©
National Instruments Corporation
3
NI PXI-2527 Specifications
Determining the System Error
To determine the system error for the NI PXI-2527/TB-2627,
first calculate the error due to thermal EMF of the
NI PXI-2527 using the following equation.
*
(1)
where
E
EMF
= error due to thermal EMF of the
NI PXI-2527
T
= temperature being measured, in degrees Celsius
T
+1
=
T
+ 1 °C
V
= voltage that corresponds to
T
V
+1
= voltage that corresponds to
T
+1
V
EMF
= thermal EMF of the NI PXI-2527
†
After you have determined the error due to thermal EMF,
calculate the system error using the following equation.
(2)
where
E
S
= system error of the NI PXI-2527/TB-2627
E
EMF
= error due to thermal EMF of the
NI PXI-2527
E
CJC
= error due to CJC temperature sensor of the
NI TB-2627
‡
Example
Measuring a K-type thermocouple at 200 °C with a
CJC temperature of 25 °C, the system error of the
NI PXI-2527/TB-2627 is calculated below.
**
Assuming typical thermal EMF (2.5
μ
V), first calculate the
error due to thermal EMF using Equation 1.
= 0.063 °C
To determine the system error, add the error due to thermal
EMF to the error due to the CJC temperature sensor using
Equation 2.
E
S
= 0.063 °C + 0.5 °C
= 0.563 °C
Determining the Thermocouple Error
Independent of the NI PXI-2527/TB-2627 system,
thermocouple error is the greater of the following values:
±a temperature range
or
±a percent of the measurement.
In the example, a standard grade K-type thermocouple is used
to measure 200 °C. The error for a standard grade K-type
thermocouple is ±2.2 °C
or
±0.75% of the measurement
temperature.
††
Because ±0.75% of 200 °C (±1.5 °C) is less
than ±2.2 °C, the error of a standard grade K-type
thermocouple is ±2.2 °C.
Determining the Total Error
The total error in thermocouple measurement is the sum of the
system error and the thermocouple error. Use the following
equation to determine the total error in thermocouple
measurement.
(3)
where
E
T
= total error in thermocouple measurement
E
S
= system error
E
Th
= thermocouple error
To determine the total error in thermocouple measurement in
the example, add the thermocouple error to the system error
using Equation 3, as illustrated below.
E
T
= 0.56 °C + 2.2 °C
= 2.76 °C
*
In thermocouple reference tables,
T
and
T
+1
are known values used to calculate the slope of the thermocouple Temperature vs.
Voltage graph. Refer to a thermocouple reference table to determine the values of
V
and
V
+1
that correspond to
T
and
T
+1
,
respectively.
†
section of this document to determine the thermal EMF value of the NI PXI-2527. For
optimal thermocouple measurement performance (V
EMF
= 2.5
μ
V), power down the latching relays of the NI PXI-2527. For
more information about powering down latching relays, refer to the Power Down Latching Relays After Debounce property
in NI-SWITCH or the Power Down Latching Relays After Settling property in NI-DAQmx.
‡
From 15 °C to 35 °C, the NI TB-2627 has an accuracy of ±0.5 °C. From 0 °C to 15 °C and 35 °C to 55 °C, the NI TB-2627
has an accuracy of ±1.0 °C. For more information about temperature sensor accuracy, refer to the
NI TB-2627 Installation
Instructions
.
**
In this example, the values of
V
and
V
+1
are found in the thermocouple reference tables of Omega Engineering’s
The Temperature Handbook
. Vol. 29. Stamford, CT: Omega Engineering Inc, 1995.
E
EMF
T
+1
T
–
V
+1
V
–
----------------
⎝
⎠
⎛
⎞
V
EMF
(
)
=
E
S
E
EMF
E
CJC
+
=
E
EMF
201
°
C
200
°
C
–
8.178 mV
8.138 mV
–
------------------------------------------------------
⎝
⎠
⎛
⎞
0.0025 mV
(
)
=
††
Omega Engineering.
The Temperature Handbook
. Vol. 29. Stamford, CT: Omega Engineering Inc, 1995.
E
T
E
S
E
Th
+
=
