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Application Note

From the Fluke Digital Library @ www.fluke.com/library

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 formu-

las and shows how to load custom constants into the Fluke

726 using a Windows PC with an RS-232 serial port.

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 manu-

factured 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. Interna-

tional standards like IEC 60751 and ASTM 1137

deﬁne the geometry and resistivity of standard

RTD’s. RTD manufacturers work to build their prod-

uct 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 tempera-

ture to resistance using Equation 1.

Equation 1: R

= R

(1 + at)

Where: t is the temperature of the sensor

is the resistance at 0 °C.

a is a constant slope that describes

resistance change per degree C.

is the resistance at temperature t,

in degrees C.

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 tempera-

tures less than 0 °C.)

Summary of content (8 pages)

PAGE 1
Remote programming and measuring uncommon RTDs with the Fluke 726 Application Note 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.
PAGE 2
300.000 280.000 Resistance (Ohms) 260.000 Rt polynomial Rt linear 240.000 220.000 200.000 180.000 200 250 300 350 400 450 500 Temperature (C) Figure 1. RTD Curves. Equation 2: Rt = R0 [1 + At + Bt 2 + C(t — 100)t 3] 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.
PAGE 3
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.
PAGE 4
Sending custom RTD constants to the Fluke 726 Once communication is established you can load custom constants into the 726. You can also send queries to the 726 to check the current settings and to verify your changes. Here is an overview of the steps to modify the constants: 1. Set the RTD type to the one of the custom types (CUST1, CUST2 or CUST3) 2. Set the minimum and maximum temperatures for the custom RTD 3. Set the constants R0, A, B, and C 4.
PAGE 5
Using custom-defined RTD’s with the Fluke 726 To use a custom RTD with the Fluke 726 you simply select it by pressing the RTD button on the front panel. This will cycle you through all of the available RTD types, including the three custom ones. Note: The cursor should be here before you save the file. Calculating the combined uncertainty of any meter and transducer Figure 7. A script used to define custom RTD constants and the results of sending it to the 726.
PAGE 6
726 Serial Command List Command Response/Actions Command Arguments Comment Ch 1 *IDN? Returns the ID string “FLUKE,726,0, Verify model {sw_rev}” where sw_rev is the number, serial firmware revision number and firmware rev. FUNC? Returns {Upper},{Lower} Answers with {Upper} responses (Channel 1) the configured DCI, DCI_LOOP, DCV, DCI_ERROR, function for the DCI_ERROR_LOOP upper and lower {Lower} responses display.
PAGE 7
26 Serial Command List cont. Command Response/Actions Command Arguments Comment Ch 1 FREQ_LEVEL 1 Argument, valid values: 1-20 V Sets the pulse output and frequency output voltage. FREQ_LEVEL? Returns the pulse output and Verify the frequency output voltage 1-20 V frequency voltage level. TRIG Toggles the pulse mode and totalize Initialize totalized trigger for read and source. pulse measurement or output.
PAGE 8
726 Serial Command List Command Response/Actions Command Arguments Comment Ch 1 CUST1_ALIAS 1 argument, sets screen name for CUST1 RTD. CUST1_ALIAS? Returns screen name for CUST1 RTD Verify RTD 1 alias. CUST2_ALIAS 1 argument, sets screen name for CUST2 RTD. CUST2_ALIAS? Returns screen name for CUST2 RTD. Verify RTD 2 alias. Ch 2 • • • • • • • • • • • • • • • • • • • • CUST3_ALIAS1 1 argument, sets screen name for CUST3 RTD.