VTI Instruments EX1000A, User Manual

Page: 101 / 320

www.vtiinstruments.com
EX10xxA Programming 101
C
ONFIGURE THE
R
ANGE
Use the vtex10xxA_set_channel_range function to set the range for EX10xxA voltage input
channel on a per channel basis. When a channel is set to perform voltage measurements, the input
connector determines the range available. For thermocouple channels, only the 67 mV range can be
used. Any other range will generate an error. Voltage channels can be set to any voltage range, with
10 V being the default value.
//sets an array of the 1032's voltage channels to 1.0 volt range
ViInt32 ex1032_volt_channels [16] = { 0, 1, 2, 3, 4 ,5 ,6 ,7 ,8 ,9 ,10 ,11 ,12 ,13 ,14 ,15};
vtex10xxA_set_channel_ranges(vi, ex1032_volt_channels, 16, 1.0);
C
ONFIGURE THE
A
DVANCED
C
ONVERSION
O
PTIONS
To facilitate its use, the EX10xxA provides a high level of hardware and software integration. A
high performance internal cold junction mechanism allows for direct connection of thermocouple
wire, and embedded polynomial coefficients for all standard thermocouple types provides
compensated temperature readings without external mathematical manipulation. It does, however,
also support the advanced configuration options of employing an external cold junction and
employing custom thermocouple conversions.
Employing an external cold junction
The EX10xxA accommodates the use of external cold junctions that are maintained and measured
by the user. In this application, the cold junction temperature in ºC is entered into the EX10xxA and
enabled on a per channel basis. That is, the use of internal and user-defined CJC inputs can be mixed
throughout
the unit. User-defined CJC temperatures are entered with the
vtex10xxA_set_user_cjc_temp function. A channel’s CJC temperature can be configured
regardless of its inclusion in the scan list, and multiple channels can be assigned to the same
temperature within one function. However, each unique temperature must be set with a separate
function.
The use of a user-defined CJC temperature is enabled on a per channel basis with the
vtex10xxA_set_user_cjc_enable function. If enabled, the user-defined CJC temperature will be
used in the thermocouple calculations instead of the internally measured one. The entry of external
CJC values and their enabling are disjoint functions. That is, the entry of a value does not
automatically enable its use, and the disabling of a previously enabled channel does not clear the
value.
NOTE These functions should only be used when the thermocouple cold junction is made external to the
EX10xxA. This feature must be used with care, as the input channel data contains no indication that
it was calculated with an external CJC value instead of an internal one.
The current user-defined CJC temperature and enable status for any channel are queried with the
vtex10xxA_get_user_cjc_temp and vtex10xxA_get_user_cjc_enable queries, respectively.
Employing custom thermocouple conversions
The EX10xxA nominally accepts standard thermocouple types and performs its thermocouple
calculations using polynomial coefficients from the NIST ITS-90 Thermocouple Database. In some
applications, however, a user may want to override the embedded coefficients with a user-defined
coefficient set. One reason to do this is if the transfer function of the specific thermocouple being
used has been completely characterized to an accuracy level that exceeds standard thermocouple
limits of error. Another reason to do this is if a non-standard thermocouple is used. Up to two unique
sets of coefficients can be entered. Specifically, the use of custom thermocouple equations requires
the user to know or generate the coefficients for two conversion polynomials.