Omega Engineering OMB-DBK-34A, User Manual

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DBK Option Cards and Modules
886995
Signal Management 1-13
Download instructions for loading the constants into DaqView were included with the DBK19. The
constants will improve the accuracy of each DBK19 channel when amplifying the thermocouple's millivolt
output, which is read by the DaqBook.
The temperature measurement system, as well as the thermocouple itself, can still have the following types
of errors:
•
Thermocouple error (departure from performance of an ideal T/C of that type)
•
CJC sensor error (±1°C maximum for the DBK19)
•
The DaqBook may have small calibration errors
In some applications, it is possible to ignore these sources of errors and still obtain useful results from the
temperature measurement system. However, the only way to optimize the performance of the system is to
perform an end-to-end calibration. This is done by applying two different temperatures to the thermocouple
connection: one temperature is close to 0°C and the other is at approximately full scale.
Then observe the two corresponding temperature readings in DaqView, and use this information to derive
the values of “m” and “b” in the line slope equation “mx + b.” In this equation, the value of “m” is usually
called the scale, and the value of “b” is usually called the offset. When both the offset and gain correction
values are applied, the system errors are reduced to their lowest level.
An Example of Two-Point Calibration
For illustration, suppose the temperature measurement system consists of a DaqBook/100, DBK19
thermocouple card, and DaqView software. (The following calibration procedure also applies to
temperature measurement systems made up of similar products, but the discussion is simplified with a
specific example.) After setting up the DBK19 card in DaqView as described previously, consider one of
the channels on the DBK19, for example, channel 2. Suppose a type T thermocouple connected to this
channel. In DaqView, select a sampling frequency of 1 Hz and an averaging factor of 200 so that the
readings you get are steady and not fluctuating rapidly.
When you look at this channel in DaqView, the default units are in degrees Celsius. With the acquisition
“off”, click on the cell in DaqView in the Units column for channel 2. You will see an engineering units
pull-down menu in a dialog box above the spreadsheet area. Click on the down arrow in this dialog box,
and then select the “mx + b” option. You will note that the default values of scale and offset are m=1 and
b=0 for engineering units of degrees C. These values need to be changed to ones that will give more
accurate temperature readings.
To do this, first place the thermocouple for channel 2 in an environment with a known temperature, for
example an ice bath. Click on the Start All Indicators button in DaqView, and observe the Readings
column for channel 2. Wait until the reading stabilizes, then write down the number in degrees C. Call this
reading the “first actual reading” or RA1, and suppose it is:
RA1 = 2.1°C
Since this reading was supposed to be 0°C, call the first correct reading RC1:
RC1 = 0°C
Place the thermocouple for channel 2 in a second known environment. This might be a thermocouple block
calibrator or a fluidized sand bath. In this example, assume that the calibrator has been set to100°C.
Observe the reading in DaqView for channel 2, wait until it stabilizes, then write down this number in
degrees Celsius. Call this reading the “second actual reading” or RA2, and suppose it is:
RA2 = 104°C
Since the correct second reading was supposed to be 100°C, write down the correct value as:
RC2 = 100°C
Stop the monitoring process in DaqView by clicking on the Stop All Indicators button.