rtd (resistance temperature detector)
DESCRIPTION
RTD (Resistance temperature detector). Varying resistance with temperature Good for measurement of small temperature differences General equation for RTD material Rt = Ro(1+ At + Bt 2 + Ct 3 - - - - - Wt 4 ) B,C - - -W are negligibly small. Rt = Ro(1+ At). - PowerPoint PPT PresentationTRANSCRIPT
1
RTD (Resistance temperature detector) Varying resistance with
temperature Good for measurement of small
temperature differences
General equation for RTD material Rt = Ro(1+ At + Bt2 + Ct3 - - - - - Wt4 )B,C - - -W are negligibly small.Rt = Ro(1+ At)
2
RTDs: Characteristics and Applications
Characteristics: Resistive device, linear Large range: -200 to +850oC for Platinum High accuracy: 0.001oC Low sensitivity: 0.39 % per oC Don’t need reference temperature
Applications: Industries and laboratories where high
accuracy of temperature measurements are required.
3
Requirements for RTD material
High temperature coefficient High resistivity to ensure small wire
length Linearity of relation between
resistance and temperature Sufficient mechanical strength Stability of electrical characteristics
4
Construction
A thin Platinum wire wound in form of a free spiral by an insulated carrier such as mica or ceramic.
Diameter of wire should be 0.02mm to 0.2mm.
Wire should be smooth, free from defects, ensurity of constancy of resistance to avoid resistance changes due to dimension changes.
Wire generally enclosed in a protective tube made of glass, quartz etc.
5
Thin-Film RTDs
6
Calendar-Van Dusen Equation
For platinum, the resistance temperature relationship is given by the following equation:
R R T T T T TT o { [ ( . )( . ) ( . )( . ) ]}1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 3
w h ere an d a re co n s tan ts , d ep en d en t o n
th e p u rity o f p la tin u m . , an d fo r
an d fo r .
, ,
1 4 9 0 0
0 11 0
. = T>
= . T<
For the U. S. calibration curve, = 0.003851/°C
(U.S. calibration curve
7
Platinum RTD: R versus T (U.S. Calibration)
0
50
100
150
200
250
300
-100 0 100 200 300 400
Temperature (C)
Res
ista
nce
(O
hm
)
8
Graph for different materials
9
RTD’s small resistance change requires
Bridge circuit: Can detect small resistance changes
R1 R2
RRTDR4
VoVs
“Supply”Voltage
10
RTD Materials
Gold & Silver: Rarely used because of low resistivity
Tungsten: Very high resistivityGold Silver alloy: Give same characs
As Platinum below 1250 C.Phosphor Bronze alloy: Better for low
temperature measurementNickel: Most used, cheaper than platinum Platinum: More suitable but expensive, high
range, high precision
11
Very accurate Very stable Standardized among vendors Large variety of packaging options
RTD AdvantagesRTD Advantages
12
RTD Disadvantages
Costly Require current source or voltage
source Low resistance/small change 4-wire measurement Slow
13
Latest development :Miniature RTDs