Platinum Resistance Thermometers Specification

A Pt100 is a sensor used to measure temperature. It is one type of sensor which falls into a group called Resistance Temperature Detectors or RTDs.

The specification of Pt100 temperature sensors is determined by a number of factors and what follows will describe each of the factors and enable you to specify a Pt100 temperature sensor with ease. Of course, if you need any assistance our engineers are always available to assist. Please contact us with your enquiry.

a PT-100 sensor with a working principle - to measure temperatures above 0°c using Platinum and other metals.

PT-100 sensor

by Process Parameters

Sensor Characteristics

Resistance Temperature Detectors

The Pt100 temperature sensor specification is heavily driven by how the sensor works and its characteristics. The Pt100 sensor falls into a group of sensors called Resistance Temperature Detectors or RTDs and they all work on the principle of a change of electrical resistance with changing temperature.

A Pt100 is just one of a range of different sensors and it is important to understand the meaning of this terminology in order to properly specify a Pt100 RTD.

Pt - the chemical symbol for Platinum

The first of the model number, Pt, is the chemical symbol for Platinum and this is the electrical conductor material used in the sensing element itself. The second part, 100, relates to the resistance value of the sensor at 0°C. In this case 100Ω. There are a number of different types of sensor and they are all specified in a similar way. Different materials can be used such as Nickel (Ni) and Copper (Cu) and different resistance values such as 50Ω, 500Ω and 1000Ω. This gives the possibility of sensors being identified Cu100, Ni120 or Pt1000.

The Pt100 variant is, however, the most commonly used. Sensors using Platinum are also by far the most common group and are often referred to as Platinum Resistance Thermometers or PRTs.

Nominal resistance

In addition to the base resistance value at 0°C, it is important to understand the resistance change characteristics for the sensor and these should be matched to the instrument you are using. Almost without exception, industrial Pt100 temperature sensors are specified to operate with a coefficient of 3.851×10-3 °C-1 as determined by the international standard IEC 60751:2008. This gives a nominal resistance at 100°C of 138.51Ω.

Custom features

In addition to this standard set of characteristics, there are others available and used in relatively rare circumstances. Often Process Parameters can supply temperature sensors with these special characteristics so please contact us with your requirement.

Interested in a Pt100 for your application?

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Sensor Tolerance – Including Pt100 Class B Specification

The tolerance of a Pt100 temperature sensor is given as a reference to a Class rather than a percentage of a measurement or of a scale. This is because the tolerance of a Pt100 is not constant for a changing temperature. The accuracy is always best at 0°C and gets worse with increasing or decreasing temperature.

The basic tolerance we use is called Class B. The Pt100 Class B specification widest tolerance sensor generally supplied. This terminology has now been used for many years and is commonly understood by any probe manufacturer. The terminology is derived from the old British Standard and subsequently the current standard BS EN 60751:2008.

Class B tolerance

A class B tolerance Pt100 sensor has a tolerance at 0°C of +/-0.3°C. As mentioned before this tolerance is not constant across the measuring range and increases with both a rising and falling temperature. For example, at 100°C the allowable tolerance is +/-0.8°C and so on. The tolerance change is linear so at 200°C the tolerance is +/-1.3°C.

All other tolerances are created in relation to the Class B tolerance and are a proportion of this tolerance band. The next level of accuracy is Pt100 Class A specification and this tolerance is +/-0.15°C at 0°C. This is half of Class B at 0°C. Again, as the temperature rises or falls the tolerance widens (by slightly better than half Class B).

British Standards

Traditionally the British Standards have only covered Class B and Class A tolerance bands so for higher accuracy sensors we have had to refer to a different standard, even though the relationship is essentially the same.

German Standards

There are three further levels of accuracy available, 1/3DIN, 1/5 DIN and 1/10 DIN. The DIN, in this case, means we are referring to the German standard. The meaning of this terminology is that the basic DIN accuracy is the same as the British Class B accuracy. The 1/3, 1/5 and 1/10 relate to the proportion of this tolerance. At 0°C this gives accuracies of: –

1/3 DIN                                +/-0.3°C / 3                         +/-0.1°C

1/5 DIN                                +/-0.3°C / 5                         +/-0.06°C

1/10 DIN                              +/-0.3°C / 10                       +/-0.03°C

1/10 DIN is the highest accuracy sensor available for industrial applications.

Tolerances bands

Tolerances for all bands are shown in the following table

The IEC standard IEC 60751:2008 has a new tolerance scheme and although the accuracy and temperature relationship remains unchanged. The standard Class B tolerance band is referred to in this standard as W0.3 and Class A as W0.15. The higher tolerance bands are also specified in relation to this W0.3 base level in a similar way as with the DIN standard. We should point out however that it is very uncommon to have a Pt100 temperature sensor specified in this way and almost exclusively manufacturers and users of such devices still refer to Class A 7 B and 1/3 DIN etc.

a table measuring the different Pt100 thermocouple classes against temperatures between -200°c and 650°c
click to enlarge

Interested in a Pt100 for your application?

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Sensor Wiring

Unlike other types of temperature sensors such as thermocouples or thermistors which have just two connection wires, the same is not necessarily true for Pt100 temperature sensors.

Whilst it is perfectly possible to connect a Pt100 with a two-wire connection it is important to recognise that this will introduce measurement errors due to the resistance of the lead wire. This is because the measuring instrument will measure the total resistance of the measuring circuit and not just that of the sensing element itself.

If you consider that the resistance changes per degree Celsius of temperature change is just 0.3851Ω, then it can be seen that by adding in connection wires with a resistance of even just 1Ω would lead to an error approximately +2.6°C. This falls outside of the published tolerances for this type of sensor.

Two wire connection is often used and Process Parameters many sensors with this type of connection. It is often used as a lower-cost solution They are however suited to a few special cases as follows: –

  • Where the application does not have a high demand for accuracy.
  • Where the sensor cable is very short.
  • Where the error is determined by testing and an offset applied to the measuring instrument.

If your application demands do not fall into these then there are standard methods of compensating for the lead resistance.

The addition of a third wire, connected to one side of the measuring element, helps to compensate for the lead resistance. It is very important that each of the three wires used in the measuring circuit are equal in terms of both conductor size and length. This is because the measured result is averaged and only gives good accuracy where all three wires have the same resistance.

The 3 wire connection specification works by measuring the resistance value through the detector and also taking a second resistance value through the pair of wires joined on one side of the detector. The subtraction of this resistance value from the total gives the resistance value for the measuring element in isolation.

Three wire connection is by far the most common of all wiring types used in Pt100 thermometry. Many instruments utilise this method of connection including temperature transmitters, temperature controllers, panel displays and data loggers.

For the greatest accuracy, you should choose a four wire Pt100 RTD specification. This measuring system is the only way of fully compensating for all lead resistance in the measuring system, even if each wire has a different resistance.

The measuring system using one pair of wires to carry the excitation current used for the measurement and the second pair is used to measure the resistance of the sensing detector by measuring the voltage drop.

As the 4-wire connection method fully compensates for all lead resistance we strongly recommend that this is used when using a high specification of Pt100 such as 1/5 or 1/10 DIN tolerance. We believe it is a false economy to specify a high tolerance temperature sensor with its associated cost and use an inferior measuring system. Four wire connection is predominantly used in laboratories and calibration applications and anywhere the highest accuracy is required.

The final method of connection is now extremely rare but is worthy of a mention. A 4-wire blind loop connection utilises a simple 2-wire connection on the sensor and a separate pair of wires joined as a closed loop connection. The functionality of this system is similar to the 3-wire system and therefore demands that each conductor has the same resistance.

Operating Temperature Range

When giving a Pt100 sensor specification it is always very important to specify the operating temperature range you require for the sensor. The international standard IEC 751 gives a notional operating range of between -200 and +850°C but for various reasons, this range is not practical for industrial applications.

Depending on the materials of construction it possible to manufacture sensors for extremes of high or low temperatures. Generally, an application will demand operation in a fairly narrow working band and this is important to use as a Pt100 sensor manufacturer. We can design the sensor in such a way that it can work in the operating range reliably and give a good service life.

In general, we offer our sensors with operating temperature ranges that fall in standard bands or ranges. These ranges are as follows:

A very limited range generally used for water applications and low temperature OEM applications.

Utilising low cost silicone rubber components this rage provides plenty of safety margin on many applications. Often used in refrigeration, deep freeze and HVAC applications. Can be supplied as a waterproof product.

This is our most common range as it covers 80-90% of industrial processes.

If you need a slightly higher range then this is a cost-effective way of achieving it. The materials used are glass fibre based and cannot therefore be waterproof.

We hold in stock special materials that allow us to manufacture fabricated sensors suitable for use to a very high temperature. Again, based on glass fibre and ceramic components so not waterproof.

We stock special sensing elements that allow a low-cost construction suitable for very low temperatures.

Cryogenic applications place special demands on sensor design that we have invested heavily in. We utilise very high-quality wire wound sensing elements and other specialised components including resins approved by NASA for use in space.

Sensor housing

Apart from some specific cases, a Pt100 should always be protected by a suitable housing. The only occasions where we do not supply sensors in housings are as follows: –

  • Our customer is fitting the element into a larger product and they will pot it accordingly.
  • The sensor is being bonded to a surface in a dry environment.
  • The sensor is being used to measure the temperature of dry air or gases. Even in this case, it is normal to use a protection sheath which is ventilated to give fast response.
Materials

The material of the housing is completely flexible but in general we supply housings in 316 Stainless Steel as this has been proven to be a good all-round material that offers a good price to performance ratio. It is easy for us to work with which also helps keep our costs down. The vast majority of our 316 stainless steel sensor housings are TIG welded to give clean strong joints between components. There are occasions where we also silver solder.

Other materials used, depending on the application, include PTFE, PEEK, Aluminium, Brass, Titanium, Inconel and other grades of Stainless Steel.

Sometimes it is not practical or economical to manufacture a sensor housing from a specialised material but there is a need to protect the sensor from the environment, usually strong acids or alkalis. In this case, it is possible to manufacture a standard 316 Stainless Steel sensor but fit a PTFE or FEP sleeve or coating over the metalwork to provide the protection.

Mechanical design

The mechanical design of the sensor is also flexible and we can usually machine or fabricate a sensor housing to suit your needs. The simplest form of sensor housing is a simple tubular sheath of a specified diameter and length. We offer tube diameters from 2mm upwards in both metric and imperial sizes. We always use a TIG welded tip to close the end of our probe sheaths.

In addition to the probe stem, we can add process connections to suit your needs. These are usually threaded connections and the most common type is BSP and BSPT British Standard Pipe Threads. Other common types include NPT (US standard National Pipe Thread) and Metric threads. Others are available by request.

For specialised applications such as brewing, dairy and pharmaceutical applications it is more common to fit a hygienic or sanitary type fitting such as Tri-Clamp flange, RJT, IDF or SMS which give a clean method of mounting the sensor with minimal dead volume and allow CIP processes to be conducted easily.

When considering the design of a sensor it is important to ensure the correct depth of immersion. All temperature sensors are “tip sensitive” and it is important to construct the sensor such that the tip is at the point where the measurement is required. The probe length can be adjusted to suit the application from perhaps 25mm long for measuring in small diameter pipes up to 2-3 metres long for large vessels and in composting applications. In some applications probe stems of many metres are required and this can be achieved using mineral insulated constructions.

Depth of immersion

The depth of immersion of the sensor is also critical to the accuracy of the measurements taken. A probe that has insufficient immersion will likely be influenced by the surrounding environment due to “stem conduction”. This is where the temperature differential between the measuring tip and the outside environment causes thermal energy to conduct either from or to the tip, influencing the measured value. This phenomenon can occur with very short sensors or sensor which are not immersed sufficiently in the fluid they are measuring.

The depth of immersion is directly linked to the diameter of the temperature probe. As a rule, the depth of immersion should be at least five times the diameter of the sensor and preferably ten times the sensor diameter. The sensing length should be added to this length. In applications demanding the highest accuracy, depths of immersion of fifteen times the stem diameter should be considered.

Operating conditions

Other considerations to take into account when designing a suitable sensor housing are the operating conditions not only of the probe within the process but also the part which sits just outside. The conditions local to the process may still be relatively arduous whether this is very high temperature, very low temperature or the presence of water, oil or other substances which may cause an issue. There are plenty of options available to deal with these and we would be happy to discuss them on a case-by-case basis.

Interested in a Pt100 for your application?

Contact us on 01628 778688 to discuss further.

Alternatively click on the button below and fill out our contact form.

Sensor cable – Pt100 Cable Specification

In many sensor designs, the probe itself is connected to an external instrument through a length of cable. The Pt100 cable specification that is used is determined by the operating conditions required by the application. There are a variety of high-performance cables available and as a sensor manufacturer, we hold stocks of the most common types. These can be summarised as follows:

 

PVC

PVC is the insulation of choice for almost any cable we see around us in our day-to-day lives. Everything from mains cable through to leads on headphones and mobile phones are invariably PVC. In general, PVC is not seen as a suitable cable for many temperature applications due to its limited operating range. Standard PVC is only suitable for use in the range -10 to +70°C. A high-temperature version more commonly used in this industry extends the upper limit to 105°C. There are still plenty of applications that fall within this range and if the application permits the use of PVC then it is the lowest cost option.

Silicone Rubber

If flexibility is required, then silicone rubber is the best choice. Any cable which uses silicone rubber is more flexible than almost any other kind plus it has a very useful operating temperature of -60 to +180°C. Special versions can be made to operate to 240°C making it a viable alternative to more expensive Teflon based products.

Silicone rubber is an excellent material to bond to and it is the material of choice for probes which are required to be moisture proof.

PTFE and PFA

Probably the most versatile cable used in temperature sensor manufacturing, PTFE and more often PFA are used to deal with a wide range of application requirements. PTFE is a tape insulation which is wrapped onto the cable and sintered whilst PFA is an extruded form. These materials form part of a group of materials called Fluoropolymers denoting the use of Fluorine in the composition of the material. Others include FEP, ETFE (Tefzel) and FPM/FKM (Viton).

The operating range for these materials is from -268°C (5K, -450°F) and up to +250°C. Notably the material retains good flexibility down to -75°C but below this, we recommend the cable is used statically.

As well as a wide temperature range, the materials are well known for being practically chemically inert and they can be used with confidence in the presence of almost any substance without risk of degradation.

The material has an extremely low coefficient of friction (the third lowest of all known materials) and is also extremely hydrophobic. This means it repels water and other substances making it extremely difficult to bond to. Whilst we possess the capability to chemically etch fluoropolymer materials prior to potting we recommend selecting other materials where possible for moisture proof assemblies.

Glass Fibre

To achieve the higher temperature ranges it is necessary to move away from polymeric materials and use glass fibre based insulations. As standard, these provide an ability to operate up to 400°C but high-temperature versions can extend this to 600°C (with Nickel conductors).

Glass fibre is a woven insulation and unlike extruded polymer insulation is a very slow process. Once woven the cable is treated with a varnish to bond the weave together and retain the integrity of the cable. Because of the woven nature of the material, it is not waterproof and should not be used in environments where water is present.

Glass fibre is also quite brittle and is susceptible to damage through flexing and abrasion. Minimising the bend radius reduces the risk of a break down in the insulation. Small radius bends should always be avoided. To improve the mechanical strength of the cable they are often supplied with a stainless steel overbraiding.

Stainless Steel Armour

In some applications it is necessary to provide additional mechanical protection to the cable. The best way of achieving this is to use a stainless steel flexible armour or conduit. This is available in various grades of stainless steel to meet cost and durability requirements. Whilst the armour provides mechanical protection it does not provide any additional waterproofing.

Cable Conductors

Unlike with thermocouple type sensors, it is permissible to use any copper-based cables as long as the number of cores maintains the measuring system from end-to-end. Therefore, if you choose a 3 wire measuring system, you should use a 3 core cable throughout.

If you require a long cable run in your system, then you should consider increasing the conductor size to reduce the resistance of the cable itself. Some temperature transmitters and instrumentation are only capable of compensating for a limited led resistance so this should be borne in mind. For long cable runs you should consider the use of a temperature transmitter to maintain accuracy.

In the cables we use, the conductors are generally plated in a material to protect the copper and give as long a life as possible. The conductors are either Tin plated, Silver plated and occasionally Nickel plated.

For very high-temperature cables where copper is not suitable, we use a pure Nickel conductor. The resistance of Nickel is much higher than that of Copper and therefore it is critical that at least a 3-wire measuring system is used.

Sensor Terminal Head

Instead of a flying lead from your Pt100 temperature sensor, you may wish to consider a “Terminal Head” instead. These heads are a form of junction box designed to fit directly to the sensor and they do offer some advantages over having a flying lead.

  • Ingress Protection – most common terminal heads supplied have a very high IP rating, often IP68, making them suitable for extreme environments where dust and moisture are an issue. This is a much easier way of achieving a high IP rating than having a flying cable which is reliant on potting and sealing being carried out by hand.
  • Easier to fit – even easier to replace. There will always come a time when a Pt100 temperature sensor will need replacing. With a terminal head sensor, this is made easier by simply detaching the connecting cable, replacing the sensor and then rewiring. If you routinely use sensors with a cable fitted you inevitably must route the cable through the machine each time.
  • Choice of materials – in general, terminal heads are made of an Aluminium Alloy which has a painted finish, but it is also possible to specify the terminal head in a variety of other materials such as 316 Stainless Steel, Glass Filled Nylon, ABS, Polypropylene or Cast Iron.
  • Choice of connections inside – the usual choice is for a terminal block to be fitted inside the terminal head. This gives a direct connection from the Pt100 sensor and allows you to connect your extension cable through the cable gland to your instrument. Our terminal blocks are generally ceramic-based but plastic blocks are also available. As an alternative, an In-Head temperature Transmitter can be fitted to provide direct conversion of the sensor to an industry-standard 4-20mA output signal.
Sensor output/transmitter

The output requirements that you specify for your Pt100 temperature sensor will be determined by a few factors and you have two basic options as follows.

  1. You could take a direct resistance output from the sensor directly to your instrument. This will involve running a length of 2, 3 or 4 core cable from the sensor to your measuring instrument. Note that to maintain accuracy you should use at least 3-wire connection and this should be maintained throughout the measurement circuit. Do not be tempted to use 2 wire connection where there is any appreciable cable length. Note that unlike thermocouples you can use copper-based cables for Pt100’s.
  2. Alternatively, you may want to convert the resistance output from the sensor to an industry-standard 4-20mA output. This has many benefits, particularly when using long cable lengths. The 4-20mA transmitter can be mounted in the terminal head if fitted (often referred to as an “In Head Temperature Transmitter”) or you may wish to use DIN rail mounted devices in a panel where cable sensors are being used.

As well as the obvious benefit of only requiring two core cable to run between the transmitter and your measuring instrument, standardising on a single sensor signal type for all measurement parameters (flow, level, humidity, pressure etc.) means you can also standardise on the measuring instrumentation you are using within the process. This gives commonality and familiarity.

Options for special applications - Pt100 RTD Specification Continued

Many Pt100 RTD sensor specifications fall into a relatively small number of standard designs which offer a wide range of flexible features but sometimes a process will have special demands which require a more bespoke type of sensor. Process Parameters excels at offering bespoke sensor manufacture and can generally meet your requirements. Talk to us to discuss your requirement. Here are some ideas for special features which give a taste of what is possible, but it is not intended to be exhaustive.

  • In some applications metals of any kind are not suitable, usually due to a lack of chemical resistance. This is not generally an issue however as we can offer sensors with an FEP covering over the standard 316 stainless steel stem. This gives the advantage of a rigid stainless steel probe but with the chemical resistance or practically inert FEP. Our high-quality coverings have welded tips which are spark tested for leak tightness.
  • If you are operating in the Dairy, Brewing, Beverage, Pharmaceutical or Cosmetic industries then you will almost certainly require hygienic or sanitary type connections. These connections meet various international standards and are therefore interchangeable from manufacturer to manufacturer. The benefits of these types of fitting are reduced dead volumes, no threads in the wetted areas and the ability to perform CIP. We take great care when welding hygienic fittings to ensure a crevice free weld. All welds are TIG welded.
  • For many of the sensors we manufacture we are asked to fit a connector to match up with our customer’s instrumentation. This could be a cable mounted inline connector for a handheld thermometer or it could be a PCB mount connector. It is perfectly permissible to use good quality connectors in a Pt100 temperature sensor measuring circuit. The connector should maintain the wiring system throughout (so use a 3 or 4 way depending on the wiring system used) and they should have a low connection resistance.
Mineral Insulated

The vast majority of Pt100 temperature sensors are of a fabricated construction which gives the greatest flexibility and can meet the requirements of the vast majority of applications. There are however cases where a Mineral Insulated construction is more suited.

Mineral Insulated construction is more widely used in Thermocouple sensing technology but more and more Pt100 sensors are now being supplied as mineral insulated.

The main benefits of Mineral Insulated Construction are as follows: –

  1. Semi-flexible Stainless Steel or Inconel sheathing in a variety of diameters from 3mm upwards.
  2. Can be supplied in almost any length whereas as a fabricated design would normally be limited to 2-3 metres.
  3. Long lengths supplied coiled for straightening on site. No special tools are required.
  4. Can be bent and formed to fit complex installations.
  5. Easily cope with higher temperatures of up to 650°C.
  6. The construction is generally more robust and can withstand shock and vibration.
The Pt100 RTD thermocouple from Process Parameters who offer bespoke sensor manufacturing for any specification

The Pt100 RTD thermocouple

from Process Parameters

Pt100 Thermocouples

We often get asked for “Pt100 Thermocouples” but sadly there is no such thing as this is a mix up of two measurement technologies. There is no common Pt100 Thermocouple specification. Pt100’s and thermocouples should not be confused even if they can externally look identical. If you were to try and use a thermocouple in place of a Pt100 or vice versa it simply would not work. If you need help with identifying the type of sensor you need don’t hesitate to contact us as we are happy to help.

Pt100 Thermistors

Again this is a confusing statement as the two technologies are treated separately. They do have a loose relationship in that both use resistance to measure temperature but apart from that should not be confused. The main differences are:-

  • A Pt100 temperature sensor will comply with International standards whereas a Thermistor will only comply with manufacturer standards. There is little or no interchangeability between different brands of thermistor. You must usually use a specific type of thermistor sensor to match your instrument.
  • A Pt100 has a linear resistance response to temperature whereas a thermistor will be non-linear.

Process Parameters manufacture thousands of thermistor sensors every year of various types. Contact us with your requirements and we do our best to help.

More about Pt100 Sensors