Fluke 726, 753, 754 Service Guide
Remote programming
and measuring
uncommon RTDs with
the Fluke 726
Using custom RTD temperature constants
The Fluke 726 Multifunction Process Calibrator can
measure temperature with most common resistance
temperature detectors (RTD’s). But what about the many
“legacy” non-standard RTD’s still in use, as well as standard
RTDs that have been specially calibrated? The 726 allows
you to enter custom RTD constants so you can measure
any RTD for which you have the constants. You can also
use custom constants to take advantage of the Fluke 726
ability to measure with 0.01° resolutions by coupling it with
a characterized RTD. This application note explains how
to use the 726 to measure non-standard or characterized
RTD’s. It covers the basic theory of RTD conversion formulas and shows how to load custom constants into the Fluke
726 using a Windows PC with an RS-232 serial port.
Application Note
RTD Temperature Curves
RTD’s take advantage of a natural property of
metals, namely that a metal’s resistance increases
with temperature. An RTD is a precisely manufactured metal wire or film, and by measuring its
resistance we can derive its temperature.
Resistance of an RTD is a function of the length
and cross sectional area of the wire or film used to
make it, and the resistivity of its metal. Resistivity
is a characteristic of a metal’s chemical makeup.
Most RTD’s are made of platinum, nickel, or copper.
The alloy of the platinum or copper must be tightly
controlled to produce precise resistivity. International standards like IEC 60751 and ASTM 1137
define the geometry and resistivity of standard
RTD’s. RTD manufacturers work to build their product to meet these standards.
In addition to defining the physical parameters of
standard RTD’s, international standards also define
equations and constants used to convert resistance
readings to temperature. Over a limited range the
relationship between temperature and resistance is
approximately linear, and you can convert temperature to resistance using Equation 1.
Equation 1: R
Where: t is the temperature of the sensor
R
a is a constant slope that describes
R
Using this simple linear equation delivers fairly
good results, especially at temperatures between 0
and 100 °C. To extend the range of the RTD and to
get more precision, IEC and ASTM standards specify
a more complex polynomial that fine-tunes the
resistance-temperature relationship. One form of
that equation is given here as Equation 2. The RTD
standards also specify values for the constants R
A, B, C. (The C constant is only used for temperatures less than 0 °C.)
= R0 (1 + at)
t
is the resistance at 0 °C.
0
resistance change per degree C.
is the resistance at temperature t,
t
in degrees C.
,
0
From the Fluke Digital Library @ www.fluke.com/library
300.000
280.000
260.000
240.000
220.000
200.000
18 0.000
200 250 300 350 400 450 500
Temperature (C)
Resistance (Ohms)
Rt polynomial
Rt linear
Figure 1. RTD Curves.
Equation 2: Rt = R0 [1 + At + Bt2 + C(t — 100)t3]
Figure 1 shows the difference between temperature 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.
Older or specialized equipment may use nonstandard 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
For example, for the common Pt 385 RTD with
, A, B and C.
0
100 W resistance at 0 °C, the constants are:
R
= 100 W
0
A = 3.9083 x 10
B = -5.775 x 10
C = -4.183 x 10
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.
-3
-7
-12
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.
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 +
Equation 4: B = —
Equation 5: C = —
a.d
100
a.d
100
a.b
100
2
4
2 Fluke Corporation Measuring uncommon RTDs with the Fluke 726
Loading custom RTD constants into the Fluke 726
Setting up communications
Figure 2.
The Fluke 700SC Serial Interface Cable (PN
667425) is used to communicate with the Fluke
726. The serial cable connects to the round multipin connector at the top of the 726 and to a 9-pin
serial port on your PC. The serial cable can only
interface with one instrument or module at a time.
After connecting the cable, make sure the 726
is turned on. Call up the Windows HyperTerminal
Program. It is usually listed in the Windows Start
Programs menu under Accessories, Communications. The HyperTerminal program will ask you
to set up a file name to store your communication
settings. You can select any icon and click OK to
continue.
Figure 4.
Next, a COM Properties box will come up. Set
baud rate (9600 in Figure 4), data bits, parity,
stop bits and flow control as shown in the Figure
4. Once your settings match Figure 4, click OK to
continue.
Figure 3.
A second dialog box, “Connect To”, will pop up.
Skip down to the bottom, where it says “Connect
Using:” Select the COM port you connected to
the 726.
3 Fluke Corporation Measuring uncommon RTDs with the Fluke 726
Figure 5.
To make it easier to read the commands and
responses, you can make some adjustments to the
way the HyperTerminal program treats screen text.
Under the File menu select Properties and click on
the Settings tab. Click on the button labeled ASCII
Setup. You will see a dialog like the one shown in
Figure 5. Match the settings shown in the figure.
Note that adding the 200 ms line delay makes it
possible to send short scripts to the 726. More on
this later.
To test the connection, type:
*idn?
After you hit Enter, the 726 should respond with
“FLUKE,726,0,X.X” where X.X is the version of the
firmware in the instrument.
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