dataTaker - Data Loggers, Powerful and Flexible Data Acquisition & Data Logging Systems

Measuring 4ñ20 mA Current Loops

Current loops provide an interface standard between a wide variety of sensors and test and measurement instruments. This interface standard is widely used in industrial and scientific applications.

The dataTaker data loggers support the 4ñ20 mA current loop standard.

There are many advantages in using current loops to interface sensors to test and measurement equipment, including

current loop provides a common interface standard to a wide variety of sensors

wiring requirements for carrying signals between sensors and the test and measurement equipment are simple

long cables can be used without loss of accuracy, because cable resistance has no influence on the signal

the interface has good immunity to electrical noise

the current loop interface can provide isolation barriers between sensors and test and measurement equipment

only a simple central power supply is required to drive the current loop system

more than one measuring device can be connected in series in a current loop

The dataTaker 50 has 5 differential analog input channels, or 15 single ended input channels, which can be used for current loop measurements.

The dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD) have 10 differential analog input channels, or 40 single ended input channels, which can be used for current loop measurements.

Any combination of these differential and single ended connections can be used for measuring current loops, depending on installation of current measuring shunts.

Whenever current loops are being measured by the dataTaker 505, dataTaker 515, dataTaker 605 and dataTaker 615 data loggers, the internal attenuators are automatically selected to provide the greatest common mode voltage range.

There are two basic techniques for measuring current loops with the dataTaker

current loop measurement using the internal current shunts provided within each analog input channel

current loop measurement using external current shunts

Basic Concepts of Current Loops

Current loop transmitters pass or actively drive the current flow in a current loop circuit, such that the current flow is proportional to the magnitude of the physical or electrical parameter being monitored by the associated sensor.

The end points for the 4ñ20 mA current loop standards are

a current of 4 mA represents the low scale (which may be a zero or negative value) output from the supported sensor

a current of 20 mA represents the full scale output from the supported sensor

For example a current loop transmitter supporting an infra red thermometer which has a range of 0 to 500 °C, will pass or drive a current of 4 mA for a temperature reading of 0 °C, and pass or drive a current of 20 mA for a temperature reading of 500 °C.

The relationship between loop current and the physical parameter sensed by the sensor is usually linear. The manufacturer's specifications for the particular current loop device will clarify this relationship.

The current flowing in the loop is independent of the loop voltage, and so unregulated power supplies and long cables can be used with no loss of measurement accuracy.

The current loop transmitter may be powered directly from the loop current if it can operate on 4 mA, or it may be separately powered and perhaps supply the loop current.

Note:  The dataTaker does not provide power for the current loop. A separate loop power supply must be used.

The block diagram for a current loop transmitter is illustrated in Figure 48 below

 

 

Figure 48 ñ Block Diagram for an Isolated Current Loop Transmitter

 

The current loop transmitters can also provide an isolation barrier between the sensor and the test and measurement equipment.

This isolation barrier offers several advantages

allows the sensor to operate at a different ground potential with respect to the test and measurement instruments (500 ñ 2500 Volts typical), without common mode voltage or ground loop problems

protects test and measurement instruments from exposure of the sensor to damaging high voltages.

There are five basic function blocks which make up a complete current loop based measurement system

a sensor, which senses the magnitude of the physical parameter being measured

an isolation barrier system

a current loop transmitter, which translates sensor output to a current loop signal

a current loop power supply

a measurement device to measure the loop current

These function blocks are illustrated for a typical current loop system below

 

 

Figure 49 ñ Current Loop Transmitter System Function Blocks

The current loop power supply is traditionally 24 Volts DC, however 12 Volts DC is becoming more common. The power supply and must have sufficient current capacity to source full scale signal current for all transmitters, plus any power drawn by the transmitters.

For example the power supply current required to support 10 current loop transmitters at full scale (20mA) is at least

Loop Power Supply Current     = 10 transmitters  x  20 mA
                                         = 200 mA total

It is good policy to over specify the loop power supply by 50 ñ 100%.

Installation of Current Loop Transmitters

The output of the current loop power supply should be isolated from the ground of its power supply. The negative side of the current loop power supply must be connected to the ground of the dataTaker.

The current loop transmitters are usually remotely located adjacent to the sensors they support, and the current loop cable is routed to the power supply and dataTaker.

Care must be taken to ensure that remote current loop transmitters are not connected to any other circuit, and that the cabling is not exposed to potential current leaks to ground. Either of these conditions will cause measurement errors and possible equipment damage.

The positive wires of all of the current loop transmitters are connected to the positive output of the current loop power supply.

It is advisable to place a 10 to 100 Ohm current limiting resistor in series with the current shunt in each current loop circuit to protect the dataTaker in the event of a short circuit in any of the current loop transmitters.

The dataTaker measures the current flow in the return wire of the current loop circuit. This avoids problems of excessive common mode voltage.

The loop current can be measured by the dataTaker either using the internal current shunts in each analog input channel, or using external current shunts (See Section II ñ Measuring Currents).

The Internal Current Shunts

Each analog input channel of the dataTaker and of the Channel Expansion Module (CEM-AD) includes an internal 100.0 Ohm 0.1% precision current shunt resistor, which is used for measurement of current and current loop signals.

When the internal current shunts are used for measurement of current loops, only one current loop can be measured with each analog input channel.

The internal current shunt within each analog input channel is internally connected between the Analog Return terminal and the dataTaker ground (See Figure 44). When current loops are measured, the voltage across this shunt is selected and measured by the dataTaker to determine current flow.

Common Mode Voltage

Current loop transmitters usually operate from a 12 or 24 Volt DC power supply.

Care must be taken when connecting current loops to the dataTakers to ensure that this voltage is not connected across the logger, otherwise the common mode range of the logger could be exceeded. This requirement applies when using either internal or external current shunts.

Exceeding the common mode voltage range will certainly produce measurement errors, and will possibly also damage the logger.

The most common circumstance for the common mode voltage range of the dataTaker to be exceeded is where the dataTaker is grounded (possibly even to the same ground as the current loop power supply), and the shunt for measuring loop current is located in the positive side of the current loop. This in effect places the current loop power supply across the logger, exceeding the common mode voltage range.

dataTaker 50, 500 and600

The dataTaker 50, dataTaker 500 and dataTaker 600 data loggers have a common mode voltage range of ±3.5 Volts.

Most Important : It is essential when measuring current loops with the dataTaker 50, dataTaker 500 and dataTaker 600 data loggers that

the negative side of the current loop power supply must be connected to the dataTaker ground

the current shunt (internal or external) must be located in the current loop return between the negative side of the power supply and the negative terminal of the current loop transmitter

if a number of current loops are being measured and more than one current loop power supply is used, then the negative side of each of these power supplies and dataTaker ground must all be connected to a common grounding point

The voltage across a 100 Ohm current shunt at the maximum loop current (20 mA) is only 2 Volts. However if this voltage is more than 3.5 Volts above or below the dataTaker 50, 500 or600 ground potential, then current loop measurement errors will occur.

Where external shunts are used, the common mode voltage can easily be measured with a multimeter between dataTaker ground and each side of the current shunt. Neither of these measurements can exceed ±3.5 Volts.

However if excessive common mode voltages cannot be avoided, then current loops can still be measured with the dataTaker 50, 500 or600 as follows

use an external current shunt for each current loop to be measured

attenuate the voltage produced across each shunt with a voltage attenuator network to reduce the common mode voltage (See Section III ñ Measuring High Level Voltages). Use at least a 10:1 attenuator for a 24 Volt loop power supply, or a 5:1 attenuator for a 12 Volt loop power supply.

use data scaling (span, polynomial or direct calculations) to correct the magnitude of the resulting data

dataTaker 505, 515, 605 and 615 

Whenever current loops are measured by the dataTaker 505, dataTaker 515, dataTaker 605 and dataTaker 615 data loggers, the internal voltage attenuator is automatically selected which increases the common mode voltage range to ±100 Volts.

Therefore the risk of exceeding the common mode voltage range of these loggers with current loops is virtually eliminated.

Sharing Current Loop Signals

Sometimes it may be necessary to measure and log current loop data from an existing transmitter and indicator/controller current loop. This may be an existing transmitter and associated panel meter readout, or existing transmitter and controller unit, etc.

Most current loop transmitters are designed to handle up to 650 Ohms of resistance in the current loop circuit. Therefore additional current shunts can be inserted into the current loop to support additional measuring devices, so long as this maximum load resistance is not exceeded. Check the specifications for the transmitter in question to determine actual capability.

It is generally acceptable to ëinsertí a dataTaker into an existing current loop, however

insert the dataTaker between the loop power supply negative terminal and the negative terminal of the transmitter

ensure that common mode voltages of the dataTaker and other readout/controller devices is not exceeded

ensure that the total loop resistance (the sum of all current shunts) does not exceed the capability of the transmitter

Measuring Current Loops Using the Internal Shunts

Current loops and loop power supplies are connected directly to the analog input channels of the dataTaker, when measuring loop currents using the internal current shunts within each of the analog input channels.

The loop current passes into the Analog Return terminal, through the internal current shunt to dataTaker ground, and out the GND terminal to the negative terminal of the loop power supply. The current loop signal is measured by the voltage produced across the internal current shunt.

The connection of current loops using the internal shunt is illustrated below

 

 

Figure 50 ñ Connecting Current Loops Using the Internal
Current Shunts

 

If more than one current loop transmitter is to share a common loop power supply, then these should be connected as follows

the positive wires each current loop transmitter is connected to the positive terminal of the current loop power supply

the negative wires from each current loop transmitter are connected to the Analog Return of the respective analog input channels

a single connection is made between the dataTaker GND terminal and the negative terminal of the loop power supply. Note that the current for all current loops being measured passes back to the loop power supply via the of the dataTaker ground.

Because the current loop being measured flows between the Analog Return terminal and dataTaker ground, and does not involve any of the +ve, ñve or T terminals of the analog input channel.

Therefore analog channels used to measure current loops with the internal shunts can also be used to measure other differential inputs such as voltage, period, frequency or analog state. Single ended measurement on these channels is also possible if the signal source can tolerate small fluctuations on the Analog Return terminal.

These unused inputs can be used to monitor additional current loops on each channel using external shunts and single ended inputs referenced to SE REF (see below).

However analog input channels used to measure internally shunted current loops cannot also be used to also measure resistance.

Current loops connected to the dataTaker using the internal current shunts are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, current loops are measured using the internal shunts by the command for example

BEGIN
 RA15M
  1#L  5#L  10#L
END

which instructs the dataTaker to measure the current loop signals connected to analog input channel 1, channel 5 and channel 10.

The # indicates that the current signals are input to the Analog Return terminal, and are to be measured using the internal current shunt. The L indicates that the signal connected to these channels is to be measured as a current loop.

Current loop data is returned as the percentage of the 4 ñ 20 mA range that the signal represents. For example a value of 50% is returned if the current loop signal is 12 mA.

Current loop signals can also be measured as channel type I, in which case data is returned in the range of 4 ñ 20 mA.

Using DeLogger, current loop signals can be measured using the internal shunts by the following Program Builder program. The current loop connections using the internal shunts are selected from the Current Wiring Configurations dialog which opens when you have selected the analog input channel.

Note that the Current Loop checkbox must be checked. If this is checked then data is returned as
0 ñ 100% data, otherwise if this is not checked then data is returned as 4 ñ 20 mA data.

 

 

The dataTaker will read the current loop signals every 15 minutes, and readings are stopped by entering a H (Halt) command.

Measuring Current Loops Using External Current Shunts

Current loop signals can also be measured by the dataTaker using external current shunt resistors, in the same manner as for measuring current signals using external shunts. (See Section II – Measuring Currents Using External Current Shunts).

The external current shunts may be of any appropriate value, however 100 Ohms is the most common. The accuracy of current loop measurements using external current shunts is determined by the accuracy of the current shunt resistors used.

The resistance of the external current shunt is declared as a channel option to the current loop input channels in schedule lists. The external current shunt resistance is declared in units of Ohms.

The default external current shunt resistance for the dataTaker is 100.0 Ohm. If an external shunt resistance is not specified, then a shunt resistance of 100.0 Ohm is assumed.

Using DeTransfer, the resistance of the external current shunt is declared in Ohms by the command for example

6L(120.0)

which specifies an external current shunt resistance of 120.0 Ohm, for the current loop signal to be measured on analog input channel 6.

Using DeLogger, the resistance of the external current shunt is declared in Ohms in the Current Wiring Configurations dialog which opens when you select the Current sensor type for an analog input channel in the Program Builder.

 

 

Note that the Current Loop checkbox must also be checked. If this is checked then data is returned as 0 ñ 100% data, otherwise if this is not checked then data is returned as 4 ñ 20 mA data.

The external current shunt is placed in the current loop return path, between the negative signal output of the current loop transmitter and the negative terminal of the loop power supply.

The loop current produces a voltage across the external current shunt, which is connected to any analog input channel as a low level voltage input. The inputs from external current loop shunts are connected to analog input channels in the same manner as differential or single ended low level voltage signals.

Differential Measurement

Current loops can be measured by passing them through external current shunts that are connected to the analog input channels for differential measurement as follows

 

 

Figure 51 ñ Connecting Current Loops Using External Current Shunts
and Differential Inputs

 

The loop current produces a voltage across the external current shunt resistor, which is measured as a differential voltage between the +ve and ñve terminals of the analog input channel.

The loop current signal being measured only flows in the external current shunt circuit (indicated by the red current path in Figure 52).

There is no current flow in the ground circuit of the dataTaker, contrary to the situation when current signals are measured using the internal current shunts.

Connecting the negative pole of the power supply to dataTaker ground is optional. However this is highly recommended, and will ensure that common mode voltage range is not exceeded, especially for dataTaker 50,500 and 600 series loggers with solid state multiplexers and relatively small common mode voltage ranges.

Current loop signals connected using external current shunts and differential inputs are sampled and data is returned when a Schedule containing the channel is executed.

Using DeTransfer, current loop signals that are connected using external current shunts and differential inputs are measured by the command for example

BEGIN
 RA2H
  3L(20.0)  10L(122.5)..8L(122.5)
END

which instructs the dataTaker to measure the current loops as follows

a current loop flowing in a 20.0 Ohm external shunt, which is connected differentially to analog input channel 3

a current loop flowing in a 122.5 Ohm external shunt, which is connected differentially to analog input channel 10

a current loop flowing in a 122.5 Ohm external shunt, which is connected differentially to analog input channel 8

The L indicates that the signal connected to these channels is to be measured as a current loop. Current loop data is returned as of the percentage of the 4ñ20 mA range that the signal represents. For example a value of 50% is returned if the current loop signal is 12 mA.

Using DeLogger, current loops can be measured using external current shunts and differential inputs by the following Program Builder program.

Differential connections and external shunt values are selected in the Current Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

The dataTaker will read the inputs every 2 hours, and readings are stopped by entering a H (Halt) command.

Single Ended Measurement Referenced to Analog Return

Current loops can be measured by passing them through external current shunts that are connected to the analog input channels for single ended measurement referenced to Analog Return as follows

 

 

Figure 52 ñ Connecting Current Loops Using External Current Shunts and
Single Ended Inputs Referenced to Analog Return

 

The current loop signal being measured only flows in the external current shunt circuit (indicated by the red current paths in Figure 53). There is effectively no current flow in the ground circuit of the dataTaker, contrary to the situation when current signals are measured using the internal current shunts. The loop currents produce a voltage across the external current shunts, which are measured as single ended voltages between the +ve, ñve or T terminals and Analog Return.

There must be negligible resistance between the current shunt resistors and Analog Return ñ especially if the shunt resistances are small in value. If the shunt resistors are located close to the dataTaker, then this will not be a concern.

However if the shunt resistors are located at some distance from the dataTaker, then these should be connected back to the logger with heavy gauge copper wire. If the remote shunts are close together, then the negative of the shunts and the negative of the loop power supply(s) should connect to a local bus bar, which in turn connects back to the logger via a heavy gauge copper wire.

These measures will ensure that any current flowing in the return path due to any ground potential differences, etc. will not be significant in the overall loop current measurements. This potential source of error can be further minimized by using external current shunts with as large a resistance value as possible ñ however do not exceed the maximum allowable loop resistance specified for the transmitters.

Up to three loop currents can be measured on each analog channel, using external current shunts and single ended input. A fourth loop current could still be measured using the internal current shunt.

The dataTaker50 does not support single ended current measurement for the Excite (T) terminal.

Current loops connected using external current shunts and single ended inputs referenced to Analog Return are sampled, and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, current loop signals that are connected using external current shunts and single ended inputs referenced to Analog Return are measured by the command for example

BEGIN
 RA1H
  4+L(100.0)  5 TL(120.0)
END

which instructs the dataTaker to measure

a current loop with a 100.0 Ohm external shunt, connected as a single ended input referenced to Analog Return to analog input channel 4+

a current loop with a 120.0 Ohm external shunt, connected as a single ended input referenced to Analog Return to analog input channel 5 T

The L indicates that the signal connected to these channels is to be measured as a current loop. Current loop data is returned as the percentage of the 4 ñ 20 mA range that the signal represents. For example a value of 50% is returned if the current loop signal is 12 mA.

Using DeLogger, current loops can be measured using external current shunts and single ended inputs referenced to Analog Return by the following Program Builder program.

Single ended connections and external shunt values are selected in the Current Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

The dataTaker will read the inputs every hour, and the readings are stopped by entering a H (Halt) command.

Single Ended Measurements Referenced to an External Common

Current loops can be measured by passing them through external current shunts that are connected to the analog input channels for single ended measurement referenced to an external common that is in turn connected to SE REF as follows

 

 

Figure 53 ñ Connecting Current Loops Using External Shunts and
Single Ended Inputs Referenced to an External Common

 

The loop current being measured only flows in the external current shunt circuit (indicated by the red current paths in Figure 54).

The loop currents produce a voltage across the external current shunts, which are measured as single ended voltages between the +ve, ñve or T  terminals and SE REF terminals.

There must be negligible resistance between the current shunt resistors and SE REF terminals ñ especially if the shunt resistances are small in value. If the shunt resistors are located close to the dataTaker, then this will not be a concern.

However if the shunt resistors are located at some distance from the dataTaker, then these should be connected back to SE REF with heavy gauge copper wire. If the remote shunts are close together, then the negative of the shunts and the negative of the loop power supply(s) should connect to a local bus bar, which in turn connects back to the logger via a heavy gauge copper wire.

These measures will ensure that any current flowing in the return path due to any ground potential differences, etc. will not be significant in the overall loop current measurements. This potential source of error can be further minimized by using external current shunts with as large a resistance value as possible ñ however do not exceed the maximum allowable loop resistance specified for the transmitters.

In this configuration, the dataTaker ground is not involved in the measurement circuit, and so a potential difference of up to ±3.5 Volts (dataTaker 50, 500, 600) or ±100 Volts (dataTaker 505, 605, 515, 615) between the common external reference point and the dataTaker ground can be permitted. If these limits are exceeded however, then dataTaker ground must also be connected to the external common reference point to overcome errors due to common mode voltage problems.

Up to three loop currents can be measured on each analog channel, using external current shunts and single ended input. A fourth loop current could still be measured using the internal current shunt.

The dataTaker 50 does not support single ended current measurement for the Excite (T) terminal.

Current loop signals connected using external current shunts and single ended inputs referenced to an external common reference point connected to SE REF are sampled, and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, current loop signals that are connected using external current shunts and single ended inputs referenced to SE REF are measured by the command for example

BEGIN
 RA30M
  2+L(X,250) 3+L(X,250)  4+L(X,250)
END

which instructs the dataTaker to measure

a current loop with a 250.0 Ohm external shunt, connected as a single ended input referenced to SE REF to analog input channel 2+

a current loop with a 250.0 Ohm external shunt, connected as a single ended input referenced to SE REF to analog input channel 3+

a current loop with a 250.0 Ohm external shunt, connected as a single ended input referenced to SE REF to analog input channel 4+

The L indicates that the signal connected to these channels is to be measured as a current loop. Current loop data is returned as the percentage of the 4 ñ 20 mA range that the signal represents. For example a value of 50% is returned if the current loop signal is 12 mA.

The X  channel option indicates that the current loop is to be measured with respect to an external common connected to the SE REF input.

Using DeLogger, current loops can be measured using external current shunts and single ended inputs referenced to SE REF by the following Program Builder program. Single ended connections and external shunt values are selected in the Current Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

The dataTaker will read the inputs every 30 minutes, and the readings are stopped by entering a H (Halt) command.

Converting Current Loop Data to Physical Units

Current loop readings can be linearized in real time to the engineering units of the physical parameter being measured, using a Span function (See Section III ñ Scaling Data, Polynomials, Spans and Functions).

Spans are defined for current loop inputs in terms of the lower and upper limits of the physical parameter measured.

For example a 4ñ20 mA thermocouple transmitter measures furnace temperatures over the range of 100 to 1000 Deg C. A loop current of 4 mA (0%) represents a temperature of 100 Deg C, and a loop current of 20 mA (100%) represents a temperature of 1000 Deg C. (The current loop transmitter performs both the cold junction compensation and linearization of the thermocouple signal).

Using DeTransfer, the Span for the thermocouple transmitter can be defined as

S1=100,1000,0,100"Deg C"

where S1 is the Span being defined.

The defined Span is assigned to the current loop input channels in the schedule lists, for example

S1=100,1000,0,100"Deg C"

BEGIN
 RA15M
  1#(S1)  2#L(S1)
END

which will return data every 15 minutes from the five 4ñ20 mA thermocouple transmitters in the units of temperature of Deg C.

Measurement Ranges and Accuracy

The accuracy of current loop measurement using internal current shunts is determined by the accuracy and tolerance of the shunt resistor (±0.1%).

The accuracy of current loop measurement using external current shunts is determined by the accuracy and tolerance of the shunt resistor provided by the user.

The measurement ranges, resolution and accuracy for current loop measurement using the internal current shunts are detailed below.

 

Input Current Loop Range

Resolution

Measurement
Accuracy

0 ñ 100%
(4 ñ 20mA)

0.001%

±0.1%

 

Error Messages

Current loop inputs using the internal current shunts which fall outside of the range of ñ25 mA to +25 mA produce an over-range reading of ñ99999.9 % or +99999.9 % respectively.

The dataTaker reports the error condition with the error message ëE11ñinput(s) out of rangeí if the Messages Switch /M is enabled.

Current loop inputs outside the range the 4ñ20 mA standard are accepted by the dataTaker, but produce data which is outside of the range of 0 to 100 %.

Page Content


Home

Title and Waranty

Go to: Section 2 | Section 3

Section 1


Construction of the dataTaker 50

Construction of the dataTaker 500 600

Construction of the CEM

Getting Started

 

Section 2


Interfacing

Powering the dataTaker

Powering Sensors from the dataTaker

The Serial Interfaces

The RS232 COMMS Serial Interface

The NETWORK Interface

Analog Process

Connect Analog

Analog Chns

Measuring Low Level Voltages

Measuring High Level Voltages

Measuring Currents

Measuring 4-20mA Current Loops

Measuring Resistance

Measuring Frequency and Period

Measuring Analog Logic State

Measuring Temperature

Measuring Temperature with Thermocouples

Measuring Temperature with RTDs

Measuring Temperature with IC Temperature Sensors

Measuring Temperature with Thermistors

Measuring Bridges and Strain Gauges

Measuring Vibrating Wire Strain Gauges

The Digital Input Channels

Monitoring Digital State

The Low Speed Counters

The Phase Encoder Counter

The High Speed Counters

The Digital Output Channels

The Channel Expansion Module

Installing The Panel Mount Display

 

Section 3


Programming the dataTaker

Communication Protocols and Commands

Entering Commands and Programs

Format of Returned Data

Specifying Channels

The Analog Input Channels

The Digital Input Channels

The Counter Channels

The Digital Output Channels

The Real Time Clock

The Internal Channels

Channel Options

Schedules

Alarms

Scaling Data - Polynomials, Spans and Functions

CVs Calcs and Histogram

Logging Data to Memory

Programming from Memory Cards

STATUS RESET TEST

Switches and Parameters

Networking

Writing Programs

Keypad and Display

Error Mess Text

Appendix A - ASCII

Appendix B - ADC Timing