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UM-0085-B09

DT80 Range User Manual

Page 308

4–20mA Current Loops

Many different sensor types provide a 4-20mA current output, where the current is proportional to the quantity being

measured.
The channel type for a 4-20mA current loop measurement is

; channel options are as for Current measurements; see

Current (P306

)

To read a current loop sensor, the DT80 first measures the current. Any of the Current wiring configurations may be

used. The measured current is then scaled so that the channel returns 0% for 4mA and 100% for 20mA.
For example, if a current loop sensor is connected as per

C3 – Independent Current Input using Internal Shunt (P307)

then it would be read using

1#L

A span is often applied to this value so that the final reported value is in the proper engineering units for the quantity

being measured.
For example, if a pressure sensor with 4-20mA output operates over a range of 100-500kPa then

BEGIN S1=100,500"kPa" RA2S 4#L(S1) END

will return a pressure reading in kPa every 2s. The loop would in this case be connected across the

and

AGND/EXT#

terminals, making use of the internal shunt resistor.
One advantage of a current loop system is that a cable fault can be readily detected. An open circuit loop will give a

negative reading, normally -25% (0mA), which can then be detected in an

ALARM

statement.



Multiple Devices in One Loop

Multiple measurement devices can be connected in series in the one current loop. For example, a current loop might

include both a DT80 and a 4-20mA digital panel display module, in series. (There can only be one 4-20mA sensor in a

given loop.)
In these situations care needs to be taken to ensure that the DT80's common mode voltage limits are not exceeded,

especially if the DT80 is placed "above" the other devices in the loop. The DT80's input attenuators may be enabled

using the

channel option, which will increase the common mode limit to ±35V, relative to

AGND/EXT#

Note:

If there are multiple devices in the loop then the loop should be continuously powered. This implies the use of an external shunt

(

C1 – Shared Terminal Current Inputs using External Shunts (P306),

as the DT80 normally disconnects the

EXT#

terminal between

measurements.

Resistance

Resistance (degree of impediment to current flow) is measured by passing a known

excitation current

through the

resistance and measuring the voltage across it. From Ohm's law, Resistance = Voltage / Current.
The

channel type returns the resistance value in ohms (Ω).

The DT80 Series 2, 3 incorporates two and Series 4 has a three precision current sources. When measuring a

resistance, one or other of these sources is switched through to the * (Excite) terminal, which is then connected to the

unknown resistance. (On Series 3 models, excitation can also be directed to the + or – terminals.)
By default, an accurately known current of approximately 200μA is generated by the DT80. This allows measurement of

resistances up to about 10kΩ. For low resistances (up to 700Ω), the

channel option can be used to select a higher

excitation current (2.5mA), which will allow a more accurate reading. In Series 4 an additional current source

III

with

current of approximately 2uA has been added to increase datataker capabilities to measure resistances up to 1MΩ.
One problem which occurs when measuring low resistance values is the fact that the wires used to connect the unknown

resistance to the logger also have resistance. As shown in the wiring diagrams below, there are various ways to

overcome this. These involve the use of separate wires to carry the excitation current and to sense the voltage.
For example, the channel

1R(4W)

will perform a

four-wire

measurement. Excitation current flows out the * terminal,

through the unknown R, and returns to the # terminal. Two separate

sense

wires connect the + and – terminals to the

unknown R. Because the DT80 has a very high input impedance (>10MΩ), negligible current will flow in these sense

wires, resulting in a negligible voltage drop. The measured voltage will therefore be the voltage across the unknown R

only, and will not include any voltage drop in the current-carrying excitation cables.
The downside of a four-wire measurement is that you need, well, four wires. The three and two wire configurations

provide for reduced cable cost, at the expense of accuracy.