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ELEKTRONIKA IR ELEKTROTECHNIKA,ISSN 1392-1215,VOL.21,NO.4,2015
1
Abstract—The present work concerns description of
additional error sources of temperature measurement done
with Pt100 sensors placed on the deflected elements. The
authors came across certain problems with temperature
measurement while working on an original signal conditioner.
For this reason it was decided to conduct more measurement
experiments. The aim of experiments was to examine the
influence of deflection of Pt100 sensors on their resistance
changes. In the article a laboratory stand and the procedure of
conducting the research are presented.
Index Terms—Temperature sensors, strain measurement,
measurement techniques.
I. INTRODUCTION
This article is a result of research on an electronic circuit
which would enable determining resistance increases of the
sensors connected to the system, such as: a strain gauge
sensor used to measure deflection or a Pt100 sensor used to
measure temperature. The constructed signal conditioner has
been described previously in [1] and as a similar circuit in
[2]. It has two current sources (J1and J2) which power the
system continuously, and four resistance sensors (R1,R2,R3
and R4) connected into a four-armed bridge (Fig. 1).
Fig. 1. Two-current-source bridge circuit.
Moreover, two reference resistors (Rr1 and Rr2) were
connected to the opposite nodes of the system. It is worth
stressing that the current efficiency of the current sources is
equal and constant in time. The bridge activity is to measure
Manuscript received December 30, 2014; accepted March 29, 2015.
This research was funded by a grant (No. S/WE/1/2013) from the
Bialystok University of Technology and by project SP2015/154,
“Development of algorithms and systems for control, measurement and
safety applications.” of Student Grant System, VSB-TU Ostrava.
the potential values in A, B, C and D nodes of the system or
the voltages between D-C and A-B nodes. It is easily
observable that the value change of one sensor – R1,R2,R3
or R4– will cause the change of the voltage difference in
nodes A, B, C and D. A system of this configuration works
similarly to a Wheatstone bridge circuit with a current
supply [3]. Therefore, there is a possibility to change two
resistances at the same time in a two-current system, which
means that two quantities can be measured simultaneously
with the use of a sensor, e.g. R1sensor can measure
deflection and R2sensor – temperature.
However, during experiments aimed to confirm the
correctness of measurement of the two mentioned non-
electrical quantities by the two-current-source bridge circuit,
certain difficulties occurred. In the experiment, where the
cantilever beam was deflected at given temperature
(e.g. 40 °C or 60 °C), unexpected values of temperature
were obtained. The occurrence of this deviation caused the
authors analyse whether the experiment had been conducted
in the appropriate way. First of all, the cantilever beam
temperature for different deflections was measured with the
use of the non-contact method. A Thermal Imaging Camera
NEC Avio InfReC Thermo Gear G100 was used for this
purpose. It helped to confirm the temperature stability
during experiment (Fig. 2).
Fig. 2. Thermal image of the cantilever beam (top view) with the sensor
position during heating process. In order to obtain clear presentation of
temperature distribution around sensors, the heater placed on the right side
was covered by a metal shield.
Evaluation of Pt100 Sensor Deflection Effect
during Strain Measurements
Wojciech Walendziuk1, Adam Idzkowski1, Zdenek Machacek2, Zdenek Slanina2
1Department of Electrical Engineering, Bialystok University of Technology,
Wiejska 45D St., 15-351 Bialystok, Poland
2Department of Cybernetics and Biomedical Engineering, FEI - Faculty of Electrical Engineering and
Computer Science,
17. listopadu 15, Ostrava - Poruba, Czech Republic
w.walendziuk@pb.edu.pl
http://dx.doi.org/10.5755/j01.eee.21.4.12776
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ELEKTRONIKA IR ELEKTROTECHNIKA,ISSN 1392-1215,VOL.21,NO.4,2015
As the next step of the experiment, the resistance changes
of the Pt100 sensor stuck next to the resistance strain gauge
in a given constant temperature were examined. It occurred
during the test that the deflecting Pt100 sensor (B) changed
its resistance, working similarly to a resistance strain gauge.
This unexpected discovery disposed us to write this
article where we especially focus on errors caused by
inappropriate sticking of sensors on deflecting elements. It is
worth stressing that authors did not find any articles about
research on Pt100 sensor deflection but the procedure was
inspired by testing sensors of other types described in
articles [4]–[7].
II. LABORATORY STAND
In order to examine the influence of deflection on
resistance of the Pt100 sensor, a laboratory stand was
created. It consists of three functional modules.
Fig. 3. General view of the laboratory stand.
One of them is a measurement data acquisition system
equipped with an NI 9219 data acquisition card. It has
4 channels, sampling rate 100 S/s per channel, analogue
input resolution of 24-bits and it supports measurement with
the use of thermocouple, RTD and other resistance
sensors [8].
Measurement data from sensors placed on the cantilever
beam is sent to a PC and processed with the use of software
created in the LabVIEW system. It has two functions: data
acquisition and processing, which means it converts the
value of current resistance of transducers into temperature
value. It also records the results on the hard disk as CSV text
files. Additionally, the program controls the drive unit of the
cantilever beam, which is the other functional module of the
measurement stand.
This unit consists of a step motor controlled by an
ATmega328 microcontroller, and transistor keys. A digital
micrometer with the range from 0 mm to 25 mm, resolution
of 0.001 mm and accuracy 0.002 mm is another element of
the unit. The step motor drives the micrometer screw which
deflects the cantilever beam of a value given by the
controlling program. It is worth stressing that the controlling
software reads the current position of the deflection position
of the cantilever beam from the micrometer with the use of a
serial interface.
A steel beam of 250 mm × 40 mm × 1 mm (Fig. 3) is
another functional element of the unit. On the top side of it a
thermocouple sensor, a strain gauge, a heater constructed on
the basis of ceramic resistors of small resistance and two
Pt100 sensors of different sizes (Table I) were placed. All
sensors were stuck on the beam with high-temperature-
resistant glue of the following chemical structure: Bisphenol
A + Epichlorohydrin > 50 %, Styrene < 12.5 %) +
Triethylenetetramine 10 % (Epidian 53 + Z1).
III. PT100 SENSORS
As it is known, RTD sensors are the elements which react
almost linearly to the influence of temperature. They are
made of platinum which is a perfect material for sensors
because of its high melting point, great temperature
coefficient, small chemical activity and stable thermometric
characteristics. Additionally, sensors of this type guarantee
great precision of temperature measurement, which is about
0.1 °C within the temperature range of 0 °C to 200 °C.
Pt100 sensors accepted for this research (Table I) fulfil the
IEC 60751 standard and its resistance is 100 Ω at 0 °C and
the Temperature Coefficient of Resistance –
0.00385 Ω/Ω/°C within the temperature range 0 °C to
100 °C [9]. The Temperature Coefficient of Resistance was
determined experimentally, according to the following
equation
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100 0
0
ppm
10 , ,
100 C
R R
R
(1)
where R0– the resistance of the sensor at 0 °C, R100 – the
resistance of the sensor at 100 °C, which equalled:
0.003764 Ω/Ω/°C for sensor A and 0.003775 Ω/Ω/°C for
sensor B.
TABLE I. RESISTIVE SENSORS PT-TYPE ON GLASS SUBSTRATE
PARAMETERS TAKEN FROM APPLICATION NOTES [10].
RTD
type
Pt100 (A)
(PROFFUSE)
Pt100 (B)
(PROFFUSE)
Tolerance
class B 0.2 %
class B 0.3 %
Body
dimensions
1.7 mm × 2.4 mm ×
1.0 mm
9.5 mm × 1.9 mm ×
0.9 mm
Operating
temperature
-50 °C to 500 °C
-70 °C to 500 °C
Temperature
coefficient
3850 ppm/°C
3850 ppm/°C
According to the manufacturer, both sensors comprise
within class B, but one of them has smaller deviation error
(0.2 %) from the ideal characteristics. It should be stressed
that the greater tolerance value, the greater variation of the
sensor from the ideal characteristics. The definition of the
mentioned class B sensor can be presented as follows
class B 0.3 0.005 , [ C].T
(2)
It results from (2) that sensor limited error for 0 °C equals
100 Ω ±0.12 Ω (0.3 °C) and, additionally, we can determine
the deviation error from ideal resistance using
standardization tables. It equals 138 Ω ±0.3 Ω (0.8 °C) for
100 °C.
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ELEKTRONIKA IR ELEKTROTECHNIKA,ISSN 1392-1215,VOL.21,NO.4,2015
IV. MEASUREMENT PROCEDURE
A four-wire method for measuring resistance was used in
the measurement procedure. Two wires of this system
conduct current to supply the Pt100 sensor and two other
wires are used to measure voltage drop on tested resistance.
This type of system enables decreasing measurement errors
in comparison with a two- or three-wire solution.
During the research, it was also noticed that there is a
possibility of self–heating of the sensor caused by the
current flowing through it. For this reason, the
recommended current value, which cannot be exceeded
during tests, is 1 mA. In the experiment described here, the
value of the current source equalled 0.5 mA.
During the tests, the cantilever beam, presented in Fig. 3
was additionally covered with a shield (Fig. 4) protecting it
from the air movement.
Fig. 4. The cantilever beam was covered by additional shield in order to
avoid air disturbances around sensors.
This occurred to be necessary because of the change in
resistance being greater than temperature, not – than
deflection. Therefore, slight air movements could cool the
Pt100 sensors. Additionally, it was decided to heat the
cantilever beam slowly, not faster than 0.5 °C per minute.
The following conditions of conducting the measurement
procedure were assumed (similarly as described in work
[11]):
Heating the cantilever beam at constant temperature in
the room and under a shield covering the laboratory stand;
Continuous monitoring of temperature by a computer
program;
Reading the resistance of sensors at given temperature
whose steady state was assumed as temperature change
read from a thermoelectric sensor, not bigger than
0.02 °C.
Additionally, it was also assumed that only change in
resistance would be measured because all the measurements
were conducted below the device precision assured by the
manufacturer of a DAQ card. For this reason, Pt100 sensors
may be used to measure values of change in resistance, not
the absolute temperature values according to the Pt100
sensor standard [9]. This will help determine the increase of
resistance changes caused by deflection, hence, similarly as
it is done with the use of resistance strain gauges.
It is worth stressing that the change in resistance is a
difference of two resistance values (3), i.e. resistance at
specific point of deflection Rxand initial value Rx0
0.
x x x
R R R
(3)
The experiment resulted in obtaining characteristics
shown in Fig. 5 and Fig. 6.
Fig. 5. The dependence of resistance increment of Pt100 sensor (B) (for
various temperatures) on the cantilever beam deflection.
It is easily observable that the change in resistance
(Fig. 5) for Pt100 sensor (B) is significantly greater at higher
temperature in comparison with the one for Pt100
sensor (A).
An interesting phenomenon was observed during the
experiment – reaction of the Pt100 (B) sensor connected
with resistance increase in relation to temperature increase
(Fig. 5). This probably derives from the fact that the
resistance of the sensor increased together with the
temperature of the cantilever beam which the sensor was
stuck on. As a result the deflection of the sensor caused
greater changes in the resistance increase deriving from
deflection. It should be stressed that Pt100 sensors were
stuck on the top of the beam with a force put from the
bottom. The deflection, then, caused the inner structures of
platinum leads lengthen, which resulted in resistance
increase.
The recorded data for Pt100 sensor (A) (Fig. 6) showed
unstable changes, which can be explained by a small change
of resistance increase.
Fig. 6. The dependence of change in resistance of Pt100 sensor (A) (for
various temperatures) on the cantilever beam deflection.
Next chart (Fig. 7) presents an example summary of
resistance increments of Pt100 sensors and a resistance
strain gauge at 20 °C. As we can observe, the characteristics
of change in resistance of the strain gauge sensor has a
different slope angle (expressed by linear regression) than
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ELEKTRONIKA IR ELEKTROTECHNIKA,ISSN 1392-1215,VOL.21,NO.4,2015
the characteristics of two Pt100 sensors. The slope angle is
related to the gauge factor (GF), which occurs to be about
three times smaller for a Pt100 sensors than for a resistance
strain gauge. It is worth stressing, however, that the
previously mentioned reaction of the Pt100 sensor to
deflection is a small disadvantage influencing the error of
precise temperature measurement. This inconvenience may
be ignored at normal applications under engineering
circumstances. Nevertheless, we must be aware of its
existence.
Fig. 7. Summary of change in resistance of Pt100 sensors and a strain
gauge sensor at 20 °C in relation to the cantilever beam deflection.
Fig. 8. Summary of change in resistance of Pt100 sensors and a strain
gauge sensor at 60 °C in relation to the cantilever beam deflection.
The next characteristics (Fig. 8) shows results of the
cantilever beam deflection experiment at 60 °C. As it is
observable, the charts for Pt100 (B) sensor and the strain
gauge have almost identical slope. The Pt100 sensor,
however, has a greater coefficient of resistance change
related to deflection in comparison to the strain gauge,
whereas the Pt100 sensor only slightly reacted to deflection,
so it can be ignored.
V. CONCLUSIONS
Presented experiment was aimed at raising engineers’
awareness of the importance of locating resistance sensors
on constructions undergoing deflection. In spite of various
imperfections of the experiment, the influence of the sensor
deflection on the change of its resistance was proved. The
authors tried to achieve possibly most precise results of
measurement through conducting numerous tests and using
an additional cover of the laboratory stand protecting it from
incidental air movements. The achieved results were
satisfactory, although the device used for measuring
resistance had a greater limited error than the value of
measured change in resistance. It was possible due to
applying a 24-bit ADC transducer in the data acquisition
card [8]. It allowed to achieve 30 µΩ resolution which
enabled observing phenomena described in this article.
It cannot be denied, however, that deflection of the Pt100
sensor may influence the value of measured temperature.
Smaller sensors are less sensitive than bigger ones.
Additionally, the track of platinum leads and substrate made
of quartz or fused silica has a great influence on the reaction
of the sensor to deflection.
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