The dataTaker supports two wire, three wire and four wire methods for measuring resistance.

The unknown resistance is connected between the Excite (T) terminal and the Analog Return terminals of the input channel. The dataTaker outputs a precise excitation current from the Excite (T) terminal during resistance measurement, which passes through the unknown resistance a produces a voltage across the unknown resistance.

The voltage produced across the unknown resistance by this excitation current is connected by various methods to the analog input channel. The measured voltage is then used to calculate the value of the unknown resistance for the known excitation current, according to Ohmís Law.

The dataTaker can output either of two precise resistance excitation currents from the Excite terminal

a precision 250.0 µA excitation current

a precision 2.500 mA excitation current

These two excitation currents combined with the three low level voltage measurement ranges of the dataTaker provide six resistance measurement ranges.

However two of the ranges overlap, such that there are effectively four ranges for measuring resistance as follows

The resistance measurement ranges for the 250.0 µA excitation current are

0 to 100.00 Ohm full scale

100 to 1000.0 Ohm full scale

1000 to 10000.0 Ohm full scale

The resistance measurement ranges for the 2.500 mA excitation current are

0 to 10.000 Ohm full scale

0 to 100.00 Ohm full scale

100 to 500.0 Ohm full scale

The excitation current to be used for resistance measurement is specified as a channel option in the input channel specification. The default excitation current is 250.0 µA, providing a resistance measurement range of 0 ñ 10000 Ohm.

The resistance ranges for each excitation current are automatically selected by the dataTaker to provide the highest measurement resolution for the unknown resistance. The resistance measurement ranges do not have to be selected by the user.

The resistance measuring ranges associated with the 250.0 µA excitation current is used for

measurement of resistances of up to 7 KOhm

measurement of thermistors, etc.

The resistance measuring ranges associated with the 2.500 mA excitation current is used for

precise measurement of resistances of up to 500 Ohm

measurement of Resistance Temperature Detectors (RTDs) whose resistance only changes from 20 to 320 Ohm over their temperature measuring range of ñ200 to 600 Deg C.

Resistance Measuring Circuit

The resistance measuring circuit of the dataTaker is shown below.

 

 

Figure 54 - dataTaker Resistance Measuring Circuit

 

The current source of 250.0 µA or 2.500 mA is output from the Excite terminal.

The voltage produced across the unknown resistance is applied between the +ve and ñve input terminals by the various methods for resistance measurement.

The compensation for lead wire resistance for the 2 wire and 3 wire methods is measured between the ñve terminal and the Analog Return terminal.

The excitation current is switched on when the channel is selected, and remains on until the analog to digital conversion is completed. Normally this is a period of 30 mS.

Specifying Excitation Current

The excitation current to be used for resistance measurement is specified as a channel option in the command to measure the resistance connected to analog input channels. If an excitation current is not specified, then the default 250.0 µA excitation current is used, providing a resistance measurement range of 0 - 10000 Ohm.

Using DeTransfer, the command for example

specifies that a resistance connected to analog channel 1 is to be measured, using the default 250.0 µA excitation current. The 250.0 µA excitation current is selected by default whenever a resistance input type is specified without excitation.

The 2.500 mA excitation current is selected by the Excite terminal channel option II. Using DeTransfer, the command for example

specifies that a resistance connected to analog channel 1 is to be measured using the 2.500 mA excitation current.

In DeLogger, the excitation for all resistance measurements defaults to 250.0 µA. If the 2.500 mA excitation current is required, then this must be enabled for each channel for which it is required. This is specified in the Program Builder via Channel Options : ExcitationÖ

 

 

Connection of Resistances 

The dataTaker supports 4 wire, 3 wire and 2 wire methods of resistance measurement, which are described in the following sections.

All cabling used to connect resistance devices to the dataTaker for measurement has innate resistance. If resistance is simply measured between two cable wires at the dataTaker end, then this value will include the resistance of the device under test, and the resistance of the cable.

The methods for resistance measurement employed by the dataTaker compensate for the resistance in the connecting cables, albeit by various means, thereby allowing use of long cables even when measuring relatively small resistances such as for RTDs.

A fourth resistance measurement method is also supported, where two resistances are placed in series. However this single ended method does not compensate for cable resistance, but allows two resistances to be measured on each analog channel.

Four Wire Resistance Measurement

The 4 wire method of resistance measurement is the most accurate, because there is effectively no current flowing in the measurement cable wires and therefore no added resistance due to the cable wires.

This method requires a 4 core cable, and has two pairs of cable wires connected across the unknown resistance. One pair of cable wires carries the excitation current, and the other pair of cable wires is used to measure the voltage that is produced across the unknown resistance by the excitation current.

This method of resistance measurement should be used where cables are long or if the individual wires are of unequal resistance.

During resistance measurement, the excitation current output from the Excite terminal passes through the unknown resistance, and returns into the Analog Return terminal. The voltage produced across the unknown resistance by the excitation current is then measured differentially between the +ve and ñve terminals.

The excitation current is flowing in the excitation circuit (shown by the red path in Figure 55 below), which is totally separate from the measurement circuit.

 

Figure 55 ñ Four Wire Resistance Measurement

 

Because of the high input impedance of the instrumentation amplifier and the analog to digital converter of the dataTaker, there is negligible current flow in the measurement circuit. Therefore there is a negligible cable wire resistance component in the voltage measured between the +ve and ñve terminals. This measured voltage is then computed to resistance for the known excitation current.

Each resistance measured by the four wire method requires all terminals of the analog input channel. Therefore a maximum of 5 resistances can be measured using this method by the dataTaker 50, and a maximum of 10 resistances can be measured using this method by the dataTaker 500/600 series loggers.

Using DeTransfer, resistance can be measured by the 4 wire method by the command

which instructs the dataTaker to measure the resistance connected to analog input channel 1 and channel 5. The R indicates that the signal is to be measured as a resistance. The 4W channel option indicates that the 4 wire method of resistance measurement is to be used.

 

Using DeLogger, resistance can be measured using the 4 wire method by the Program Builder program above. The 4 wire connections are selected from the Resistance Wiring Configurations dialog which opens when you select the analog input channel.

The resistance data is returned in units of Ohms.

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

Four Wire Resistance Measurement with External Common

The 4 wire method of resistance measurement can be used with an external common which is common to all of the resistances being measured. The common point is established near the resistances, away from the dataTaker.

Each resistance has a pair of cable wires running from the dataTaker to one end, and the other end is connected to the local common point. A single pair of cable wires returns from the common point back to the logger.

This method is particularly useful when a number of resistances are to be measured at a point that is some distance away from the dataTaker, and almost halves the amount of cabling required.

Resistances are connected to the analog channels for 4 wire resistance measurement referenced to an external common as follows

 

 

Figure 56 ñ Four Wire Resistance Measurement With an External Common

 

During resistance measurement, the excitation current from each Excite terminal passes through the unknown resistance and returns into the GND terminal via the common return wire.

The voltage produced across the unknown resistances is measured between the +ve input terminal and the common SE REF input terminal.

Up to 5 resistances can be measured by the 4 wire method with an external common by the dataTaker 50, and up to 10 resistances can be measured by the dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD) .

Resistances connected to the analog channels using the four wire method with an external common are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the resistances connected to analog input channels 1+, 2+ and 3+ using an external common point.

The R indicates that resistance is to be measured. The resistance data is returned in units of Ohms.

The 4W channel option indicates that the resistances are measured by the four wire resistance measurement method for cable compensation.

The X channel option indicates that the resistances are to be measured with respect to the external common connected to the SE REF terminal.

The II channel option indicates that the 2.500 mA excitation current is to be used for measuring the resistance connected to channel 10. The default 250.0 µA excitation current is used for measuring the resistances connected to channel 1.

DeLogger does not directly support this method of resistance measurement. However the User channel type (DeLogger Version 4.2.15 or later) can be used, and the channel specifications shown above for DeTransfer can be entered.

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

Three Wire Resistance Measurement

Most resistance measurements are made using the 3 wire resistance measurement method, which also compensates for cable wire resistances in the measurement circuit provided that the two current carrying leads are of the same resistance.

In this method one cable wire is connected to one end of the unknown resistance, and two cable wires are connected to the other end.

The two cable wires connected to the lower end of the unknown resistance are used to  measure the resistance of the cable wire, which is then used to compensate the overall reading of resistance.

Resistances are connected to the analog input channels for 3 wire method of resistance measurement as follows

 

 

Figure 57 ñ Three Wire Resistance Measurement

 

During resistance measurement, the excitation current from the Excite terminal passes via the link and upper cable wire, then through the unknown resistance and returns into the Analog Return terminal (shown by the red path).

The ñve terminal has a much higher input impedance than the Analog Return terminal, and so the excitation current returns via the cable wire connected to Analog Return. There is no appreciable current flowing in the cable wire connected to the ñve terminal.

The voltage is measured between the +ve and ñve terminals, which combines both the voltage produced by the unknown resistance and voltage produced by the cable wire resistance.

The cable wire connecting to the ñve terminal is used to accurately measure the voltage due to the resistance of the return cable wire. The dataTaker then uses this voltage to correct the overall voltage measured, such that the result is the voltage produced by the unknown resistance. This is then computed to resistance for the known excitation current.

This technique for correcting for cable wire resistance assumes that the three cable wires are of equal resistance. This in turn implies that cables used for connecting resistance inputs should be of equal length and gauge of wire.

Up to 5 resistances can be measured using the 3 wire method by the dataTaker 50, and up to 10 resistances can be measured by the dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD) .

Resistances connected using the three wire method are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the resistances connected to analog input channels 3 and 5.

The R indicates that the signal applied to these channels is to be measured as a resistance. The resistance data is returned in units of Ohms.

The II channel option indicates that the 2.500 mA excitation current is to be used for measuring the resistances connected to channel 3. The default 250.0 µA excitation current is to be used for measuring the resistance connected to channel 5.

Using DeLogger, resistance can be measured using the 3 wire method by the following Program Builder program. The 3 wire connections are selected from the Resistance 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.

Two Wire Resistance Measurement

The 2 wire resistance measurement method is the simplest method for measuring resistance with the dataTaker.

Only a single pair cable is required to connect the unknown resistance to the logger, and carries both the excitation current and the signal. Links are installed between the Excite and +ve terminals, and between the Analog Return and ñve terminal, to provide excitation current and return.

This method can be used wherever an existing sensor with just two cable wires must be connected to the dataTaker. However this method has the disadvantage of including the cable wire resistances in the measured resistance.

During resistance measurement, the excitation current output from the Excite terminal passes through the unknown resistance R and cable wires, and returns into the Analog Return terminal.

The voltage produced across the unknown resistance and the cable wire resistances is measured between the +ve and ñve terminals.

The connection of resistances to the analog input channels for 2 wire resistance measurement is illustrated in Figure 58a below

 

 

Figure 58 ñ Two Wire Resistance Measurement

 

Cable wire compensation is performed during measurement of the resistance, but is of little significance since the resistance of the links is very much less than that of the cable wires.

Since the resistance of the cable wires connecting the unknown resistance to the logger are included in the measurement, the two wire resistance measurement method is best used where the unknown resistance is large relative to the resistance of the cable wires.

However cable wire resistance compensation can be approximated by replacing the link between the ñve terminal and Analog Return with a resistor of equal value to that of the cable resistance, as shown in Figure 58b above.

Alternatively the error produced by the constant cable resistance could be corrected using a Polynomial.

Up to 5 resistances can be measured using the 2 wire method by the dataTaker 50, and up to 10 resistances can be measured by the dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD) .

Resistances connected using the 2 wire method are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the resistances connected to analog input channels 1 through 5 inclusive, and analog input channel 10.

The excitation current option is not specified, and so the default 250.0 µA excitation current is used.

The R indicates that the signal applied to these channels is to be measured as a resistance. The data is returned in units of Ohms.

Using DeLogger, resistance can be measured using the 2 wire method by the following Program Builder program.

The 2 wire connections are selected from the Resistance Wiring Configurations dialog which opens when you select the analog input channel.

 

 

The dataTaker will read the inputs every 5 minutes, and readings are stopped by entering a H (Halt) command.

Measuring Resistance Using Single Ended Measurement

Two resistances can be measured using a combination of 4 wire input and a single ended input for each of the analog input channels.

Note : This method does not provide cable wire compensation for the resistance connected as a single ended input.

In this method, pairs of unknown resistances are connected to each analog input channel.

Up to 10 resistances can be measured using this method by the dataTaker 50, and up to 20 resistances can be measured by the dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD) .

Resistances are connected to the analog input channels as single ended inputs as follows

 

 

Figure 59 ñ Connecting Resistances for Single Ended Inputs

 

During resistance measurement, the excitation current from the Excite terminal passes through the unknown resistances and returns into the Analog Return terminal (shown by the red path).

The voltage produced across the two unknown resistances as a result of this current is measured between the +ve terminal and ñve terminal, and between the ñve terminal and Analog Return.

Resistances connected to an analog input channel as single ended inputs are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

instructs the dataTaker to measure the two resistances as follows

the resistance connected between the +ve and the ñve terminal of analog input channel 1 (R1 in Figure 59) is measured by the 4 wire method

the resistance connected between the –ve terminal of analog input channel 1 and Analog Return 1 (R2 in Figure 59) is measured uncompensated

The R indicates that the signal applied to these channels is to be measured as a resistance. The data is returned in units of Ohms.

The 4W channel option indicates that the resistance is measured by the four wire resistance measurement procedure.

The excitation current channel option is not specified, and so the default 250.0 µA excitation current is used.

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

DeLogger does not directly support this method of resistance measurement. However the User channel type (DeLogger Version 4.2.15 or later) can be used, and the channel specifications shown above for DeTransfer can be entered.

Measurement Ranges and Accuracy

The dataTaker has three resistance measurement ranges for each of the 250.0 µA and 2.500 mA Excite terminal excitation currents.

The resistance measuring ranges for of the each excitation currents are selected automatically by the dataTaker.

The following table summarizes the measurement ranges, resolution and accuracy for resistance measurement for each excitation current.

 

Resistance Range Ohms	Resolution	Measurement Accuracy
250 µA Excitation
100 500 10000	5 mOhm 50 mOhm 500 mOhm	±0.1% ±0.2% ±0.2%
2.5 mA Excitation
10 100 500	0.5 mOhm 5 mOhm 50 mOhm	±0.2% ±0.1% ±0.2%

 

Error Messages

Resistances which exceed the maximum value of 10000 Ohm produce an over-range reading of 99999.9 Ohm.

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