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

Measuring Currents

The dataTaker measures current signals both as direct currents, and as 4ñ20 mA current loops. This section describes measurement of direct current (DC) signals with the dataTaker. Alternating current (AC) signals cannot be measured directly with the dataTaker, but require external rectification and smoothing.

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

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

There are two basic techniques for measuring currents with the dataTaker

using the internal current shunts provided within each analog input channel

using external current shunts

The internal current shunts provided within each analog input channel are also used for measuring signals to the 4ñ20 mA current loop standard.

The Internal Current Shunts

Each analog input channel of the dataTaker and the Channel Expansion Module has an internal 100.0 Ohm 0.1% precision current shunt resistor, which is used for the measurement of current signals. Only one current signal can be measured with each analog input channel.

The internal current shunt in each analog input channel is permanently connected between the Analog Return terminal and the dataTaker ground (See Figure 45). When currents are measured, the voltage drop across this shunt is selected and measured by the dataTaker to determine current flow.

Measuring Current Using the Internal Current Shunts

Currents in the range of ±25 mA can be connected directly to the analog input channels of the dataTaker, and measured using the internal current shunts. Connection of current signals using the internal shunt is illustrated below

 

 

Figure 44 ñ Connecting Current Signals Using the Internal Current Shunts

The current signal passes into the Analog Return terminal, and flows through the internal current shunt to dataTaker ground. The current signal is measured by the voltage drop produced across the internal current shunt.

The current signal being measured flows between the Analog Return terminal Ground, and does not involve any of the +ve, ñve or T terminals of the analog input channel.

Therefore analog channels used to measure currents via the internal current shunts can also be used to measure other differential inputs such as voltage, frequency, period 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.

Unused input terminals can be used to monitor more current signals on each channel using external shunts and externally referenced single ended inputs (see below). However analog input channels used to measure internally shunted current signals cannot also be used to measure resistance.

Current signals 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 signals are measured using the internal shunts by the command for example

BEGIN
 RA15M
  5#I  8#I  10#I
END

which instructs the dataTaker to measure the current signals connected to analog input channel 5, channel 8 and channel 10.

The # indicates that the current signals are input to the Analog Return terminal, and measured using the internal current shunt. The I indicates that the signal connected to these channels is to be measured as a current. The data is returned in units of mA.

Using DeLogger, current signals can be measured using the internal shunts by the following Program Builder program.

 

 

The current signal connections using internal shunts are selected from the Current Wiring Configurations dialog which opens when you select the analog input channel.

The dataTaker will read the inputs every 10 minutes, and readings are stopped by entering a H (Halt) command.

Measuring Current Using External Current Shunts

Current signals can also be measured in both differential and single ended modes using external current shunts. A separate external current shunt is required for each current signal to be measured.

The current shunt can be of any value, and allows very large or very small DC currents to be measured. For example

a current shunt of 0.1 Ohm will allow currents of up to 25 Amp to be measured

a current shunt of 1000 Ohm will allow currents as small as 10 nA to be measured

The important factor when selecting the shunt value is that the voltage drop across the shunt at maximum current to be measured should not exceed 3250mV ñ the maximum measurable voltage on the ±2500 mV range of the dataTaker. According to Ohmís Law

Shunt Resistance (Ohms) = 3250 (mV) / Max Current (mA)

The following table lists some suitable external shunt resistance values for measuring different current ranges

 

Current Range

Shunt Resistance

0 ñ 25mA

100.0 Ohm

0 ñ 100mA

20.0 Ohm

0 ñ 1000mA

2.00 Ohm

0 ñ 10A

200.0 mOhm

0 ñ 100A

20.00 mOhm

 

The lower value shunts must be precision resistances, and are available from specialist test and measurement suppliers. The power rating for the shunt must be appropriate for the magnitude of the current to be measured as follows

   Power Rating (Watts) = Max Current (mA)2  x  Shunt Resistance (Ohms)

The shunt resistor is best installed near the source of the current signal, especially if the current signal is large, or separation between the current signal and the dataTaker is appreciable.

In this case the long cable run carries the voltage produced by the current shunt, and signal loss is small because only a small bias current flows in this part of the circuit because of the high input impedance of the analog input channels. Alternatively, the external current shunt could be installed directly across the current signal input lines at the screw terminals of the dataTaker. This is appropriate if the signal source is close to the logger.

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

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

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

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

5I(250.0)
6I(249.5)

which specifies that the dataTaker is to measure

the current signal on analog channel 5, with a current shunt of 250.0 Ohm

the current signal on analog channel 6, with a current shunt of 249.5 Ohm

Note that the current shunts do not have to be the same resistance for all channels. The dataTaker can be programmed on each channel for a different shunt resistance.

The I indicates that the signal connected to these channels is to be measured as a current. The data is returned in units of mA.

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.

 

 

The accuracy of current measurement using external shunts is determined by the accuracy of the current shunt resistors used.

Differential Measurement

Current signals can be measured by passing through external current shunts that are connected to the analog input channels for differential measurement as illustrated in Figure 45 below.

The current signal being measured only flows in the external current shunt circuit (indicated by the red current path in Figure 46). The current signal produces a voltage across the external current shunt, which is measured as a differential voltage between the +ve and ñve terminals of the analog input channel.

 

 

Figure 45 ñ Connecting Current Signals Using External Current Shunts
and Differential Inputs

 

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

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

BEGIN
 RA2H
  5I(50.0)  8I(100.8)
END

which instructs the dataTaker to measure the current signals passing through

an external 50.0 Ohm shunt differentially connected to analog channel 5

an external 100.8 Ohm shunt differentially connected to analog channel 8

The I indicates that the signal connected to these channels is to be measured as a current. The data is returned in units of mA.

Using DeLogger, current signals 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 signals 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 46 ñ Connecting Current Signals Using External Current Shunts
and Single Ended Inputs Referenced to Analog Return

 

The current signal being measured only flows in the external current shunt circuit (indicated by the red current paths in Figure 47).

The current signals produce a voltage across the current shunts, which are measured as single ended voltages between the +ve, ñve or T terminals and Analog Return.

There should be no resistance, or least resistance as possible, between the common point of the external shunt resistors, and the Analog Return terminal ñ especially if the shunt resistances are of smaller value.

Ideally this connection should be a heavy gauge copper wire, or even a bus bar. This will ensure that any current flowing in this path due to any ground potential differences, etc. will not be significant in the overall current measurements.

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 external common point and the dataTaker ground can be permitted.

If these limits are exceeded then the dataTaker ground must also be connected to the external common point to overcome errors due to common mode voltage problems.

Up to three currents can be measured on each analog channel, using external current shunts and single ended input. A fourth 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 signals 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 signals that are connected using external current shunts and single ended inputs referenced to Analog Return are measured by the command for example

BEGIN
 RA30M
  5+I(70.5)  10 TI(101)
END

which instructs the dataTaker to measure the current signals

passing through an external 70.5 Ohm shunt resistor connected between the +ve terminal of analog input channel 5 (5+) and Analog Return

passing through an external 101.0 Ohm shunt resistor connected between the T terminal of analog input channel 10 (10 T) and Analog Return

The I indicates that the signal connected to these channels is to be measured as a current. The data is returned in units of mA.

Using DeLogger, current signals 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.

Be sure not to check the Current Loop check box, otherwise this will result in data not being returned in units of mA (see Section II ñ Measuring 4-20 mA Current Loops).

 

 

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

Single Ended Measurements Referenced to an External Common

Current signals 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 47 ñ Connecting Current Signals Using External Shunts and Single Ended Inputs
Referenced to an External Common

 

The current signal being measured only flows in the external current shunt circuit (indicated by the red current paths in Figure 48).

The current signals 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 terminal.

There should be no resistance, or least resistance as possible, between the external common point of the external shunt resistors, and the SE REF terminal ñ especially if the shunt resistances are of smaller value.

Ideally this connection should be a heavy gauge copper wire, or even a bus bar. This will ensure that any current flowing in this path will not be significant in the overall current measurements.

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, then the dataTaker ground must also be connected to the external common reference point to overcome errors due to common mode voltage problems.

Up to three currents can be measured on each analog channel, using external current shunts and single ended input. A fourth 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 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 signals that are connected using external current shunts and single ended inputs referenced to SE REF are measured by the command for example

BEGIN
 RA1M
  2+I(55.0,X)  5-I(105.0,X)
END

which instructs the dataTaker to measure the current signals

passing through an external 55.0 Ohm shunt resistor connected between the +ve terminal of analog channel 2 (2+) and SE REF

passing through an external 105.0 Ohm shunt resistor connected between the ñve terminal of analog channel 5 (5ñ) and SE REF

The I indicates that the signal connected to these channels is to be measured as a current. The data is returned in units of mA.

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

Using DeLogger, current signals 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.

Be sure not to check the Current Loop check box, otherwise this will result in data not being returned in units of mA (see Section II ñ Measuring 4-20 mA Current Loops).

 

 

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

Measurement Ranges and Accuracy

The measurement range for current signals using 100 Ohm internal current shunts is ñ25 mA to +25 mA.

The accuracy of current measurement using the 100 Ohm internal current shunts is 0.1%, which is set by the accuracy of the internal shunts.

The measurement range for current signals using external current shunt resistors can be calculated by

±(3250 / shunt resistance) mA

The accuracy of current measurement using external current shunts is determined by the accuracy of the external shunt resistor used. High tolerance resistors can be used as external current shunts if greater accuracies are required.

External current shunts can be calibrated by measuring a steady state current in the external circuit using an accurate multimeter, and adjusting the shunt value declared as a channel option to the input channel until the dataTaker returns the correct value.

If there are common mode or offset voltages on the current signals that are causing measurement problems, then the following corrections can be used

external attenuation of the voltage signal from across external shunts (see Figure 42 for connection details)

select the internal attenuators of the dataTaker 505, 515, 605 and 615 by the A channel option in the input channel specification

The following table summarizes the measurement ranges, resolution and accuracies for current measurement using the internal current shunts.

 

Input Current Range

Resolution

Measurement
Accuracy

±0.25mA

200nA

±0.12%

±2.50mA

1µA

±0.2%

±25.00mA

10µA

±0.12%

 

Error Messages

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

The dataTaker also reports the out of range error condition with the error message ‘E11ñinput(s) out of rangeí if the Messages Switch /M is enabled

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