPACER
Figure 3 illustrates the accuracy specification of the
TMP117 vs an RTD. The graph clearly shows that
accuracy specification for the TMP117 can easily
outperform that of a Class AA RTD.
SPACER
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RTD replacement in heat meters using digital temperature sensors
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The TMP117, with a comparable accuracy as the
Class AA thin-film RTD while consuming only a
fraction of the power required for temperature
measurement, is designed for a variety Heat Meter
applications.
0.5
TMP117
Class AA RTD
0.4
Temperature Error (qC)
0.3
0.2
Figure 4 shows the same block diagram of Heat Meter
system using the TMP117 to replace the RTDs. By
using the TMP117 instead of an RTD, designers can
simplify both their software and system architecture to
save time, board space, and costs.
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
20
40
60
80
100
Temperature (qC)
120
140
Accu
Figure 3. TMP117 vs. RTD Class-AA Accuracy
The TMP117 features a shutdown mode where the
device aborts the active running conversion and enters
a low-power shutdown mode where it typically
consumes 250 nA of current. It can perform quick,
15.5-ms temperature conversions using the one-shot
conversion mode an active current as low as 3.5 μA
for a duty cycle of 1 Hz. After completing a one-shot
conversion, the device returns to low-power shutdown
mode.
Also, the device features an offset register that
automatically applies a user-defined offset to the
measurement results prior to an MCU read. As the
TMP117 provides additional simplicity over an RTD, it
eliminates the need for costly calibration, external
circuitry, matched traces, and Kelvin connections
easing the system designers task for accurate
measurement.
Finally the TMP117 features a fast mode (400 kHz)
I2C communication. These specs make the TMP117
excellent for low-power consumption requirements, as
well as quick power on-off cycling necessary in Heat
Meter systems.
2
Figure 4. Fluid Temperature Measurement With
TMP117
Overall, the high-accuracy TMP117 temperature
sensor with digital interface, fast conversion, and
extremely low-power shutdown mode eliminates the
need for multiple narrow-tolerance discrete
components and integrated devices, which can save
PCB space, complexity, and cost in Heat Meters.
COLLATERAL
DESCRIPTION
TMP117
±0.1°C Accurate Digital Temperature
Sensor with Integrated NV Memory
TMP116
±0.2°C Accurate Digital Temperature
Sensor with Integrated NV Memory
Application Report
RTD Class-AA Replacement With HighAccuracy Digital Temperature Sensors in
Field Transmitters
Application Report
Replacing Resistance Temperature
Detectors With the TMP116 Temp Sensor
Application Report
Precise Temperature Measurements with
TMP116
RTD replacement in heat meters using digital temperature sensors
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