Signal Conditioning

Introduction

Pico signal conditioning is designed to provide a simple method of interfacing a wide range of sensors and transmitters (temperature, humidity, pressure etc) to data loggers and oscilloscopes. The conditioners provide power or excitation and amplify the sensor output.

One or two signal conditioners can be plugged into an adaptor CM001 (picture right) which provides outputs from two standard BNC connectors, so that sensors can be interfaced to Pico PC based oscilloscope modules, and other manufacturers dataloggers, multimeters and chart recorders.

The CM002 Adaptor accepts four signal conditioning modules and interfaces to the ADC-16 (PC based data logger with four differential inputs).

Adaptor EL036 is available for Enviromon (stand alone data logging)

The data logging software will then automatically detect which signal conditioning module is connected and scales the results into the correct units. For example, if a PT100 temperature conditioner is connected, data will be displayed and recorded in °C. If a current clamp is connected then readings will be in Amperes.

Signal Conditioners

There are nine signal conditioners available (the black modules in picture above), each suitable for connecting different sensors:

CM004: Temperature (PT100)

CM005: Temperature (Type K thermocouple)

CM006: Temperature & resistance

CM013: Humidity & temperature: sensor on 5m lead

CM007: 4-20mA current loop (Powered)

CM008: 4-20mA current loop (Isolated)

CM015: 10V bridge (for pressure/load)

CM018: Current clamp (LEM-Heme)

CM019: Current clamp (AC mV)

Signal Conditioner Adaptors

CM001 (see picture above) accepts two signal conditioning modules and provides two standard BNC connector outputs. The unit is ideal for use with Pico PC based oscilloscopes, but can also be used to connect the signal conditioners to other oscilloscopes, data loggers, chart recorders and multimeters. Please note however that the automatic scaling/linearisation only applies when using PicoScope or PicoLog software. $180+GST

CM002 provides sockets for four signal conditioner modules and connects to an ADC-16 PC based data logger
which has four differential inputs. $180+GST

Signal Conditioner details

CM004 : Temperature (PT100)

The PT100 platinum resistance signal conditioner works with standard 2, 3 or 4 wire sensors. It provides high accuracy, high stability temperature measurement over a wide range.
See also our technical note on choosing and using PT100 sensors. $180+GST

Technical Specification

CM005 : Temperature (Type K thermocouple)

The CM005 works with any type K thermocouple. It provides good accuracy over the full temperature range. See also our technical note on choosing and using thermocouple sensors. $180+GST

Technical Specification

CM006 : Temperature and Resistance
The CM006 is primarily designed for use with a precision thermistor, but can also be used with any resistor whose resistance varies between 10k and 1M. $180+GST

May need CM001 Conditioner adaptor to adapt 25-pin output connector of CM006 to BNC connector.

Technical Specification CM006

Technical specification : EL015 Temperature Sensor

CM013 : Temperature and Humidity
The CM013 comes complete with a calibrated humidity/temperature sensor on a 5m lead. Note that as the conditioner measures both temperature and humidity, two channels are required. One CM013 can be used per Enviromon / Oscilloscope adapter. Two CM013s can be used per ADC-16 adapter. $280+GST

Technical Specification

CM007 : 4-20 mA Current Loop (Powered)
The 4-20mA signal conditioner works with two-wire transmitters that produce a 4-20mA output. This type of transmitter is very practical because the two-wire circuit provides both sensor power and signal return: being current- based, it offers very good immunity to electrical noise. The conditioner provides power for the current loop. $180+GST

Technical Specification

CM008 : 4-20 mA Current Loop (Isolated)
The CM008 is designed to be used in existing current loops, or where the sensor / transducer has its own power supply. The CM008 is opto isolated to prevent ground loops and increase safety. $

Technical Specification

CM015 : 10V Bridge (for pressure / load / flow)
The 10 Volt bridge signal conditioner works with standard 4 or 6 wire pressure sensors, load cells and air flow sensors. It provides excitation at 10 volts, and accepts input voltages up to 100mV. $180+GST

Technical Specification

CM018 : Current clamp
The CM018 signal conditioner works with the LEM-Heme range of AC and DC current clamps. There are several current ranges, and both DC and RMS versions are available. The DC version is suitable for DC current measurement and for AC signal quality measurement. RMS versions are suitable for power measurement with mains signals. When working with AC, EnviroMon and the ADC16 are suitable only for use with the RMS versions. $180+GST

Technical Specification

CM019 : Current transformer
The CM019 works with a wide range of AC current clamp transformers. It uses a true RMS converter to produce a DC output. It is ideal for AC power and current monitoring. $TBA

Technical Specification

Sensor power

Conditioner Adapters

Conditioner adaptors interface Pico data loggers and oscilloscopes to the signal conditioners.

The EL036 signal conditioner accepts two signal conditioning modules. It plugs into the Enviromon data logger network for stand alone data logging. The EL036 draws its power from the Enviromon network. $164

The CM002 connects to an ADC-16 PC based data logger, and provides sockets for four signal conditioner modules. $164

The CM001 (see picture right) accepts two signals conditioning modules and provides two standard BNC connector outputs.
The unit is ideal for use with Pico PC based oscilloscopes, but can also be used to connect the signal conditioners to other oscilloscopes, data loggers, chart recorders and multimeters. Please note however that the automatic scaling/linearisation only applies when using PicoScope or PicoLog software. $180+GST

Technical Notes

Platinum resistance thermometers

Platinum resistance thermometers (PRTs or RTDs) offer excellent accuracy over a wide temperature range (from -200 to 850 C). Sensors are interchangeable between different manufacturers, and are available in various accuracy ratings in packages to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor.

The principle of operation is to measure the resistance of a platinum element. The most common type (Pt100) has a resistance of 100 ohms at 0 C and 138.4 ohms at 100 C. There are also Pt1000 sensors that have a resistance of 25 ohms and 1000 ohms respectively at 0 C.

The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 C range, the error at 50C is 0.4 C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90). This linearisation is done automatically, in software, when using Pico signal conditioners. The linearisation equation is

Rt = R0 * (1 + A* t + B*t2 +C*(t-100)* t3)

A = 3.9083 E-3
B = -5.775 E-7
C = (below 0 C) -4.183 E -12
(Above 0 C) zero

For a Pt100 sensor, a 1 C temperature change will cause a 0.384ohm change in resistance, so even a small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor) can cause a large error in the measurement of the temperature. For precision work, sensors have four wires- two to carry the sense current, and two to measure the voltage across the sensor element. It is also possible to obtain three-wire sensors, although these operate on the (not necessarily valid) assumption that the resistance of each of the three wires is the same.

The current through the sensor will cause some heating: for example, a sense current of 1mA through a 100 ohm resistor will generate 100uW of heat. If the sensor element is unable to dissipate this heat, it will report an artificially high temperature. This effect can be reduced by either using a large sensor element, or by making sure that it is in good thermal contact with its environment.

Using a 1mA sense current will give a signal of only 100mV. Because the change in resistance for a degree celsius is very small, even a small error in the measurement of the voltage across the sensor will produce a large error in the temperature measurement. For example, a 100uV voltage measurement error will give a 0.4 C error in the temperature reading. Similarly, a 1uA error in the sense current will give 0.4 C temperature error.

Because of the low signal levels, it is important to keep any cables away from electric cables, motors, switchgear and other devices that may emit electrical noise. Using screened cable, with the screen grounded at one end, may help to reduce interference. When using long cables, it is necessary to check that the measuring equipment is capable of handling the resistance of the cables. Most equipment can cope with up to 100 ohms per core.

The type of probe and cable should be chosen carefully to suit the application. The main issues are the temperature range and exposure to fluids (corrosive or conductive) or metals. Clearly, normal solder junctions on cables should not be used at temperatures above about 170 C.

Sensor manufacturers offer a wide range of sensors that comply with BS1904 class B (DIN 43760): these sensors offer an accuracy of ±0.3 C at 0 C. For increased accuracy, BS1904 class A (±0.15 C) or tenth-DIN sensors (±0.03 C). Companies like Isotech can provide standards with 0.001 C accuracy. Please note that these accuracy specifications relate to the SENSOR ONLY: it is necessary to add on any error in the measuring system as well.

Related standards are IEC751 and JISC1604-1989. IEC751 also defines the colour coding for PRT sensor cables: the one or two wires atttached to one end of the sensor are red, and the one or two wires at the other end are white.

The Pico Technology CM004 signal conditioner works with 4-wire sensors and offers ±0.2 C accuracy. It can be used either with a Pico ADC16 converter, or with the EnviroMon data logging system.

Thermocouples

Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve.

How they work

In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients. The diagram below shows a K type thermocouple, which is the most popular:

Standard tables show the voltage produced by thermocouples at any given temperature, so for example in the above diagram, the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is not possible to simply connect up a voltmeter to the thermocouple to measure this voltage, because the connection of the voltmeter leads will make a second, undesired thermocouple junction. To make accurate measurements, this must be compensated for, a technique known as cold junction compensation (CJC). In case you are wondering why connecting a voltmeter to a thermocouple does not make two additional thermocouple junctions (one for each lead), the law of intermediate metals states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions are at the same temperature. This law is also important in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by soldering the two metals together as the solder will not affect the reading. In practice, however, thermocouples junctions are made by welding the two metals together (usually by capacitive discharge) as this ensures that the performance is not limited by the melting point of solder.

All standard thermocouple tables allow for this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this was done with a carefully constructed ice bath (hence the term 'cold' junction compensation). Maintaining a ice batch is not practical for most measurement applications, so instead the actual temperature at the point of connection of the thermocouple wires to the measuring instrument is recorded.

Typically cold junction temperature is sensed by a precision thermistor in good thermal contact with the input connectors of the measuring instrument. This second temperature reading, along with the reading from the thermocouple itself is used by the measuring instrument to calculate the true temperature at the thermocouple tip. For less critical applications, the CJC is performed by a semiconductor temperature sensor. By combining the signal from this semiconductor with the signal from the thermocouple, the correct reading can be obtained without the need or expense to record two temperatures. Understanding of cold junction compensation is important; any error in the measurement of cold junction temperature will lead to the same error in the measured temperature from the thermocouple tip.

Linearisation

As well as dealing with CJC, the measuring instrument must also allow for the fact that the thermocouple output is non linear. The relationship between temperature and output voltage is a complex polynomial equation (5^th to 9^th order depending on thermocouple type). Analogue methods of linearisation are used in low cost themocouple meters. High accuracy instruments such as the Pico TC-08 store thermocouple tables in computer memory to eliminate this source of error.

Thermocouples Types

Thermocouples are available either as bare wire 'bead' thermocouples which offer low cost and fast response times, or built into probes. A wide variety of probes are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes take care to ensure they have the correct type of connector. The two common types of connector are 'standard' with round pins and 'miniature' with flat pins, this causes some confusion as 'miniature' connectors are more popular than 'standard' types.

When choosing a thermocouple consideration should be given to both the thermocouple type, insulation and probe construction. All of these will have an effect on the measurable temperature range, accuracy and reliability of the readings. Listed below is our (somewhat subjective) guide to thermocouple types.

Type K (Chromel / Alumel)

Type K is the 'general purpose' thermocouple. It is low cost, and owing to its popularity it is available in a wide variety of probes. Thermocouples are available in the -200°C to +1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you have a good reason not to.

Type E (Chromel / Constantan)

Type E has a high output (68uV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that is non-magnetic.

Type J (Iron / Constantan)

Limited range (-40 to +750°C) makes type J less popular than type K. Main application is with old equipment that can not accept 'modern' thermocouples. J types should not be used above 760°C as an abrupt magnetic transformation will cause permanent decalibration.

Type N (Nicrosil / Nisil)

High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an 'improved' type K, it is becoming more popular.

Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10uV/⁰C) there are usually only used for high temperature measurement (>300°C).

Type B (Platinum / Rhodium)

Suited for high temperature measurements up to 1800°C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0°C and 42°C. This makes them useless below 50°C.

Type R (Platinum / Rhodium)

Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost makes them unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/vC) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43°C).

When selecting thermocouple types, ensure that your measuring equipment does not limit the range of temperatures that can be measured. Listed below is the range of temperatures that the 8 channel Pico TC-08 can measure. Note that thermocouples with low sensitivity (B, R and S) have a correspondingly lower resolution.

Thermocouple type	Overall Range (°C)	0.1°C Resolution	0.025°C Resolution
B	100..1800	1030..1800	-
E	-270..790	-240..790	-140..790
J	-210..1050	-210..1050	-120..1050
K	-270..1370	-220..1370	-20..1150
N	-260..1300	-210..1300	340..1260
R	-50..1760	330..1760	-
S	-50..1760	250..1760	-
T	-270..400	-230..400	-20..400

Precautions and Considerations for Using Thermocouples

Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Listed below are some of the more common problems and pitfalls to be aware of.

Connection problems. Many measurement errors are caused by unintentional thermocouple junctions. Remember that any junction of two different metals will cause a junction. If you need to increase the length of the leads from your thermocouple, you must use the correct type of thermocouple extension wire (eg type K for type K thermocouples). Using any other type of wire will introduce a thermocouple junction. Any connectors used must be made of the correct thermocouple material and correct polarity must be observed.

Lead Resistance. To minimise thermal shunting and increase response times, thermocouples are made of thin wire (in the case of platinum types cost is also a consideration). This can cause the thermocouple to have a high resistance which can make it sensitive to noise and can also cause errors due to the input impedance of the measuring instrument. A typical exposed junction thermocouple with 32AWG wire (0.25mm diameter) will have a resistance of about 15 ohms / meter. The Pico TC-08 has an input impedance of 200kW so will have an error of less than 0.1% for 12 meters of such cable. If thermocouples with thin leads or long cables are needed, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so has a lower resistance) to run between the thermocouple and measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.

Decalibration is the process of unintentionally altering the makeup of thermocouple wire. The usual cause is the diffusion of atmospheric particles into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specifications of the probe insulation.

Noise. The output from a thermocouple is a small signal, so it is prone to electrical noise pick up. Most measuring instruments (such as the TC-08) reject any common mode noise (signals that are the same on both wires) so noise can be minimised by twisting the cable together to help ensure both wires pick up the same noise signal. Additionally, the TC-08 uses an integrating analog to digital converter which helps average out any remaining noise. If operating in an extremely noisy environment, (such as near a large motor) it is worthwhile considering using a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see if the reading changes.

Common Mode Voltage. Although thermocouple signal are very small, much larger voltages often exist at the input to the measuring instrument. These voltages can be caused either by inductive pick up (a problem when testing the temperature of motor windings and transformers) or by 'earthed' junctions. A typical example of an 'earthed' junction would be measuring the temperature of a hot water pipe with a non insulated thermocouple. If there are any poor earth connections a few volts may exist between the pipe and the earth of the measuring instrument. These signals are again common mode (the same in both thermocouple wires) so will not cause a problem with most instruments provided they are not too large. For example, the TC-08 has a common mode input range of -4V to +4V. If the common mode voltage is greater than this then measurement errors will result. Common mode voltages can be minimised using the same cabling precautions outlined for noise, and also by using insulated thermocouples.

Thermal Shunting. All thermocouples have some mass. Heating this mass takes energy so will affect the temperature you are trying to measure. Consider for example measuring the temperature of liquid in a test tube: there are two potential problems. The first is that heat energy will travel up the thermocouple wire and dissipate to the atmosphere so reducing the temperature of the liquid around the wires. A similar problem can occur if the thermocouple is not sufficiently immersed in the liquid, due to the cooler ambient air temperature on the wires, thermal conduction may cause the thermocouple junction to be a different temperature to the liquid itself. In the above example a thermocouple with thinner wires may help, as it will cause a steeper gradient of temperature along the thermocouple wire at the junction between the liquid and ambient air. If thermocouples with thin wires are used, consideration must be paid to lead resistance. The use of a thermocouple with thin wires connected to much thicker thermocouple extension wire often offers the best compromise.

Pico Products for measuring Thermocouples

Pico has two products designed to allow temperature to be measured and recorded using thermocouples. The TC-08 interfaces up to 8 thermocouples of any type to a PC. The CM005 K Type thermocouple signal conditioner provides a simple way of connecting a thermocouple to the ADC16 data logger, the Enviromon network or any of our PC based oscilloscopes.