TEMPERATURE
AGRON 901: Page 1
RTD (Resistance Temperature Detector)
Highlighted Sensor: 100 ohm PRT, DIN 43760, α = 0.00385, (e.g., Omega TFD or 1PT100FR828)
General Principles of Operation and Theory
A Resistance Temperature Detector (RTD) is a type of sensor element that has a temperature-dependent
electrical resistance. The resistance of a RTD increases with temperature in a linear manner over the
terrestrial temperature range (i.e., -50 to 100C). RTDs are made from metals or mixtures of metals, with
platinum being the most common. Platinum RTDs are often called PRTs (Platinum Resistance
Thermometers).
RTD: Resistance vs. Temperature
Ro=100 ohms, α=0.00385 ohm/ohm-C
150
140
Resistance (ohms)
130
120
110
100
90
80
-20
0
20
40
60
80
100
120
Temperature (C)
RTDs can be purchased in many shapes and sizes, but in general are not a small as thermistors. Omega
Engineering (Stamford, Connecticut) sells a wide array of RTDs well suited for air and soil temperature
measurements (see attachment). For example the 1PT100FR828 (8 mm long, 2.8 mm diam.) is good
choice for putting inside a soil probe or water bath, while the very small TFD sensor (9.5 x 1.9 x 0.9
mm) is great for air temperature sensing.
RTD specifications include:
1. Sensor material (e.g., platinum, nickel-iron)
2. Temperature coefficient (TC), or alpha value (α) (e.g., 0.00385 ohm/ohm-C)
3. Reference resistance, Ro (Resistance an 0 C)
4. Wiring configuration, optional (e.g., 3-wire bridge)
The most common RTDs are made of platinum, have a reference resistance of 100 ohms at 0 C, and
have a temperature coefficient of resistance, α, of 0.00385 ohm/ohm-C. The alpha value is a property of
the metal or alloy used to make the RTD element and describes the average resistance change per unit
temperature from the ice point to 100 C. A probe with an Ro of 100 ohms and α = 0.00385 will have a
resistance of 100 ohms at 0 C and 138.5 ohms at 100 C. Notice this change is much less than that of a
thermistor. The alpha value for a typical thermistor might be -6.0 %/C.
TEMPERATURE
AGRON 901: Page 2
Standards for RTDs have been established. The most commons standards are the DIN 43760 and the
BS1904. These standards are essentially the same and state that Ro must be within a 0.1 % of the target
value and α must be 0.00385 ohm/ohm-C. Also there are “Class A” and “Class B” RTDs, a designation
that defines their tolerance and accuracy. The accuracy of the sensors can be computed as
Class A: ± (0.15 + 0.002|T|)
Class B: ± (0.3 + 0.005|T|)
Most RTDs are Class B; thus, the expected sensor-based accuracy at 25 C is ± 0.42 C.
The most common RTD is the 100 ohm PRT (Ro=100 ohms) made from platinum conforming to the
DIN 43760 standard (α of 0.00385 ohm/ohm-C). An RTD of this type should always be chosen if
possible because many dataloggers are preconfigured to read these sensors.
Like thermistors, RTDs must be wired to a datalogger using an excitation and sensing bridge. The best
and most common bridge for a 100 ohm PRT is the 3 wire half bridge (see attachment from the
Campbell Sci. manual). This bridge compensates for lead wire resistance if all three wires running to the
sensor have the same resistance. Also, a 1% 10K reference resistor is needed to limit and measure
current flow. Passing too much current through an RTD can cause self heating. The heat generation in
the RTD (or thermistor) is equal to I2R or V2/R, where I is current in amps and V is the voltage drop
across the RTD.
A good general description of RTDs is provided by Thermometrics and Omega Engineering
http://www.thermometricscorp.com/rtdsensors.html
http://www.omega.com/rtd.html
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AGRON 901: Page 3
Using a 100 ohm PRT with a Campbell Datalogger.
The wiring and programming of a 100 PRT for a Campbell Sci. logger using a three-wire bridge is
described in the CR10X and 23X manuals. The P7 bridge command is used to read the ratio of the
probe resistance (Rs) to the reference resistor (Rf), which in this case is 10 K (1%). Thus, if we used a
multiplier of 1.0 in the P7 command we will get Rs/10,000 as the result. However, the P16 command,
which calculates temperature, requires the ratio of Rs to Ro or Rs/100. Thus we need a multiplier of 100
in location 7 of the P7 command to return Rs/Ro.
The accuracy of an RTD can be increased substantially by calibrating the probe in an ice bath and
making a small change to the multiplier. Calibrate the probes as follows:
1. Wire and program the sensor as described in the Campbell manual.
2. Use a multiplier of 100 in location 7 in the P7 command.
3. Place the sensor in a beaker of ice water and stir it slowly. Use a lot of ice in the beaker
because its hard to get the beaker water to 0 C. (I’ll say it again, USE A LOT OF ICE !)
4. Read the value of Rs/Ro measured by the P7 command. A perfect sensor would have a
resistance of 100 and the result would be 1.0. However, the real sensor will result in a number
slightly different than unity, for example Rs/Ro_measured = 1.02.
5. Take the reciprocal of the result found in Step 4 and multiply by 100, this is your new
multiplier (loc 7) in the P7 command. For example, 100/1.028 = 98.04. Now the probe is
calibrated and you should get 1.0 for Rs/Ro when stirred in the ice bath.
Campbell Scientific Datalogger Program for Reading a RTD
Reading an RTD in a 3-wire bridge only requires the use of commands P7 and P16 as described
in the manual. The program assumes a 10K (0.1%) reference resistor is used in the RTD circuit. It is also
possible to add the RTD program to the thermistor program used in the previous laboratory exercise.
Resources:
Campbell Scientific, Logan, UT; http://www.campbellsci.com/home.html
RTDCompany; http://www.rtdcompany.com/items/index.cfm?CAT_ID=34
Omega Engineering, Stamford, Connecticut; http://www.omega.com/
Mouser Electronics, http://www.mouser.com/
Newark Electronics, http://www.newark.com/
TEMPERATURE
AGRON 901: Page 4
TEMPERATURE
From Campbell Sci. CR10 Manual
AGRON 901: Page 5
TEMPERATURE
AGRON 901: Page 6
CR23X Program for Reading a 107 probe Thermistor and a 100-ohm platinum RTD
The 107 program was written with shortcut utility.
then modified using Edlog to include the RTD.
*Table 1 Program
01: 1.0000
Execution Interval (seconds)
1: Batt Voltage (P10)
1: 1
Loc [ Batt_Volt ]
2:
1:
2:
3:
If time is (P92)
0
Minutes (Seconds --) into a
1440
Interval (same units as above)
30
Then Do
3: Signature (P19)
1: 2
Loc [ Prog_Sig
]
4:
End (P95)
5:
1:
2:
3:
4:
5:
6:
Temp (107) (P11)
1
Reps
1
SE Channel
21
Excite all reps w/E1, 60Hz, 10ms delay
3
Loc [ T107_C
]
1.0
Multiplier
0.0
Offset
;Add RTD **********
6:
1:
2:
3:
4:
5:
6:
7:
8:
3W Half Bridge (P7)
1
Reps
23
200 mV, 60 Hz Reject, Slow Range
3
SE Channel
2
Excite all reps w/Exchan 2
2100
mV Excitation
4
Loc [ R_Ro
]
100
Multiplier
0.0
Offset
7:
1:
2:
3:
4:
5:
Temperature RTD (P16)
1
Reps
4
R/R0 Loc [ R_Ro
5
Loc [ T_RTD
]
1.0
Multiplier
0.0
Offset
]
This code was
TEMPERATURE
8:
1:
2:
3:
AGRON 901: Page 7
If time is (P92)
0
Minutes (Seconds --) into a
60
Interval (same units as above)
10
Set Output Flag High (Flag 0)
9: Set Active Storage Area (P80)
1: 1
Final Storage Area 1
2: 101
Array ID
10: Real Time (P77)
1: 1220
Year,Day,Hour/Minute (midnight = 2400)
11: Average (P71)
1: 1
Reps
2: 3
Loc [ T107_C
]
;Add RTD output **********
12: Average (P71)
1: 1
Reps
2: 5
Loc [ T_RTD
13: If time
1: 0
2: 1440
3: 10
]
is (P92)
Minutes (Seconds --) into a
Interval (same units as above)
Set Output Flag High (Flag 0)
14: Set Active Storage Area (P80)
1: 1
Final Storage Area 1
2: 102
Array ID
15: Real Time (P77)
1: 1220
Year,Day,Hour/Minute (midnight = 2400)
16: Minimum
1: 1
2: 0
3: 1
(P74)
Reps
Value Only
Loc [ Batt_Volt ]
17: Sample (P70)
1: 1
Reps
2: 2
Loc [ Prog_Sig
]
*Table 2 Program
01: 10.0000
Execution Interval (seconds)
1: Serial Out (P96)
1: 71
Destination Output
1
0
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