Tag Archive for 4/20mA

Wireless Control of Life Support System in an Exotic Aquarium for Show Animals

Wireless Control of Life Support System in an Exotic Aquarium for Show Animals

APPLICATION

Park engineers have an application sending a 4/20mA signal from an ORP (Oxidation Reduction Potential) sensor to a Control Room approximately 500 feet away.

The ORP sensor is used to electronically monitor the amounts of sanitizer and other chemicals in the water, before it enters the large holding tank.

Rain or shine, a ladder is climbed to the top of a pump house building several times everyday to take readings. This posed a safety and health risk to the operator as well as only provided sporadic measurement cycles.

The EPA requires monitoring of this chemical system for both human and animal safety.

More Info: Frequency Hopping Spread Spectrum Technology

More Info: Value of Isolated Signal Conditioners

USER REQUIREMENTS

The ORP monitor where the reading was taken is in a hard to reach location at the top of a pump house on the side of the tank.

Due to a large amount of sidewalks and other obstacles, it was not cost effective to run wires from the sensors to the Control Room.

WIRELESS POSSIBILITY

Checking the 500 foot line of sight between the Transmitter and Receiver locations indicated there was a 50 foot elevation difference. The check of the antenna path also indicated there was a large building near the Receiver. There are also several trees in the antenna path.

More Info: How to calculate your antenna needs

WILKERSON INSTRUMENT’ SOLUTION

DR9011 With 3 Element Yagi Antenna

DR9011 With 3 Element Yagi Antenna

Use a 900 MHz Band Transmitter (DR9011, 2 analog and 4 switch inputs) at the top of the building to monitor the 4/20mA signal from the ORP sensor.

The DR9011 was installed inside a NEMA 4X housing. A small 3” high 2.1dBi gain vertical antenna was mounted on the top of the NEMA 4X housing.

Use a 900 MHz Band Receiver (DR9021, 2 analog and 4 switch outputs) at the LSS Control to receive the data coming from the DR9011 Transmitter.

This unit was installed inside a pre-existing control panel. Along with a wall mount 3 element directional Yagi antenna (8dBi gain) to the top of the LSS (Life Support System) Control building.

The use of the Omni directional antenna at the sensor end allowed the signal to be transmitted across a wider path. In using the Yagi antenna at the receiver side, the install crew was able to choose the best location and path for pointing the Yagi antenna.

The trees and building posed difficulty by either blocking part of or potentially causing a reflected signal in the Fersnel Zone. The Yagi helped eliminate this due to its narrow beam width.

This system has been reliably sending data since June 2007.

How Loop Powered Isolators Work

How Loop Powered Isolators Work

Loop powered isolators are necessary when an extra isolated 4/20mA output is required in areas where no power is available.

A loop powered isolator’s power is provided by its 4/20mA input current.  This makes it possible for an existing 4/20mA loop to be used to provide another isolated 4/20mA output through a loop powered isolator.

dm4391 loop powered isolator

DM4391-2 Dual channel DC isolators With and without front cover

A loop powered isolator is basically a center tapped transformer which has the input 4/20mA current flowing through its center tap on the primary. An oscillator circuit alternately switches the current through each half of the primary.  This alternating current creates a current in the secondary of the transformer. The voltage out of the secondary is rectified and filtered.

The turns ratio on the transformer is designed with a slight step down so the current out of the secondary is slightly greater than the input current.

This is a simple form of a loop power isolator, but it has shortcomings in that the transformer is not perfectly linear over the current range. The current is also sensitive to the load resistance on the secondary.

A more complex version of a loop powered isolator has basically the same transformer circuit, but it has a circuit which monitors the current into the unit. The circuit passes this data through another isolation circuit to the secondary side of the unit.  This circuit controls the current out of the isolator so it exactly matches the input current.  This control circuit makes the loop powered isolator as accurate and linear as any powered transmitter.The DM4391 utilizes this feed forward circuit to provide an accurate and linear 4/20mA output.

The DM4391 can be connected anywhere in an existing 4/20mA loop. It is available as a single channel or dual channel unit.

For specifications and information about the DM4391 see our Catalog Page (pdf) or Installation / Operation Manual (pdf)

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