Application Note

2 Fluke Corporation Measuring uncommon RTDs with the Fluke 726
Older or specialized equipment may use non-
standard RTDs. Well-equipped standards labs can
improve the accuracy of an RTD by adjusting the
constants to suit a particular sensor. The Fluke 726
allows you to check less common RTD’s or take
advantage of RTD’s that have been characterized,
by specifying your own values for R
0
, A, B and C.
For example, for the common Pt 385 RTD with
100 W resistance at 0 °C, the constants are:
R
0
= 100 W
A = 3.9083 x 10
-3
B = -5.775 x 10
-7
C = -4.183 x 10
-12
Note: Instruments installed before and up to the early 1990s used a
slightly different temperature scale (IPTS-68) than newer instruments
(ITS-90). When a calibrator using ITS-90 RTD curves is used to
calibrate an instrument using IPTS-68 errors will result. The errors
are extremely small at room temperature and increase to a maximum
of 0.36 °C at 760 °C.
Another common form of Equation 2, the Call-
endar Van Dusen or CVD, uses the constants a, d,
and b. This alternative form is directly derived from
Equation 1 and uses the same constant a. And even
though the two polynomials produce the same
results, the constants are different. The Fluke 726
uses the constants A, B, and C. If you know the a,
d, and b constants for an RTD you can convert them
to A, B, C constants by using Equations 3, 4 and 5.
Equation 3: A = a +
a.d
100
Equation 4: B = —
a.d
100
2
Equation 5: C = —
a.b
100
4
300.000
280.000
260.000
240.000
220.000
200.000
180.000
200 250 300 350 400 450 500
Temperature (C)
Resistance (Ohms)
Rt polynomial
Rt linear
Equation 2: R
t
= R
0
[1 + At + Bt
2
+ C(t — 100)t
3
]
Figure 1 shows the difference between tempera-
ture values produced by the linear equation and
the polynomial between 200 and 500 °C. Note that
at 240 ohms, the polynomial curve of Equation 2
gives a more accurate temperature 16 °C higher
than the linear equation.
The Fluke 726 uses the polynomial in Equation
2 and it has built-in constants to support most
common RTD’s. Standard RTD’s supported by the
Fluke 726 are shown in Table 1.
RTD Type Reference R0, Metal a Range
Ohms W/W/°C °C
Pt 100 (3916) 100 Platinum 0.003916 -200 to 630
Pt 100 (385)* 100 Platinum 0.00385 -200 to 800
Pt 200 (385) 200 Platinum 0.00385 -200 to 630
Pt 500 (385) 500 Platinum 0.00385 -200 to 630
Pt 1000 (385) 1000 Platinum 0.00385 -200 to 630
Pt 100 (3926) 100 Platinum 0.003926 -200 to 630
Ni 120 (672) 120 Nickel 0.00672 -80 to 260
Cu 10 (42) 10 Copper 0.0042 -10 to 250
Table 1: Standard RTDs types included in the Fluke 726.
Pt 100 (385) is the IEC and ASTM standard.
Figure 1. RTD Curves.