Dwyer DL8 - Manual (Page 9)

Non-Standard Ranges
You may want to use a special transducer or tie in an exist-
ing process signal loop that does not match the DL8’s stan-
dard input ranges. The following step-by-step procedure
will help you choose which channel is best.
1. Determine the maximum output signal (M) and the zero
offset signal (Z) of the transmitter or circuit you wish to tie
into. For example, a transducer with a specified range of 1
to 6 volts DC will have a maximum output signal of 6 volts
and a zero offset signal of 1 volt. A 4 to 20 milliamp trans-
mitter will have a maximum output signal of 20 mA and a
zero offset signal of 4 mA.
2. Determine the transducer’s full scale output (F). You can
do this simply by subtracting Z from M. The full scale out-
put of the 1 to 6 volt transducer is 5 volts. The full scale out-
put of the 4-20 mA transmitter is 16 mA.
3. Compare M with the channels available on the logger and
choose a channel with an input equal or greater to this value
(if available). If no such channel exists, then modify one to
suit by referring to the Customizing Input Ranges section.
4. Determine the resolution (R) of the channel you chose in
Step # 2 will be adequate. You can determine this, in per-
cent, by using the following equation:
Where:
C = the input range of the channel to use (in either
millivolts, volts or milliamps).
F = the full scale output of your transducer (M-Z)
measured in the same units as for C.
R = the resolution expressed in a percentage (%)
of full scale output.
For example, a 0.5 to 2.5 volt transducer connected to the
2.5V channel will be logged with a resolution of 0.5%. A 4
to 20 mA transducer hooked up to the 25 mA channel will
log with a resolution of 0.6%.
Generally, it is good practice to keep resolution to within 1%
for most applications. However, you may be able to accept
far coarser resolution in some applications and, in others,
require far better. You’ll be able to determine your own
requirements after you’d be become familiar with the results
you can expect with different arrangements. If your require-
ments demand greater resolution than what you calculated,
then you should set up your inputs with resistors as
described in the Customizing Input Ranges section.
5. To find out in what actual steps (S) your logger will collect
data, first determine the input (I) of your transducer or trans-
mitter. You can do this by subtracting the lowest level input
from the high. A thermocouple transmitter, for example, with
a range of -20° to 600° will have an input span of 620° (600-
(-20)). A pressure transducer with a range of 0 to 100 psi will
have an input span of 100 psi.
6. To calculate S, measured in the same units as the input
span (I) above, use this equations:
For example, a 4 to 20 mA Relative Humidity transmitter
with an input span (I) of 90% R.H. and a calculated resolu-
tion (R) of 0.6% will record data in steps of 0.54% R.H.
CUSTOMIZING INPUT RANGES
The standard input ranges on the DL8 should be suitable for
most process signal applications, but occasionally you may
require measurement of non-standard input levels where
resolution is a primary concern (see the Choosing Input
Channels section). Ranges can be adapted easily by using
either one or two external resistors. Trendreader Software’s
Expert, Equations function can then be used to scale the
data to the new input levels.
The resistor value that you will require to modify each log-
ger’s input channel is selected by inserting your special
input requirements into a simple equation. The following
sections detail these equations and the method of tying in
these resistors.
10 Volt Channel
The 10 volt channel can be modified to span higher ranges
(greater than 0 volts DC) by adding a single resistor to the
“10V” terminal shown in Figure 8. The value of the resistor
R must be calculated by the following formula based on the
desired voltage input range E
in
:
R = (4,090 x E
in
) - 40,900
For example, if you wish to measure up to 20 volts, the
resistor value should be:
(4,090 x 20) - 40,900 = 40,900 ohms.
Since you won’t always be able to get the exact resistor
value that you calculated, you can determine the logger’s
actual input range by plugging the value of the resistor (R)
you obtain back into the equation. For example, if the clos-
est resistor you could obtain was 42K ohms, then the actu-
al voltage input range would be:
R = C x 0.4
F
S = R x 1
100
E
in
= 42,000 + 40,900
4,090
= 20.27 volts
Bulletin E-90-DL8 6/22/05 3:56 PM Page 9