1). THERMOCOUPLES

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2).
The advantages of 2, 3 & 4 wire pT100 measurements.
3).
Introduction to Thermocople measurements.
4).
Basic RTD measurements

The thermocouple is frequently used as the sensing element in a thermal sensor or switch. The principle is that two dissimilar metals always have a contact potential between them, and this contact potential changes as the temperature changes.

Fig. 1.1


The contact potential is not measurable for a single connection (or junction), but when two junctions are in a circuit with the junctions at different temperatures then a voltage of a few millivolts can be detected (Fig. 1.1). This voltage will be zero
if the junctions are at the same temperature, and will increase as the temperature of one junction relative to the other is changed until a peak is reached.

Fig. 1.2


Fig. 1.2 A thermocouple characteristic, showing the typical curvature and the transition point at which the characteristic reverses. A few combinations of metals (like copper/silver) have no transition, but have a very low output.

The shape of the typical characteristic is shown in Fig. 1.2, from which you can see that the thermocouple is useful only over a limited range of temperature due to the non-linear shape of the characteristic and the reversal that takes place at temperatures higher than the turn-over point.

The output from a thermocouple is small, of the order of millivolts for a 10°C temperature difference, and Fig. 1.3 shows typical sensitivity and useful range for a variety of the common types. Of these, the copper/constantan type is used mainly for the lower range of temperatures and the platinum! rhodium type for the higher temperatures.

Fig.1.3


Because of the small voltage output, amplification is usually needed unless the thermocouple is used for temperature measurement along with a sensitive millivoitmeter. If the output of the thermocouple is required to drive anything more
than a meter movement, then DC amplification will be needed, using an operational amplifier or chopper amplifier.

The type of amplifier that is used needs to be carefully selected, because good drift stability is necessary unless the device is recalibrated at frequent intervals. This
makes the chopper type of amplifier preferable for most applications.

If an on/off switching action is required, the thermocouple must be used along with a controller that uses a Schmitt trigger type of circuit which also permits adjustment of bias so that the switching temperature can be preset. The usual circuitry includes amplification, because the lower ranges of thermocouple outputs are comparable with the contact potentials (the same type of effect) in amplifier circuits,
and attempting to use very small inputs for switching invariably leads to problems of hysteresis and excessive sensitivity.

One particular advantage of thermocouples is that the sensing elements themselves are very small, allowing thermocouples to be inserted into very small spaces and to respond to rapidly changing temperatures. The electrical nature of the process
means that the circuitry for reading the thermocouple output can be remote from the sensor itself. Note that thermocouple effects will be encountered wherever one metallic conductor meets another, so that temperature differences along circuit
boards can also give rise to voltages which are comparable with the output from thermocouples.

The form of construction of amplifiers for thermocouples is therefore important, and some form of zero-setting is needed.

With acknowledgments to 'Sensors & Transducers',
A Guide for Technicians by Ian R. Sinclair - ISBN 0-632-02069-5
Copyright acceptance to be applied for.




PLATINUM RESISTANCE Pt100 SENSORS

The resistance to the flow of electricity in metallic materials varies with temperature. This can be used to good effect in platinum resistance detectors. Platinum is particularly stable both electrically and mechanically and is also stable with respect to time, producing a relatively linear change in resistance versus temperature.

Because the output resistance change to temperature is relatively small, it follows that lead lengths and resistances are therefore important features. In general when lead lengths are short, or can be considered as an acceptable additive content, two wire configuration is sufficient.

Three wire is the most commonly used and unless otherwise specified is supplied as standard, the third wire is the compensator for lead length and providing that all three wire have equal resistance, compensates for any ZERO or SPAN errors. (Not true for all bridges).

Four wire provides for high precision and is recommended for use with Zener Barriers.



CONNECTION CONFIGURATION


Platinum resistors are most commonly 100 ohms at 0 °C and 138.51 ohms at 100 °C. They are available in different grades according to the accuracy required and can be supplied as duplex sensors, two independent sensors on a single former.

The respective accuracies of the three main specification types, BS.EN 60751 Class A, BS.EN 60751 Class B and 1/10th Class B. Pt100 Platinum Resistance sensors are as shown in the tolerance table below.

 Resistance at 38.51 ohms fundamental interval

 Tolerances for 100 ohms Thermometers

 Temperature
 

 Class A

 Class B

 1/10 Class B

 °C

  °F

 Nominal Value

 ± °C

± ohm

± °C

 ± ohm

± °C

 ± ohm

 -200

 -328
  18.52   0.55   0.24

  1.3

 0.56

  0.13

  0.06

 -100

 -148

 60.26

 0.35

  0.14

  0.8

 0.32

 0.08

 0.03

 0

 32

100.00

 0.15

  0.06

 0.3

 0.12

 0.03

  0.01

 100

  212
  138.51

 0.35

 0.13

 0.8

 0.30

  0.08

 0.03

 200

 392
  175.86

  0.55

 0.20

 1.3

  0.48

  0.13

 0.05

 300

 572
 212.05

  0.75

 0.27

  1.8

 0.64

  0.18

 0.06

 400

 752
 247.09

  0.95

 0.33

  2.3

 0.79

  0.23

  0.08

 500

 932
 280.98

 1.15

  0.38

 2.8

 0.93

 0.28

 0.09

 600
  1112  313.71

 1.35

 0.43

 3.3

  1.06

  0.33

 0.10

 700
  1292  345.28

 -

  -

 3.8

 1.17

  -

  -

 800
  1472  375.70

 -

  -

 4.3

  1.28

 -

  -




TOLERANCE VALUES AS A FUNCTION OF TEMPERATURE FOR 100 ohms THERMOMETERS


LEADS
Pt100 sensors are supplied with 2, 3 or 4 wire connections and, unless otherwise specified, will be supplied as 3 wire type 7 x 0.2 mm Cu PTFE insulated, with two red wires indicating one end of the element and one white wire indicating the other. Alternative types of wire insulation can be supplied.

 Material  Maximum Range  Application
 PVC   -10 to +105 °C  Low cost, moisture resistant, short lengths
 PTFE  -60 to +250 °C  Abrasion resistant, long lengths
 Woven Asbestos  to +700 °C   Fireclay impregnated
 Woven Silica  to +1000 °C  
 Aluminous Porcelain  to +1400 °C  Electrical resistance declines above 900 °C
 Recrystalised Aluminia   to +1950 °C   Electrical resistance declines above 900 °C


Flying lead sensors can be supplied with an optional stainless steel overbraid or convolute sheathing for more arduous environments.




2). The advantages of 2, 3 and 4 wire
Pt100 measurement.

INTRODUCTION

Two, Three and Four wire measurement techniques have been developed for measuring accurately the resistance of resistive temperature detectors (RTD). This application note looks at the new techniques being adopted in ‘Smart’ instruments where, using the intelligence of a microprocessor, the traditional drawbacks of 3 wire systems no longer apply.

THREE WIRE MEASUREMENT
The traditional method of accurately measuring a resistance, is to incorporate the resistance into a Wheatstone bridge circuit (see figure below).



A voltage excites the bridge and the voltage across the bridge is proportional to the resistance of the RTD.

A problem occurs when we introduce lead resistances (See figure below). It is apparent that any resistance in the lead looks as though there is additional resistance in the element to be measured.





To minimise these errors the three wire compensated bridge was introduced (see figure below).



This has the effect of removing the error introduced by the lead resistance as long as lead resistances RL1 and RL3 are matched.

However the effect of the lead resistance can be to cause less current to flow in the detector leg and hence introduce a small but possibly significant span error. This can be eliminated by exciting the bridge from a constant current source rather than a constant voltage and so whatever the lead resistance, the same current always flows through the detector. With this method there are no lead resistance errors introduced as long as the lead resistances are equally matched. In practice they are very closely matched as long as the wire used is part of the same multi-core cable.

The exception to this is when the sensor is used in a Hazardous area and connected to the bridge circuit via a Zener Barrier. Here any mis-match in the resistance of the two legs of the Zener barrier can appear as a sensor error. Although still small, this error can be as much as 0.15 ohm or approx. 0.3°C.(MTL 155 Barrier).

For analogue transmitters Status Instruments Ltd. have traditionally used a variation of this technique using an in house active bridge circuit. The exception being the new ‘Smart’ series of instruments which use a different technique which will be explained later.

Another way of measuring Pt100 elements is to use a 4 wire current and voltage method (see figure below).



Here the detector is excited by a constant current and the voltage across the detector measured by an amplifier with a high impedance input. If the current source is perfect and the input impedance of the voltage measuring circuit is infinite, then there is no error whatsoever introduced by the lead resistances even if they are mismatched.

THE ‘SMART’ WAY FORWARD
This paragraph describes the method used to measure Pt100 on the new DM3000 series instruments.

The current trend for so called SMART instruments is to have a universal input capable of supporting a wide range of inputs. It is inconvenient (and unnecessary) to dedicate input pins and electronics to support a constant current supply and a bridge arrangement. The input circuit measures voltages to a high degree of accuracy and the microprocessor performs the calculation in the figure below.



Rc is used solely to limit the current flowing and Rs is a stable reference resistor.

Having computed the resistance, the microprocessor applies the corrections required and translates the resistance to an accurate temperature reading.

In addition, the microprocessor can determine which if any, of the RTD inputs has become disconnected and detect other errors such as RTD short circuit. This is an improvement over both conventional three and four wire circuits because you can now have a predictable failure mode which does not depend upon which of the three wires has become disconnected.

This technique removes lead resistance effects as long as they are equal. Again, we have the problem when using Zener barriers, in that if the legs of the barrier are not accurately matched, then a small error could be introduced.



With acknowledgement to Status Instruments Ltd.
Tel. +44 1684 296818 Fax +44 1684 293746
E-mail: sales@status.co.uk Web:
www.status.co.uk



3). Introduction To Thermocouple Measurements

Introduction
The most common devices used for sensing temperature include thermocouples, resistance temperature detectors (RTDs), and thermistors. Each has unique characteristics and properties that make one more suitable than another for a certain application.

Thermocouples are the most widely used device for sensing temperature, and probably the least understood. They are simple and efficient, and provide a small voltage signal proportional to the temperature difference between two junctions in a closed thermoelectric circuit. In its most basic configuration, one junction is held at a constant reference temperature while the other is placed in contact with the medium to be measured.

This medium can be gas, liquid, or solid, but in all cases, the medium shall not be allowed to chemically, electrically, or physically contaminate or alter the thermocouple junction. For special applications or to protect them from the environment, thermocouples are available with protective coatings and shields or sheaths. RTDs are composed of metals with a high positive temperature coefficient of resistance. Most RTDs are simply wire-wound or thin-film resistors made of wire with a known resistance vs. temperature relationship. Platinum is one of the most widely used materials for RTDs. They come in a broad range of accuracies, and the most accurate are also used as NIST (National Institute of Standards and Technology) temperature standards.

Thermistors are similar to RTDs in that they also change resistance between their terminals with a change in temperature. However, they can be made with either a positive or negative temperature coefficient. In addition, they have a much higher ratio of resistance change per degree C (several %) than RTDs, which makes them more sensitive.

The Gradient Nature of Thermocouples
Thermocouple junctions alone do not generate voltages. The output or potential difference that develops at the open end is a function of both the closed junction and the open end temperatures. The principle of operation depends on the unique value of thermal emf generated between the open ends of the leads and the junction of two dissimilar metals held at a specific temperature. The principle is called the Seebeck Effect, named after the discoverer. The amount of voltage generated at the open ends of the sensor and the range of temperatures the device can measure depend on the Seebeck coefficient, which in turn depends upon the chemical composition of the materials comprising the thermocouple wire.

In principle, a TC can be made from any two dissimilar metals such as nickel and iron. In practice, however, only a few TC types have become standard because their temperature coefficients are highly repeatable, they are rugged, and they generate relatively large output voltages. The most common thermocouple types are called J, K, T, and E, followed by N28, N14, S, R, and B. The junction temperature could be inferred from the Seebeck voltage by consulting standard tables. However, this voltage cannot be used directly because the thermocouple wire connection to the copper terminal at the measurement device itself constitutes a thermocouple junction (unless the TC lead is also copper) and generates another emf that must be compensated.

Cold Junction Compensation
The classical method used to compensate the emf at the instrument terminals is a thermocouple immersed in an actual ice-water bath which in turn connects in series with the measuring thermocouple. The ice and water combination holds the temperature bath to a constant and accurate 0°C (32°F). NIST’s thermocouple emf tables list the emf output of a thermocouple based on a corresponding reference thermocouple junction held at 0°C.

Software Compensation
Ice baths and multiple reference junctions in large test fixtures are nuisances to set up and maintain, and fortunately they all can be eliminated. The ice bath can be ignored when the temperature of the lead wires and the reference junction points (isothermal terminal block at the instrument) are the same. The emf correction needed at the terminals can be referenced and compensated to the NIST standards through computer software. When ice baths are eliminated, cold junction compensation (CJC) is still necessary in order to obtain accurate thermocouple measurements. The software has to read the isothermal block temperature. One common technique uses a thermistor, mounted close to the isothermal terminal block that connects to the external thermocouple leads. No temperature gradients are allowed in the region containing the thermistor and terminals.
The type of thermocouple employed is pre-programmed for its respective channel, and the dynamic input data for the software includes the isothermal block temperature and the measured environmental temperature. The software uses the isothermal block temperature and type of thermocouple to look up the value of the measured temperature corresponding to its voltage in a table, or it calculates the temperature with a polynomial equation. The latter method allows numerous channels of thermocouples of various types to be connected simultaneously while the computer handles all the conversions automatically.

Hardware Compensation
Although a polynomial approach is faster than a look-up table, a hardware method is even faster, because the correct voltage is immediately available to be scanned. One method uses a battery in the circuit to null the offset voltage from the reference junction so the net effect equals a 0°C junction. A more practical approach is an "electronic ice point reference," which generates a compensating voltage as a function of the temperature sensing circuit powered by a battery or similar voltage source. The voltage then corresponds to an equivalent reference junction at 0°C.

Type Mixing
Thermocouple test systems often measure tens to hundreds of points simultaneously. In order to conveniently handle such large numbers of channels without the complication of separate, unique compensation TCs for each, thermocouple-scanning modules come with multiple input channels and can accept any of the various types of thermocouples on any channel, simultaneously. They contain special copper-based input terminal blocks with numerous cold junction compensation sensors to ensure accurate readings, regardless of the sensor type used. Moreover, the module contains a built-in automatic zeroing channel as well as the cold-junction compensation channel. Although measurement speed is relatively slower than most other types of scanning modules, the readings are captured in ms, they contain less noise, and they are more accurate and stable. For example, one TC channel can be measured in 3 ms, 14 channels in 16 ms, and 56 channels in 61 ms. Typical measurement accuracies are better than 0.7°C, with channel-to-channel variation typically less than 0.5°C.

Linearization
After setting up the equivalent ice point reference emf in either hardware or software, the measured thermocouple voltage must be converted to a temperature reading. Thermocouple output voltage is proportional to the temperature of the TC junction, but it is not perfectly linear over a very wide range.
The standard method for obtaining high conversion accuracy for any temperature uses the value of the measured thermocouple voltage plugged into a characteristic equation for that particular type thermocouple. The equation is a polynomial with an order of six to ten. The computer automatically handles the calculation, but high-order polynomials take considerable time to process. In order to accelerate the calculation, the thermocouple characteristic curve is divided into several segments. Each segment is then approximated by a lower order polynomial.
Analogue circuits are employed occasionally to linearize the curves, but when the polynomial method is not used, the thermocouple output voltage frequently connects to the input of an analogue to digital converter (ADC) where the correct voltage to temperature match is obtained from a table stored in the computer’s memory. For example, one data acquisition system TC card includes a software driver that contains a temperature conversion library that changes raw binary TC channels and CJC information into temperature readings. Some software packages supply CJC information and automatically linearize the thermocouples connected to the system.

Thermocouple Measurement Pitfalls
Noisy Environments

Because thermocouples generate a relatively small voltage, noise is always an issue. The most common source of noise is the utility power lines (50 or 60 Hz). Thermocouple bandwidth is lower than 50 Hz, so a simple filter in each channel can reduce the interfering ac line noise. Common filters include resistors and capacitors and active filters built around op amps. Although a passive RC filter is inexpensive and works well for analogue circuits, it’s not recommended for a multiplexed front end because the multiplexer’s load can change the filter’s characteristics. On the other hand, an active filter composed of an op amp and a few passive components works well, but it’s more expensive and complex. Moreover, each channel must be calibrated to compensate for gain and offset errors.

Additional Concerns
Thermocouple Assembly
Thermocouples are twisted pairs of dissimilar wires and soldered or welded together at the junction. When not assembled properly, they can produce a variety of errors. For example, wires should not be twisted together to form a junction; they should be soldered or welded. But solder is sufficient only at relatively low temperatures, usually less than 200°C. And although soldering also introduces a third metal, such as a lead/tin alloy, it will not likely introduce errors if both sides of the junction are at the same temperature. Welding the junction is preferred, but it must be done without changing the wires’ characteristics. Commercially manufactured thermocouple junctions are typically joined with capacitive discharge welders that ensure uniformity and prevent contamination. Thermocouples can become un-calibrated and indicate thewrong temperature when the physical makeup of the wire is altered. Then it cannot meet the NIST standards. The change can come from a variety of sources, including exposure to temperature extremes, cold working the metal, stress placed on the cable when installed, vibration, or temperature gradients.

The output of the thermocouple also can change when its insulation resistance decreases as the temperature increases. The change is exponential and can produce a leakage resistance so low that it bypasses an open-thermocouple wire detector circuit. In high-temperature applications using thin thermocouple wire, the insulation can degrade to the point of forming a virtual junction. The data acquisition system will then measure the output voltage of the virtual junction instead of the true junction.

In addition, high temperatures can release impurities and chemicals within the thermocouple wire insulation that diffuse into the thermocouple metal and change its characteristics. Then, the temperature vs. voltage relationship deviates from the published values. Choose protective insulation intended for high-temperature operation to minimize these problems.

Isolation
Thermocouple isolation reduces noise and errors typically introduced by ground loops. This is especially troublesome where numerous thermocouples with long leads fasten directly between an engine block (or another large metal object) and the thermocouple-measurement instrument. They may reference different grounds, and without isolation, the ground loop can introduce relatively large errors in the readings.

Auto-Zero Correction
Subtracting the output of a shorted channel from the measurement channel’s readings can minimize the effects of time and temperature drift on the system’s analogue circuitry. Although extremely small, this drift can become a significant part of the low-level voltage supplied by a thermocouple. One effective method of subtracting the offset due to drift is done in two steps. First, the internal channel sequencer switches to a reference node and stores the offset error voltage on a capacitor. Next, as the thermocouple channel switches onto the analogue path, the stored error voltage is applied to the offset correction input of a differential amplifier and automatically nulls out the offset. See Figure 9.

Open Thermocouple Detection
Detecting open thermocouples easily and quickly is especially critical in systems with numerous channels. Thermocouples tend to break or increase in resistance when exposed to vibration, poor handling, and long service time. A simple open thermocouple detection circuit comprises a small capacitor placed across the thermocouple leads and driven with a low level current. The low impedance of the intact thermocouple presents a virtual short circuit across the capacitor so it cannot charge. But when a thermocouple opens or significantly changes resistance, the capacitor charges and drives the input to one of the voltage rails, which positively indicates a defective thermocouple.

Galvanic Action
Some thermocouple insulating materials contain dyes that form an electrolyte in the presence of water. The electrolyte generates a galvanic voltage between the leads, which in turn, produces output signals hundreds of times greater than the net open circuit voltage. Thus, good installation practice calls for shielding the thermocouple wires from high humidity and all liquids to avoid such problems.

Thermal Shunting
An ideal thermocouple does not affect the temperature of the device being measured, but a real thermocouple comprises a mass that when added to the device under test can alter the temperature measurement. Thermocouple mass can be minimized with small diameter wires, but smaller wire is more susceptible to contamination, annealing, strain, and shunt
impedance. One solution to help ease this problem is to use the small thermocouple wire at the junction but add special, heavier thermocouple extension wire to cover long distances. The material used in these extension wires has net open-circuit voltage coefficients similar to specific thermocouple types. Its series resistance is relatively low over long distances, and it can be pulled through conduit more easily than premium grade
thermocouple wire. In addition to its practical size advantage, extension wire is less expensive than standard thermocouple wire, especially platinum.
Despite these advantages, extension wire generally operates over a much narrower temperature range and is more likely to receive mechanical stress. For these reasons, temperature gradients across the extension wire should be kept to a minimum to ensure accurate temperature measurements.

Improving Wire Calibration Accuracy
Thermocouple wire is manufactured to NIST specifications. Often, these specifications can be more closely met when the wire is calibrated on site against a known temperature standard.


With acknowledgement to IPC Systems Ltd.
Tel: +44(0) 1905 338989
4). Basic RTD Measurements
Click to download a PDF file on 'Basic RTD Measurements'.

With acknowledgement to IPC Systems Ltd.
Tel: +44(0) 1905 338989

 



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