RTD (Resistance Temperature Detector) TEMPERATURE AGRON 901: Page 1 RTD (Resistance Temperature...
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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
-20 0 20 40 60 80 100 120
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.
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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
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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/
http://www.campbellsci.com/home.html http://www.omega.com/ http://www.mouser.com/ http://www.newark.com/
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From Campbell Sci. CR10 Manual
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CR23X Program for Reading a 107 probe Thermistor and a 100-ohm platinum RTD The 107 program was written with shortcut utility. This code was 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: If time is (P92) 1: 0 Minutes (Seconds --) into a 2: 1440 Interval (same units as above) 3: 30 Then Do 3: Signature (P19) 1: 2 Loc [ Prog_Sig ] 4: End (P95) 5: Temp (107) (P11) 1: 1 Reps 2: 1 SE Channel 3: 21 Excite all reps w/E1, 60Hz, 10ms delay 4: 3 Loc [ T107_C ] 5: 1.0 Multiplier 6: 0.0 Offset ;Add RTD ********** 6: 3W Half Bridge (P7) 1: 1 Reps 2: 23 200 mV, 60 Hz Reject, Slow Range 3: 3 SE Channel 4: 2 Excite all reps w/Exchan 2 5: 2100 mV Excitation 6: 4 Loc [ R_Ro ] 7: 100 Multiplier 8: 0.0 Offset 7: Temperature RTD (P16) 1: 1 Reps 2: 4 R/R0 Loc [ R_Ro ] 3: 5 Loc [ T_RTD ] 4: 1.0 Multiplier 5: 0.0 Offset
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8: If time is (P92) 1: 0 Minutes (Seconds --) into a 2: 60 Interval (same units as above) 3: 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 is (P92) 1: 0 Minutes (Seconds --) into a 2: 1440 Interval (same units as above) 3: 10 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 (P74) 1: 1 Reps 2: 0 Value Only 3: 1 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 ----- ------ ---