Dwyer instruments, inc – Dwyer DL8 User Manual

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DWYER INSTRUMENTS, INC.

Phone: 219/879-8000

www.dwyer-inst.com

P.O. Box 373 • Michigan City, IN 46361-0373, U.S.A.

Fax: 219/872-9057

e-mail: [email protected]

Printed in U.S.A. 2/99

FR R1-443087-00

Determining how long your batteries will last
To estimate how long your batteries will successfully power
your external sensors and transducers, you will need to
know both the capacity of the batteries as well as the pro-
posed resistance presented to them by the load (your own
transducers).

Battery capacity, usually expressed in milliamperes hours
(mAh), is the total amount of electrical charge a cell can
store or deliver. Unfortunately, most commercially-available
batteries are sold without this information.

As a general guide, most good quality nine volt batteries
have a capacity of approximately 500 mAh. This means, at
an average continuous current draw of 50 mA it will deliver
approximately ten hours of service.

The load you impose on your battery will depend on how
many transducers you intend to have in your logging circuit.
The greater the number of transducers, the greater the
power draw from the battery. If you are logging from a 4 to
20mA transducer, the maximum current it will draw will be
approximately 20mA. If you power it continuously, your 500
mAh battery should not run out until you’d had at least
twenty-five hours of service. Since your transducer proba-
bly will not always draw maximum current (12mA might be
a more reasonable figure), you can probably expect up to
forty-two hours of service.

The battery-saving switch contacts can increase the lifetime
of your external batteries significantly. Since the batteries
will be powered only for eight seconds out of every logging
interval, the power draw on the battery will be reduced con-
siderably.

Example
Problem: You want to log pressure once every thirty min-
utes from a 0 to 300 psig transducer. The output of the
transducer is 4 to 20mA. The excitation voltage is listed at
between 9 and 40 volts DC.

Solution: From this information we know that a 4 mA sig-
nal will represent 0 psig and a 20 mA signal will represent
300 psig. Since the minimum excitation voltage is 9 volts,
we should supply at least 18 volts initially from our batteries.
We can do this by simply connecting two nine volt 500 mAh
batteries in series.

Without the battery-saving switch enabled, our minimum
expected battery life would be approximately twenty-five
hours. With the switch enabled, and thus closing only eight
out of every 1,800 seconds (or thirty minutes), the life will be
extended by a factor of 225 (or 1800/8). This means that we
should be able to log from this transducer for over 5,625

Regulated Voltages
The unstable voltage characteristics of batteries makes
them, by themselves, incompatible for use with transducers
that required a regulated voltage supply. You can, however,
add your own voltage regulator to your transducer circuit to
compensate for this instability (see Figure 12). Voltage reg-
ulators are readily available at most electronic parts supply
stores (such as Radio Shack).

Make sure that your supply voltage to your regulator is at
least two volts higher than the level to which you want to
regulate to. To do this, you can easily increase the voltage
of your batteries by hooking them up in series as shown in
Figure 12.

If the wires between the voltage regulator, batteries, and
transducers are long (more than a few inches) the regulator
may oscillate and produce an unstable voltage. You can
correct this by connecting one 0.1 microfarad capacitor to
each of the regulator’s outside pins then wiring them to the
regulator’s center pin. Make sure to keep the capacitor
leads as short as possible.

MAINTENANCE
No routine maintenance is required on the Series DL data
loggers. Periodic checks of connections and mounting is
recommended. Please contact Dwyer Instruments, Inc.
before returning unit for repair to review information relative
to your application and obtain a return authorization num-
ber. When returning a product to the factory, carefully pack-
age and ship freight prepaid. Be sure to include a complete
description of the application and problem and identify any
hazardous material used with the product.

Figure 12: Regulating Battery Supply Voltage

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