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Measuring Temperature with Thermocouples

Thermocouples are the most widely used temperature sensors, because of their simplicity, ruggedness, low cost, wide range and interchangability.

The dataTaker supports thermocouple types B, C, D, E, G, J, K, N, R, S and T, and provides reference junction compensation and linearization over the temperature range for each of the thermocouple types. Any mix of thermocouples can be used in the same project.

The thermocouple linearizations support the ITS-90 thermocouple standard.

Temperature data from thermocouple inputs is returned in the selected temperature units, with a resolution of 0.1 Deg C and an accuracy of better than 0.5 Deg C.

The accuracy of temperature measurement with thermocouples is a function of the accuracy of the thermocouple, and not of the dataTaker.

Thermocouples are available in a wide range of grades, varying from ±0.1 Deg C accuracy to ±5 Deg C accuracy. Thermocouple wire type should be chosen carefully, rather than simply using the cheapest thermocouple wire available which is often only extension grade wire of ±2 Deg C accuracy.

The decreasing price of RTDs is challenging thermocouples as an economical and accurate temperature measuring device. If temperature measurement is performed in the biological or environmental range (-10 to +100 Deg C), then RTDs should preferably be used for greatest accuracy.

Thermocouples are still the cheapest method for measuring high temperatures, and accuracy of even ±5 Deg C is often not a problem when measuring temperatures greater than several hundred Deg C.

Basic Concepts of Thermocouples

A practical thermocouple consists of two wires of dissimilar metals that are electrically joined at one end (the measurement junction), and thermally joined at the other end (the reference junction).

A low DC voltage is produced by the thermocouple when the two junctions are at different temperatures. This voltage is referred to as the thermoelectric voltage.

This thermoelectric voltage is produced by the temperature gradient along the thermocouple wires. The thermoelectric voltage is not produced by the junctions themselves.

It is important that the purity of the two thermocouple wires is rigidly maintained, particularly at points where there are significant temperature gradients.

A typical thermocouple circuit, in this case a type J thermocouple, is illustrated below

 

 

Figure 71 ñ A Typical Thermocouple Type J Circuit

 

Thermocouple Alloys and Combinations

The alloy combination, polarity and measurement range for the thermocouple types supported by the dataTaker is listed in the table below

 

Type

Alloy of

+ve wire

Alloy of

-ve wire

Temperature Range

B

Platinum 30% Rhodium

(70% Pt - 30% Rh)

Platinum 6% Rhodium

(94% Pt - 6% Rh)

0 to 1700 Deg C

32 to 3100 Deg F

C

Tungsten 5% Rhenium

(74% W - 26% Re)

Tungsten 26% Rhenium

(95% W - 5% Re)

0 to 2320 Deg C

32 to 4200 Deg F

D

Tungsten 3% Rhenium

(97% W - 3% Re)

Tungsten 25% Rhenium

(75% W - 25% Re)

0 to 2320 Deg C

32 to 4200 Deg F

E

Chromel

(55% Cu - 45% Ni)

Constantan

(90% Ni - 10% Cr)

-200 to 900 Deg C

-330 to 2280 Deg F

G

Tungsten

(100% W)

Tungsten 26% Rhenium

(74% W - 26% Re)

0 to 2320 Deg C

32 to 4200 Deg F

J

Iron

(100% Fe)

Constantan

(55% Cu - 45% Ni)

0 to 750 Deg C

32 to 1380 Deg F

K

Chromel

(90% Ni - 10% Cr)

               Alumel

(96% Ni - 2% Mn - 2% Al)

500 to 1250 Deg C

-330 to 2280 Deg F

N

Nicrosil

(Ni-Cr-Si)

Nisil

(Ni-Si-Mg)

-270 to 1350 Deg C

-450 to 2370 Deg F

R

Platinum 13% Rhodium

(87% Pt - 13% Rh)

Platinum

(100% Pt)

0 to 1450 Deg C

32 to 2640 Deg F

S

Platinum 10% Rhodium

(90% Pt - 10% Rh)

Platinum

(100% Pt)

0 to 1450 Deg C

32 to 2640 Deg F

T

Copper

(100% Cu)

Constantan

(55% Cu - 45% Ni)

-500 to 350 Deg C

-330 to 660 Deg F

 

Thermocouple Wire Colour

There are a number of standards for colour coding thermocouple wires. Each standard specifies a colour for each wire of each thermocouple type, and a colour for the outer wrap for each thermocouple type. Consult the standards appropriate to your source of thermocouple wire to identify the positive and negative poles of the wire.

Thermocouple Reference Junction

In practice it is often difficult to identify a single point in the thermocouple circuit as the reference junction. The reference junction is often two junctions as illustrated in Figure 72.

In Figure 72 there are three metals in the thermocouple circuit ó iron, constantan and copper. However for practical purposes the IronñCopper and ConstantanñCopper junctions can in fact be considered collectively as the reference junction, provided these are held at the same temperature.

Thermocouple Reference Junction Temperature

The reference junctions of thermocouples are traditionally maintained at 0°C. This is assumed in thermocouple calibration tables.

While this requirement can be provided by maintaining the reference junctions in melting ice baths or electronically cooled enclosures, this is impractical for portable or remote applications.

An alternative and more practical approach is to maintain the thermocouple reference junctions at ambient temperature, and to allow to drift with ambient temperature.

In this approach, the thermocouple reference junctions are all maintained at an equal temperature by placing them in close thermal proximity to a heat conductor, and ideally enclosed in thermal insulation. The heat conductor is generally referred to as an isothermal block, and is usually a block of aluminium, copper or similar material with a high thermal conductivity.

This approach requires that the temperature of the thermocouple reference junctions (actually the difference between the temperature of the reference junctions and 0°C) is known. This temperature is then used to correct the temperature measured by the thermocouple.

This correction overcomes the errors produced by a non-zero thermocouple reference junction temperature, referred to as reference junction temperature compensation.

The dataTaker has a metal chassis around the analog input channel terminals, and optionally a substantial steel case. These contribute towards creating an isothermal environment around the junctions of directly connected thermocouples. However if accurate and stable thermocouple measurements are required, then an external isothermal block should be used.

Thermocouple Zero Voltage

Another potential source of error in measurement of thermocouple inputs are the various thermoelectric voltages produced by the effects of temperature gradients on the mixture of metals in screw terminals, connectors, cables, printed circuit board tracks, etc. in the thermocouple circuit.

Thermoelectric voltages can be produced between the thermocouple reference junctions and the inner circuits of the measuring instrument.

These thermoelectric voltages electrically add to the signal voltage of the thermocouple, producing errors in the measured temperature. Correct thermocouple measurement requires that these voltages be measured and then used to correct the signal voltage read from the thermocouple measurement junction. This correction is referred to as zero voltage compensation.

The thermocouple zero voltage is usually measured from a thermocouple mounted in thermal contact with the thermocouple reference junctions.

Voltage to Temperature Conversion

The relationship between temperature and measured voltage for a thermocouple is not linear. The temperature-voltage relationship of all thermocouples has been accurately measured and published. These are used to calculate the thermocouple temperature from a measured voltage by a technique called thermocouple linearization.

Thermocouple Support by the dataTaker

The dataTaker supports the thermocouple types B, C, D, E, G, J, K, N, R, S and T. When a thermocouple type is measured by the dataTaker, the dataTaker automatically performs the reference junction temperature compensation, zero voltage compensation and linearization calculations.

Readings are returned directly in degrees Celsius, Fahrenheit, Kelvin or Rankine with a resolution of 0.1 Deg C and accuracy of better than 0.5 Deg C, depending on the thermocouple.

Reference Junction Temperature Compensation

The thermocouple support by the dataTaker provides for the thermocouple reference junction temperature to be measured with any ëabsoluteí temperature sensor, such as a AD590, AD592, LM335, LM35, LM34, RTD or thermistor temperature sensor.

The reference junction temperature is used to compensate the thermocouple signal voltage measurements.

The reference junction temperature sensor is normally mounted in thermal contact with the thermocouple reference junction terminals, and is connected to any analog input channel.

Reference junction temperature compensation is accurate to within 0.1 Deg C over the temperature range of ñ20 to +65 Deg C. Errors due to the thermocouple are in addition to this error.

If a reference junction temperature sensor is not implemented, then the dataTaker will use its internal temperature as the default reference junction temperature. The internal temperature is measured by an LM35 sensor, located within the body of the dataTaker.

The dataTaker also supports thermocouple interfaces which have reference junction compensation, and output a voltage referenced to 0 Deg C. In this case reference junction compensation by the dataTaker is not required, and only zero voltage compensation and linearization is performed.

Zero Voltage Compensation

The thermocouple support implemented by the dataTaker also provides for the thermocouple zero voltage to be measured.

The zero voltage is usually measured with a thermocouple mounted in thermal contact with the thermocouple terminals, and connected to any analog input channel.

If a zero voltage thermocouple is not implemented, then the dataTaker uses the analog to digital converter calibration zero voltage channel as the default thermocouple zero voltage.

The zero voltage is used to correct or compensate the thermocouple signal voltage measurements for any stray thermoelectric voltages in the thermocouple circuit, between the reference junctions and the analog to digital converter.

Linearization of Thermocouple Inputs

Linearization of the measured thermocouple voltage is carried out automatically by the dataTaker, by applying polynomial functions for the particular thermocouple type.

The coefficients for these polynomials are stored permanently in the dataTaker ROM. The thermocouple linearization errors are less than 0.1 Deg C over the temperature measuring range for each thermocouple type.

The thermocouple linearizations support the ITS-90 thermocouple standard.

Calculation of Thermocouple Temperature

The temperatures sensed by thermocouples connected to the analog input channels of the dataTaker are calculated by the following procedure

the measured reference junction temperature is converted to a voltage equivalent for the thermocouple type being used, and subtracted from the measured thermocouple signal

the measured thermocouple zero voltage is subtracted from this reference junction temperature compensated thermocouple signal

the compensated thermocouple signal is linearized by applying the appropriate polynomials to calculate the junction temperature of the thermocouple.

Directly Connecting Thermocouples to the dataTaker

The simplest method for monitoring the signal from thermocouples with the dataTaker is to connect the thermocouple wires directly to the analog input channel screw terminals.

In this configuration, a number of defaults are implemented as follows

the screw terminals of the analog input channels become the isothermal reference junction

the case temperature of the dataTaker and the Channel Expansion Module (CEM-AD) is used as the reference junction temperature. The case temperature is measured by an LM35 temperature sensor located amongst the analog input channel terminals. The accuracy of this sensor is approximately 0.5 Deg C, and may be a limitation to thermocouple measurement accuracy for directly connected thermocouples

the thermocouple zero voltage reference is measured by internal analog channel 2%V of the dataTaker, which has its input shorted

This method of thermocouple connection to the dataTaker can be subject to small errors due to temperature gradients around the analog input board, and between the analog input channel screw terminals.

Temperature gradients can occur within the dataTaker or the Channel Expansion Module (CEM-AD) case as a result of exposing the cases to uneven temperatures.

Thermocouple wires can be connected directly to the screw terminals of the dataTaker and the Channel Expansion Module (CEM-AD) analog input channels, either as differential or as single ended inputs.

Installing Thermocouples

When installing thermocouples, you must ensure that the thermocouple tips are not electrically connected to the system or structure being temperature monitored. If the thermocouple tip electrically contacts the system or structure, small noise currents can enter the thermocouple and produce significant measurement errors.

If you need to attach the thermocouple tip to the structure or system, then place an electrical insulation material between the thermocouple tip and the structure.

There are various materials available for this purpose, which have good electrical insulation properties, and goo thermal conduction properties. Consult your thermocouple supplier for details and materials.

Measuring Thermocouples as Differential Inputs

Thermocouples can be connected directly to the analog input channels of the dataTaker as differential inputs as follows

 

 

Figure 72 ñ Connection of Thermocouples as Differential Inputs

 

The thermocouple measurement junctions must be isolated from ground.

The polarity of the leads for the thermocouple types supported by the dataTaker are listed in the Thermocouple Alloys table at the beginning of this chapter.

The differential mode is the preferred method for measuring thermocouples directly connected to the dataTaker.

Thermocouples which are directly connected to the analog channels as differential inputs are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

BEGIN
 RA5M
  5TT  10TK
END

instructs the dataTaker to measure thermocouple temperatures as follows

for type T thermocouples connected between +ve and ñve of analog channel 5

for a type K thermocouple connected between +ve and ñve of analog channel 10

the junction temperature and zero voltage for thermocouple compensation is measured from the internal channels

The TT specifies type T thermocouples, and TK specifies type K thermocouples.

Using DeLogger, differential thermocouple signals can be measured by the following Program Builder program. The differential thermocouple connections are selected from the Thermocouple Wiring Configurations dialog which opens when you have selected the analog input channel. Also select the type of thermocouple that is being used.

 

 

The temperature data is returned in the units of temperature defined by the Parameter36 command.

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

Measuring Thermocouples as Single Ended Inputs 

Thermocouples can be directly connected to the dataTaker analog input channels for measurement as single ended inputs referenced to Analog Return, or as single ended inputs referenced to an external common which is connected to the SE REF input terminal.

Single ended inputs have the advantage of also being able to detect open circuit thermocouples. If a thermocouple becomes open circuit, the data is returned as 99999.9 Deg C and can easily be detected in Alarms.

Single Ended Inputs Referenced to Analog Return

Thermocouples can also be directly connected to the analog input channels as single ended inputs referenced to Analog Return as follows

 

 

Figure 73 ñ Connection of Thermocouples as Single Ended Inputs
Referenced to Analog Return

 

The thermocouple measurement junctions must be isolated from ground.

The polarity of the leads for the thermocouple types supported by the dataTaker are listed in the Thermocouple Alloys table at the beginning of this chapter.

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

Thermocouples which are directly connected to the analog input channels as single ended inputs referenced to Analog Return are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

BEGIN
 RA5M
  1+TK  5TTK
END

instructs the logger to measure thermocouple temperatures as follows

for type K thermocouples connected between channel 1+ and Analog Return

for a type K thermocouple connected between channel 5T and Analog Return

the junction temperature and zero voltage for thermocouple compensation is measured from the internal channels

The TK specifies type K thermocouples.

Using DeLogger, single ended thermocouple signals referenced to Analog Return can be measured by the following Program Builder program. The single ended thermocouple connections are selected from the Thermocouple Wiring Configurations dialog which opens when you have selected the analog input channel. Also select the type of thermocouple that is being used. The default is type T.

 

 

The temperature data is returned in the units of temperature defined by the Parameter36 command. The dataTaker will read the thermocouples every 5 minutes, and the readings are stopped by entering a H (Halt) command.

Single Ended Inputs Referenced to an External Common

Thermocouples can also be directly connected to the analog input channels as single ended inputs referenced to external common as follows

 

 

Figure 74 ñ Connection of Thermocouples as Single Ended Inputs
Referenced to an External Common

 

The thermocouple measurement junctions must be isolated from ground.

This method of connection is only of practical use for external isothermal blocks.

The polarity of the leads for the thermocouple types supported by the dataTaker are listed in the Thermocouple Alloys table at the beginning of this chapter.

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

Thermocouples which are directly connected to the analog input channels as single ended inputs referenced to an external common are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

BEGIN
 RA6M
  1+TR(X)  1-TR(X)
END

instructs the logger to measure thermocouple temperatures as follows

for type R thermocouples connected between single ended channels 1+ and 1ñreferenced to SE REF

the junction temperature and zero voltage for thermocouple compensation is measured from the internal channels 

The TR specifies type R thermocouples, and TB specifies type B thermocouples. The X channel option indicates that the thermocouple voltage is to be measured with respect to an external common connected to the SE REF input terminal.

Using DeLogger, single ended thermocouple signals referenced to an external common can be measured by the following Program Builder program. The single ended thermocouple connections are selected from the Thermocouple Wiring Configurations dialog which opens when you select the analog input channel.

 

 

The temperature data is returned in the units of temperature defined by the Parameter36 command.

The dataTaker will read the thermocouples every 6 minutes, and the readings are stopped by entering a H (Halt) command.

Using an External Isothermal Block 

An isothermal block is a group of thermocouple terminations which are maintained at the same temperature (isothermal). These terminations are the reference junctions for the connected thermocouples.

The construction of an isothermal block has all of the thermocouple terminations in close thermal contact with a (usually copper) mass of high thermal conductivity.

The temperature of the isothermal block can follow ambient temperature. However the isothermal block should not be exposed to uneven heating (eg. a source of radiant heat) which will heat one side of the block relative to the other.

The temperature of the isothermal block and reference junctions is measured by an RTD or solid state temperature sensor connected to the dataTaker. This sensor must be declared to the dataTaker with the appropriate commands.

Isothermal blocks can be purpose built for particular thermocouple applications, or can be purchased as an optional accessory from your dataTaker supplier.

A correctly constructed and implemented external isothermal block can increase the accuracy of thermocouple measurement to the order of 0.3 Deg C.

Reference Junction Temperature Sensor

The temperature of the isothermal block and thermocouple reference junctions is measured by an ëabsoluteí type temperature sensor. The dataTaker requires that the reference junction temperature is measured with one of AD590, LM335, LM35, LM34, RTD or thermistor temperature sensors. Note that a thermocouple cannot be used as the reference temperature sensor!

The reference junction temperature sensor is specified to the dataTaker by a channel option to the particular sensor channel in the general format

nsensor(TR)

n                       is the analog channel for the reference junction temperature sensor
sensor                 is the sensor identifier for the sensor type used
(TR)                    indicates that the sensor is the reference junction temperature sensor

Temperature sensors that can be used for measuring thermocouple reference junction temperature are listed in the following table

 

Sensor Type

Description

nAD590(TR)

AD590 Kelvin sensor

nLM335(TR)

LM335 Kelvin sensor

nLM34(TR)

LM34 Fahrenheit sensor

nLM35(TR)

LM35 Celsius sensor

nPT385(TR)

Platinum RTD, 0.003850 Ohm/Ohm/Deg C

nPT392(TR)

Platinum RTD, 0.003916 Ohm/Ohm/Deg C

nNI(TR)

Nickel RTD, 0.005001 Ohm/Ohm/Deg C

nCU(TR)

Copper RTD, 0.0039 Ohm/Ohm/Deg C

nYSxx(TR)

YSI thermistor type xx

11SV(TR)

No reference junction temperature compensation

 

Using DeTransfer, the commands for example

10LM35(TR)

specifies that the reference junction temperature sensor is an LM35 device, which is differentially connected to analog input channel 10

1:5PT392(4W,TR)

specifies that the reference junction temperature sensor is a platinum RTD, which is measured as a 4 wire input on analog input channel 5 of the Channel Expansion Module 1

8YS02(TR,4W)

specifies that the reference junction temperature sensor is a YSI thermistor, which is measured as a 4 wire input on analog input channel 8.

Channel options can also be declared in the normal manner to define how the reference junction temperature sensor is to be managed when scanned.

Using DeLogger, firstly create the reference junction temperature sensor channel in the Program Builder, then click Channel Options:ReferenceÖ and select Thermocouple Reference Temperature radio button.

 

 

 

The use of each of the temperature sensor types is discussed in detail elsewhere in this manual.

The reference junction temperature sensor channel does not return data to the host when it is read. The data is saved internally for use in subsequent thermocouple compensation.

The reference junction temperature sensor must be declared in the Schedule schedule lists before the thermocouple channels.

If thermocouples from an external isothermal block are included in more than one Schedule, then the reference junction temperature sensor channel must be declared in each Schedule.

Several isothermal blocks can be simultaneously supported by the dataTaker. However the reference junction temperature sensor declaration for each isothermal block must precede the thermocouple channels associated with each block.

The dataTaker default reference junction temperature sensor is the internal temperature sensor, which is an LM35 device connected to internal channel 1% and is specified as 1%LM35(TR).

The thermocouple reference junction temperature sensor can be connected to any analog input channel as a differential or single ended input.

No Reference Junction Temperature Compensation

A number of thermocouple interfaces are available which internally provide for reference junction compensation and output a corrected thermocouple signal, and so compensation is not required by the dataTaker.

The requirement for no reference junction temperature sensor is declared as 11SV(TR), which is a system zero.

If this is not declared, then the dataTaker will use the internal temperature as the default reference junction temperature.

Zero Voltage Reference

A thermocouple in close thermal contact with the isothermal block terminations is required for measuring the thermocouple zero voltage reference.

The dataTaker uses the zero voltage reference thermocouple to measure and compensate for thermoelectric voltages induced by temperature gradients outside of the thermocouple circuit.

The zero reference voltage thermocouple can be connected as a differential or single ended input to any analog input channel, and is defined by the general format

nV(TZ)

n        is the analog input channel to which the zero voltage reference
         thermocouple is connected.
V       indicates a voltage input
(TZ)    channel option which indicates that this sensor is the zero reference voltage thermocouple

Using DeTransfer, the commands for example

4V(TZ)

specifies that the zero voltage reference thermocouple is connected as a differential input to analog input channel 4.

9+V(TZ)

specifies that the zero voltage reference thermocouple is connected as a single ended input to analog input channel 9+

Using DeLogger, firstly create the reference junction temperature sensor channel in the Program Builder, then click Channel Options:ReferenceÖ and select Thermocouple Reference Zero radio button.

 

The zero voltage reference channel does not return data to the host when it is scanned. The data is saved internally for use for subsequent thermocouple compensation.

The zero reference voltage must be declared in the Schedule schedule lists before the thermocouple channels.

Several isothermal blocks can be simultaneously supported by the dataTaker. However the zero voltage reference channel for each of the isothermal blocks must precede the thermocouple channels associated with each block.

If thermocouples from an external isothermal block are included in more than one Schedule, the zero reference voltage channel must be declared in each Schedule.

The dataTaker default zero voltage reference channel is the internal zero voltage reference, which is connected to internal channel 2% and is specified as 2%V(TZ).

Connecting an Isothermal Block 

A schematic for a full isothermal block implementation is illustrated overleaf.

The reference junction temperature sensor is an AD590 powered by the dataTaker. Refer to Section II ñ Measuring Temperature with IC Temperature Sensors for details of the use of this sensor type.

The zero voltage reference is a thermocouple of the same type as the measurement thermocouples, but which is in thermal contact with the thermal mass, and so is at the same temperature as the thermal mass.

 

Figure 75 ñ A Full Isothermal Block Implementation

 

Reading a Thermocouple Isothermal Block

Using DeTransfer, typical commands for configuring an isothermal block, and reading a group of thermocouples supported by the isothermal block, are as follows

P36=0
BEGIN
 RA10M
  9#AD590(TR)  9V(TZ)  1..5TT
END

P36=0                 temperature date returned in degrees Celsius

9#AD590(TR)        junction temperature sensor is an AD590 powered by the
dataTaker, measured using the internal shunt of channel 9

9V(TZ)                 zero voltage thermocouple is connected differentially to channel 9

1..5TT                 five type T thermocouples terminated on the isothermal block, and connected to analog channels 1 through 5

Using DeLogger, the program is created in the Program Builder.

Firstly create the two reference channels, for the thermocouple temperature reference sensor, and the thermocouple zero voltage reference. These two reference channels must be created above the actual thermocouple channels for the isothermal block.

When the reference channels are in place, then add the measurement thermocouple channels.

The finished program should be similar to the following

 

 

The reference options have been set for the first two channels, however this does not show on the icons in the Program Builder window.

Grounding of Thermocouples

When thermocouple inputs are measured by the dataTaker, the analog input channels are by default internally terminated to the dataTaker ground via 1 MOhm resistors. The thermocouple measurement junctions must not be grounded externally.

When externally grounded thermocouples are measured, the internal termination of inputs can be disabled or Un-terminated by the U channel option.

The thermocouple channel specification for example

1..5TT(U)

specifies the thermocouple inputs are not to be terminated.

Thermocouples connected to the dataTaker as single ended inputs are not terminated, because these thermocouples are grounded via the Analog Common.

Thermocouple Measurement Accuracy

The differential thermocouple measurement errors of the dataTaker are small. The thermocouple errors are summarized below for all thermocouple types supported. These errors include voltage measurement and linearization errors.

The error envelope does not include errors produced by the method of thermocouple connection to the dataTaker, such as implementation and accuracy of the external isothermal reference junctions.

 

 

Figure 76 ñ Thermocouple Measurement Errors

 

If proper care is taken to implement thermocouples with respect to the various electrical, mechanical and thermal considerations discussed in the preceding sections, thermocouple measurement accuracies of 0.3°C can be achieved.

The thermocouple type G has a ±1 Deg C error band, which is greater than for the other thermocouple types supported.

The temperature measurement accuracy of thermocouples is dependent on a number of other factors including

grade of thermocouple wire  ñ several grades of thermocouple wire are available, ranging from high accuracy measurement wire to low accuracy extension wire

method of junction construction ñ many methods of junction construction are available, including twisted and soldered junctions, crimped junctions and welded junctions.

Soldering of junctions is not recommended, because this introduces additional metals to the thermocouple junction and produces measurement errors.

Error Messages

Over-ranging may occur if external offset voltages are present in the thermocouple voltage signal.

Thermocouple signals which fall outside the linearization range are returned as temperature data, which is extrapolated from the linearization functions.

However the dataTaker also reports the error condition with the error message ‘E16ñLinearization errorí 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