Dwyer DL8 - Manual (Page 8)

Avoiding Ground Loop Problems
Take special care to avoid ground loop problems when you
use your DL8 Process Data Logger. A ground loop can
occur when there is more than one path to ground in your
logger-transducer circuit. Ground loops can damage your
DL8 Process Data Logger as well as your transducers. To
avoid ground-loop problems your DL8 circuit:
• Do not use more than one grounded power sup
ply to power your transducers.
• Do not connect your logger to your computer in
real time readings unless:
• your computer is battery-operated (i.e. not
grounded); or
• your transducer power supply is not grounded.
If you must use more than one grounded power supply in
your circuit, each transducer you use must be isolated. If
you must use non-isolated transducers, then you must use
a Signal Repeater/Loop Isolator between the transducer
output and the DL8 input.
Wiring Notes and Considerations
1. The Series DL8 Process Data Logger has two common
inputs (labeled “-Common”). These are not isolated from
each other and can be used as the “-” connection for the
power supply (or battery) and any of the transducers being
logged by the Series DL8.
2. For permanent applications, wiring connections can first
be made to the Series DL8 removable terminal block. The
terminal block can then be attached (with glue, for example)
to a permanent fixture. When it comes time to analyze data,
the data logger can simply be unplugged from the terminal
block and brought back to your computer. After backing up
the data, the logger can be returned and plugged back into
the terminal block. Logging will then automatically continue.
3. The 2.5 V and 200 mV channels “float” when no con-
nections are made to them. They can thus be expected to
read a positive voltage when left both active and uncon-
nected. When connected, however, they will read the cor-
rect input voltage.
4. The “25 mA” channels read positive current only. Make
sure to observe polarity. If you are using both channels
simultaneously, ensure that both 4-20 mA transmitters
source a positive current through the logger to a common
(-).
CHOOSING INPUT CHANNELS
To choose which of the DL8’s seven external input channels
to use for your particular applications and understanding of
resolution is required.
Maximizing Resolution
The resolution of your DL8 Logger is eight bits. This means
that it can resolve analog signals, with a defined range, to
256 discrete steps (2 to the power of 8). When logging from
the 2.5V channel, for example, the DL8 will record digital
values in increments of 2.5 volts divided by 256, or approx-
imately 10 millivolts.
Resolution is usually not a significant factor until you have
logging applications that will produce input signals far less
than that of the range of the logger’s channel itself.
The 2.5V channel will have a resolution of 0.4% over a
range of 0 to 2.5 volts (.01/2.5 x 100) which should be more
than adequate for most applications. If, however, you are
intending to measure signals ranging from 50 to 150 milli-
volts (with the same 2.5V channel), resolution will be much
coarser (10%). By switching the 50 to 150 millivolt input to
the 200mV channel, resolution can be improved to approx-
imately 1%.
Choosing Input Channels
It is important to maximize resolution when measuring and
recording from the DL8’s current and voltage channels.
Usually you can do this simply by choosing the channels
that match or approximate the input signals you will be
using. The table below lists several possible input signal
ranges and the recommended channels to use. It also lists
alternative channels that can be used to add additional
simultaneous monitoring capability. These alternative chan-
nels will, however, require the addition of external resistors
(see Customizing Input Ranges).
In the above table, the Range of Input column lists exam-
ples of input signals that you may wish to monitor using
your logger. The Standard Solution column lists the channel
you would normally choose to monitor that particular signal.
In this table, 200 refers to the 200mV channel, 2.5 the 0 to
2.5V channel, 5 to either of the two 0 to 5V channels, 10 to
the 0 to 10V channel, and 25 to either of the two 0 to 25mA
channels. The Alternative Solution column lists other chan-
nels that can be used to monitor the same input signals. In
most cases, these channels will need to be fitted with exter-
nal resistors
.
Range
of Input
0 to 500 mV
0 to 1.0 V
0 to 2.0 V
0 to 3.0 V
0 to 6.0 V
0 to 20 V
0 to 2.0 mA
4 to 20 mA
0 to 50 mA
0 to 100 mA
Standard
Solution
2.5
2.5
2.5
5
10
N/A
25
25
N/A
N/A
Alternative
Solution(s)
200
200
200
200
5, 2.5, 200
10, 5, 2.5, 200
200
2.5, 200
25, 5
25, 10
Bulletin E-90-DL8 6/22/05 3:56 PM Page 8