The dataTaker data loggers support several types of the Yellow Springs Instrument Inc. range of thermistors.

The thermistor is a thermo-resistive device, which changes resistance with changes in temperature. The thermistors have a negative temperature coefficient, such that the resistance of the device decreases as temperature increases.

The dataTaker accurately measures the changes in resistance of the element, and linearizes the measured resistance to units of temperature.

Thermistor Support by the dataTaker

The dataTaker supports a number thermistors manufactured by Yellow Springs Instrument Inc. The YSI thermistors supported are essentially two wire types, which have a nominal resistance of 10 KOhm or less at 25 Deg C.

The YSI Thermilinear range of thermistors is not supported by the dataTaker, because of increased wiring complexity and the requirement for a precision resistor network. However because of their linear response, these thermistors can be supported by the polynomials or span limits (See Section III ñ Scaling Data, Polynomials, Spans and Functions).

There is no direct support provided for thermistors from other manufacturers. However other thermistors can be used with the dataTaker if the appropriate polynomials are available, and applied to the thermistor readings in the normal manner (See Section III ñ Scaling Data, Polynomials, Spans and Functions).

The dataTaker measures YSI thermistors directly as resistive devices. The resistance of the thermistor element is measured by any of the 2 wire, 3 wire or 4 wire resistance measurement methods (See Section II ñ Measuring Resistance).

The lower resistance types of thermistors, and those being used near the upper end of their respective temperature range (small resistances), will generally benefit from the 4 wire and 3 wire methods of resistance measurement. This is particularly true if long cables are being used.

However because thermistors have a relatively high resistance and sensitivity, satisfactory measurements can also be made using the 2 wire method of resistance measurement. This is because the resistance of the thermistor is generally very much greater than the resistance of the cables, particularly if shorter cables are used.

The resistance of the thermistor element is measured, and then linearized to units of temperature using internal linearization functions.

Minimum Temperature Measured

Thermistors have a negative temperature coefficient, in that the resistance of the device decreases as temperature increases. The resistance of thermistors is highest at lower temperatures.

Therefore the minimum temperatures which can be measured using thermistors with the dataTaker data loggers is limited by the maximum resistance that the logger can measure. The dataTaker can measure resistance to approximately 10000 Ohm (See Section II ñ Measuring Resistance).

This maximum resistance that can be measured translates to a different minimum temperature for the different types of YSI thermistors supported. The minimum temperatures which can be measured using the dataTaker for the different types of YSI thermistors supported are summarized in the table overleaf.

In contrast the maximum temperatures that can be measured using the YSI thermistors with the dataTaker are limited only by the manufacturers specification.

Extending the Minimum Temperature

The YSI thermistors which exceed the resistance measurement limit of approximately 10000 Ohm for the dataTaker, can however be supported by placing a 'linearizing' resistor in parallel with the thermistor. This fixed resistor reduces the total resistance from the thermistor, and allows the dataTaker to measure lower temperatures.

The connection of this optional parallel resistor is illustrated below

 

 

Figure 96 ñ Connection of Optional Parallel Resistor Rp

 

This fixed resistor can be located at the sensor, or be installed across the analog input channel screw terminals of the dataTaker.

The optimal value for the linearizing resistor is approximately equal to the resistance of the thermistor in the middle of the expected temperature range to be measured. However any value is allowable.

If the resistor value is too high, it will have little effect. If the resistor value is too low, it will reduce the sensitivity of the thermistor at lower temperatures.

When a linearizing resistor is used to extend the minimum temperature measurement capability for thermistors, then the value of the resistor is declared as a scaling factor to the analog signal type.

Using DeTransfer, the command for example

specifies that a YSI type 5 thermistor (see table below) is connected to analog input channel 2, which has a 3300 Ohm linearizing resistor Rp connected in parallel.

Using DeLogger, resistance of the linearizing resistor is specified as the Scale Factor in the Resistance Wiring Configurations dialog, which opens after you select the YS temperature sensor type

 

 

YSI Thermistor Types

The YSI thermistor types supported by the dataTaker are listed in the table on the next page. The YSI thermistor types have been grouped into nine different analog channel types according to thermistor sensitivity, and the minimum measurable temperature.

When any of these YSI thermistor types are used, they are specified to the dataTaker by the analog channel type identifier. For example the YSI thermistor types 44003A, 44035 and 44101A are all identified to the dataTaker as analog input type YS03.

The table also lists

the minimum temperature that can be measured for each thermistor type with
the dataTaker when no linearizing resistor is used

the maximum temperature that can be measured with the dataTaker

The dataTaker measures YSI thermistors by any of the 4 wire, 3 wire or 2 wire methods of resistance measurement. The method of resistance measurement recommended for each of the YSI thermistor types are also listed in the table.

Excitation Current

The dataTaker outputs a precise constant current from the Excite terminal of the analog channel during measurement of a resistance. This excitation or measurement current is pulsed on for 30 mS during the measurement of thermistors.

The thermistors are connected between the Excite terminal of the analog input channels and the Analog Return terminals, such that this excitation current passes through the sensor element.

The voltage produced across the thermistor element by the excitation current is then connected by various methods to the analog input channels. The measured voltage is used to calculate the value of the resistance of the thermistor element.

The dataTaker can output a 250.0 µA or a 2.500 mA excitation current from the Excite terminal (See Section II ñ Measuring Resistance). The 250.0 µA excitation current is used by default for thermistors, although the 2.500 mA excitation current can be used.

While the temperature resolution obtained for the two currents is generally the same, the lower current is preferred because of potential errors due to self heating at the higher current.

Using DeTransfer, the channel specification for example

specifies that the thermistor is to be measured using a 2.500 mA excitation current from the Excite terminal, as defined by the II channel option 

 

Using DeLogger, the excitation current can be specified in the Channel Properties dialog which opens when you select Channel Options:ExcitationÖ as shown above

The 250.0 µA excitation current is selected by default whenever a thermistor input type is specified, and does not need to be specifically selected.

Yellow Springs Inc Thermistors

 

Yellow Springs Inc Thermistors
dataTaker Channel Type	YSI Sensor Catalogue Number		Minimum Temp (no Rp) Deg C	Maximum Temp Deg C	Preferred Measuring Method
YS01	44001A		ñ65	100	3, 4 wire
YS02	44002A	44102A	ñ45	100	3, 4 wire
YS03	44003A 44035	44101A	ñ20 ñ20	100	3, 4 wire
YS04	44004 44033 45004 46033 44901 44902	44104 46005 46040	1 1 1 1 1 1	150 75 200 200 90 70	2, 3, 4 wire 2, 3, 4 wire 3, 4 wire 3, 4 wire 2, 3, 4 wire 2, 3, 4 wire
YS05	44005 44030 45005 46030 44903 44904	44105 46005 46040	7 7 7 7 7 7	150 75 200 200 90 70	2, 3, 4 wire 2, 3, 4 wire 3, 4 wire 3, 4 wire 2, 3, 4 wire 2, 3, 4 wire
YS07	44007 44034 45007 46034 44905 44906	44107 46007 46044	18 18 18 18 18 18	150 75 200 200 90 70	2, 3, 4 wire 2, 3, 4 wire 3, 4 wire 3, 4 wire 2, 3, 4 wire 2, 3, 4 wire
YS17	44017 45017 46017 46037	46047	22 22 22 22	150 250 200 200	2, 3, 4 wire 3, 4 wire 3, 4 wire 3, 4 wire
YS16	44016 44036 46036		34 34 34	150 75 200	3, 4 wire 2, 3, 4 wire 3, 4 wire
YS06	44006 44031 45006 46006 46031 44907 44908	44106 46041	35 35 35 35 35 35 35	150 75 250 200 200 90 70	3, 4 wire 2, 3, 4 wire 3, 4 wire 3, 4 wire 3, 4 wire 2, 3, 4 wire 2, 3, 4 wire

 

Connection and Measurement of Thermistors 

YSI thermistors can be connected to the analog input channels by three similar methods, which provide for varying degrees of compensation for cable wire resistance.

The three connection methods are the 4 wire, 3 wire and 2 wire methods of resistance measurement, and are described in the following sections.

These thermistor measurement methods use all four terminals of the analog input channels. Therefore a total of 5 thermistors can be connected to the dataTaker 50, and a total of 10 thermistors can be connected to the dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD).

Four Wire Thermistor Measurement 

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

This method has two cable wires are connected to each end of the thermistor. One pair of cable wires carries the excitation current, and the other pair of cable wires is used to measure the voltage across the unknown resistance.

This method of thermistor measurement should be used where cables are necessarily long or of unequal resistance.

During resistance measurement of the thermistor, the excitation current output from the Excite terminal of the analog channel passes through the thermistor and cable wires, and returns into the Analog Return terminal.

The voltage produced across the thermistor as a result of the excitation current is measured differentially between the +ve and ñve terminals.

Thermistor are connected to the analog input channels for 4 wire resistance measurement as follows

 

 

Figure 97 ñ Connection for Four Wire Thermistor Measurement

 

The excitation current is flowing in the excitation circuit (shown above by the red path), which is totally separate from the measurement circuit. Because of the high input impedance of the analog to digital converter, there is negligible current flow in the measurement circuit. Therefore there is a negligible cable wire resistance component in the voltage measured across the thermistor.

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

Thermistors connected to the analog channels using the 4 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 thermistor connected to each analog input channel as follows

a YSI type 44001A thermistor connected to analog channel 1

a YSI type 44035 thermistor connected to analog channel 2. This thermistor
has a 2200 Ohm resistor connected in parallel across it

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

The YS01 and YS03 specifies that the signals applied to these channels are from YSI thermistors. The 4W channel option indicates that the 4 wire method of resistance measurement is to be used.

Using DeLogger, thermistors connected by the 4 wire method can be measured by the following Program Builder program.

The 4 wire connections are selected from the Resistance Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

The measured resistance is linearized, and data is returned in units of temperature.

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

Three Wire Thermistor Measurement

Most thermistor measurements are made using the 3 wire resistance measurement method, which compensates for cable wire resistances in the measurement circuit (assuming that the two current carrying wires are of the same resistance).

In this method one cable wire is connected to the upper end of the thermistor, and two cable wires are connected to the lower end. The third cable wire is used to compensate for cable wire resistance.

Thermistors are connected to the analog input channels for 3 wire measurement as follows

 

 

Figure 98 ñ Connection for Three Wire Thermistor Measurement

 

During resistance measurement, the excitation current output from the Excite terminal of the analog channel passes through the thermistor and cable wires, and returns into the Analog Return terminal (shown above by the red path).

The voltage produced across the thermistor and the cable wire resistance as a result of the excitation current is measured between the +ve and ñve terminals.

Since all cable wires have resistance, the voltage produced across these is included in the total voltage drop measured. This introduces resistance offset errors, especially when long cable wires are used.

There is a much higher impedance for the –ve input terminal than for the Analog Return terminal, causing the excitation current to return via the cable wire connected to Analog Return. As a result there is no appreciable current flowing the third cable wire connected to the ñve terminal.

The third cable wire is used to accurately measure the voltage drop due to the resistance of the return cable wire in the resistance measurement circuit. The dataTaker doubles this voltage and subtracts it from the voltage measured across the thermistor and the cable wires, before calculating the temperature.

This technique for correcting for cable wire resistance assumes that the two current carrying cable wires are of equal resistance. This in turn implies that cables used for connecting thermistors to the dataTaker should be of equal length, gauge and type of wire.

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

Thermistors connected using the 3 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 two YSI thermistors connected to analog channels 3 and 4.

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

The YS06 specifies that the signal applied to these channels is from YSI thermistors.

Using DeLogger, Thermistors connected by the 3 wire method can be measured by the following Program Builder program.

The 3 wire connections are selected from the Resistance Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

The measured resistance is linearized and data is returned in units of temperature.

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

Two Wire Thermistor Measurement

The 2 wire thermistor measurement method is the simplest method for measuring thermistors with the dataTaker. The 2 wire method only requires a single pair cable to connect the thermistor to the logger. This cable pair carries both the excitation current and the signal current.

However this method also has the disadvantage of including the effects of cable wire resistances in the measured temperature.

Thermistors are connected to the analog input channels for 2 wire measurement by installing links between Excite and +ve terminals, and between Analog Return and ñve terminal. The two cable wires connect the thermistor to the +ve and ñve terminals.

During measurement, the excitation current passes from the Excite terminal through the thermistor and cable wires, and returns into the Analog Return terminal. The voltage produced across the thermistor and the cable wire resistances is measured between the +ve and ñve terminals.

This configuration is only recommended for measuring thermistors with resistances greater than 500 Ohm.

Thermistors are connected to analog input channels for 2 wire measurement as follows

 

 

Figure 99a ñ Connection for Two Wire Thermistor Measurement

 

Figure 99b ñ Connection for Two Wire Thermistor Measurement

 

Cable wire compensation is performed during measurement of the thermistor, 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 thermistor to the analog input channel are included in the measurement, the two wire resistance measurement method is best used where the thermistor resistance is large relative to the resistance of the cable wires.

Cable wire compensation is performed during measurement of the RTD, but is of little significance since the resistance of the links is very much less than that of the cable wires (Figure 99a).

Where cables with only two cable wires are already installed and must be used, cable wire resistance compensation can be obtained by replacing the link between the ñve terminal and Analog Return with a resistor (Figure 99b) of value equal to that of the total cable resistance.

However using a compensation resistor does not compensate for temperature effects on the cables.

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

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

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

Using DeTransfer, the command for example

instructs the dataTaker to measure the three YSI thermistors connected to analog channels 1 and 2.

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

The YS03 specifies that the signal applied to these channels is from YSI thermistors

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

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

 

 

The measured resistance is linearized, and data is returned in units of temperature.

The dataTaker reads the sensors every 10 minutes, and readings are stopped by a H (Halt) command.

Measurement Ranges and Accuracy

The measurement accuracy for thermistors is the same as that for standard resistance measurement, and is discussed in Section II ñ Resistance Measurement.

Greater accuracy of thermistor measurement is achieved when 3 or 4 wire methods are used, and the 250.0 µA thermistor excitation current is used.

Sources of Error

There are three sources of error in thermistor measurements by the dataTaker.

The first is due to errors in resistance measurement (excluding cable wire effects), which are ±0.1% over the logger temperature range of 10 to 40 Deg C. This systematic error can however be calibrated out.

The second source of error is due to self heating of thermistors during measurement. The 250.0 µA measurement current will heat the thermistor bead. The worst case power that can be dissipated in the thermistor is approximately 450µW (a 7 KOhm thermistor without a parallel resistor). In open air this can result in a 0.45 Deg C error if the thermistor is continually excited. In practice a thermistor is normally excited for 30 mS each reading, resulting in an error of less than 0.007 Deg C in still air.

The third source of error is linearization error. The dataTaker uses the Steinhart and Hart equation recommended by YSI, with coefficients as derived from resistances measured at 0, 50 and 100 Deg C. Over the ñ20 to +120 Deg C range the deviation is quoted as +0.83, ñ0.067 Deg C. The higher value only occurs at the lower extreme.

Error Messages

Thermistor resistances which fall outside of the linearization range of ñ80 to 275 Deg C are returned as -99999.9 Deg C or +99999.9 Deg C.

When this occurs, the dataTaker reports the error condition by returning the error message ëE16 -linearization errorí if the Messages Switch /M is enabled