Application Note

5 Fluke Corporation Measuring uncommon RTDs with the Fluke 726
Figure 7 shows a script to change the constants
for the CUST1 type RTD. The script can be devel-
oped in Windows Notepad, just like the previous
example. Here’s how it works:
•
The script activates the RTD measurement and
sets the RTD type to CUST1.
•
It sends commands to establish new min and
max limits for the new RTD type.
•
The next three commands set the A, B, and C
constants. In this example we are limiting the
sensor to temperatures of 0 °C or greater, and
setting the C coefficient to 0.
•
The alias command will name the new RTD
PT040.
Note that numbers may be in integer, decimal or scientific notation.
Once the file is prepared, click on Transfer, Send
Text File in HyperTerminal to send the file to the
726. You can verify that the 726 has the correct
constants by using the query script from Figure 6.
Your 726 is now ready to make measurements
using your custom RTD curve.
Calculating the combined uncertainty of any
meter and transducer
The combination of the Fluke 726 and an RTD will have an
uncertainty that considers both the meter and the transducer.
Since the uncertainties of the meter and probe are indepen-
dent of each other, rather than simply adding the uncertain-
ties, we can take the square root of the sum of the squares.
The uncertainty of the Fluke 726 is expressed as a percent-
age of reading with a noise floor that is determined by the
range. The temperature specifications are 0.15 °C uncertainty
for the meter, 0.1 °C uncertainty for the probe. Here’s an
example of how to calculate the uncertainty for the Fluke 726
in °C at 100 °C.
Once you have the uncertainties of both the meter and the
probe at a particular point, you can combine these uncer-
tainties to come up with the uncertainty of the system. The
formula for combining these independent uncertainties is:
U
system
= √U
2
probe
+ U
2
meter
*
For example, the uncertainty of the Fluke 726 at 100 °C is
0.15 °C as calculated above. If we have an RTD whose uncer-
tainty at 100 °C is 0.1 °C, then the combined accuracy will be
plus or minus
√0.1
2
+ 0.15
2
= 0.18 °C
*The above equation assumes that the U values follow a normal distribution. A
metrology standpoint would recommend dividing the ‘U’ values by √3 and then
doing the root sum square, when using the manufacture’s specification.
Figure 7. A script used to define custom RTD constants and the
results of sending it to the 726.
Note: The cursor should be
here before you save the file.
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 avail-
able RTD types, including the three custom ones.