VTI Instruments EX1000A, User Manual

Page: 30 / 320

VTI Instruments Corp.
30 EX10xxA Introduction
E
XPLANATION OF
S
PECIFICATIONS
The base accuracy of the EX10xxA is specified over an ambient operating temperature range of
20 ºC to 30 ºC and within one year of full calibration. This accuracy is shown in Table 1-2.
Significant performance improvement over the base accuracy can be realized, however, using
periodic self-calibration. Note, however, that the base ambient operating temperature range and full
calibration time interval restrictions still apply.
The EX10xxA’s thermocouple accuracy table provides the user with an easy-to-use description of
the unit’s capabilities at several points over the valid input dynamic range of the instrument. Each
specific thermocouple type is represented by a different row. Each column refers to an input
temperature of that value. For example,, an input temperature of 100 ºC on a type K thermocouple
has a maximum instrument uncertainty of ±0.20 ºC, exclusive of thermocouple errors.
Exclusive of thermocouple errors
The “exclusive of thermocouple errors” qualification refers to two inherent sources of error present
in all thermocouples to some extent. The first, and most obvious, error source is the accuracy of the
thermocouple itself. This refers to the extent to which the specific thermocouple potentially deviates
from the standardized International Temperature Scale of 1990 (ITS-90) characteristic for its
thermocouple type. The second, less obvious error source relates to the resistance of the
thermocouple wire, which creates a voltage drop against the input bias current of the EX10xxA. In
most applications, this error effect is negligible. However, if the length and wire gauge of the
employed thermocouple represents a resistance over 250 Ω, its effect should be analyzed for
significance.
As an example, 650 ft of 24 gauge type T wire has a resistance of about 500 Ω. Against the
EX10xxA’s typical input bias current of 7.5 nA, this creates a voltage error of:
V 3.75 nA 7.5
Ω
500

 
To convert this error to its representative temperature error, it is then divided by the slope of the
thermocouple characteristic at the temperature of interest. For example, the slope of the type T
characteristic at 0 ºC is 39
µV/ºC. The error at this point is then:
C 0.1 C V/ 9 3 V 3.75
   
 
Common mode rejection
The common mode rejection characteristics of the EX10xxA are specified in terms of dB for both
dc and (50/60) Hz interference. It should be noted that these specifications refer to the distortion of
the measurement in terms of voltage, not temperature. The following example illustrates how to
determine the possible temperature error for a T type thermocouple measuring 0 ºC due to a 1 V dc
common mode signal.
The EX10xxA has a minimum CMRR at dc of 100 dB. A 1 V common mode voltage creates a
maximum differential voltage error of:
V 10
10
V 1
20
100
 






As before, this voltage error converts to a temperature error by dividing by the appropriate
thermocouple slope to yield a maximum error of:
C 0.26 C V/ 39 V 10
   
 