Probe Sensor Guide

Make sure you select the right probe for your business

Because we have such a vast range of temperature probes and sensors to choose from, ETI has produced a series of guides to help customers make informed purchases.

Below you will find detailed information on ETI’s major product ranges – Thermocouple probes, Thermistor probes and the Therma series of handheld devices. These guides contain all the information you need to ensure you select the right solution that matches the needs of your business.

If you have any additional questions that are not answered in the guides, please do not hesitate to contact our team.

Thermocouple Probes

ETI manufactures an extensive range of type K or T thermocouple probes to compliment our range of thermometers, data-loggers and instrumentation.

Thermocouple probe factors
to think of:

The response time is the time taken for the sensor to reach 2/3rds of the final reading and is the industry standard means of measuring probe response times. Five times the quoted response time is the figure normally required to obtain 100 % of the reading. Response times are dependent upon the substance being measured and in the case of liquid or gas, upon the degree of agitation. It is therefore difficult to quote an accurate response time without knowledge of the application. The results given in this catalogue were obtained in a stirred oil bath and may differ from those obtained under other conditions but can be used as a
general guide.

PVC straight lead is a general purpose lead and available in lengths up to 100 metres. As standard and where appropriate, each probe is supplied with a one metre straight PVC lead and a connector.
As an alternative, a one metre coiled PU lead is available, simply replace the first digit (1) of the order code with the number 3. Maximum temperature for both PVC and PU coiled leads is 80 °C.

Some industrial and high temperature probes are available with a stainless steel, fibreglass insulated over-braided lead. As standard and where appropriate, each probe is supplied with a two metre stainless steel lead and a connector. Maximum temperature of 350 °C.

Applications quoted are typical for the specific probe, although there are many alternative uses for which the probe could be equally suitable. For advice on a specific probe for a particular application, please contact us (+44 (0)1903 202151 / Where requirements cannot be met from the existing standard range of probes, then bespoke designs can be manufactured.

RTD/PT100 Probes

ETI manufactures an extensive range of PT100 (RTD) probes to compliment our range of thermometers, data-loggers and instrumentation.

Resistance temperature detector probes are slower to respond to changes in temperature than thermocouple probes, but are generally more accurate.

RTD/PT100 Probe factors to think of:

Resistance temperature detector (PT100 or RTD) probes consist of flat film or a wire wound platinum resistance sensor element. The measurement resistance value changes in line with the temperature being measured.

PVC lead is a general purpose lead and available in lengths up to 100 metres. Stainless steel braided lead is a high temperature (fibreglass insulated) lead with a maximum temperature of 350 °C. As standard and where appropriate, each probe is supplied with a one metre straight PVC lead and a connector. Maximum temperature for both PVC and PU leads is 80 °C

Applications quoted are typical for the specific probe, although there are many alternative uses for which the probe could be equally suitable. For advice on a specific probe for a particular application, please contact us (+44 (0)1903 202151 / Where requirements cannot be met from the existing standard range of probes, then bespoke designs can be manufactured.

Benefits of Thermocouple and RTD's in five categories

RTDs and thermocouples are both designed to measure temperature yet are designed differently, which can make it difficult to determine whether one should be used over another.


RTD sensors, for example, are used where higher accuracy, stability and repeatability are needed and can be suited to lower temperatures. Thermocouples can measure higher temperatures and are more cost-effective, but they are less accurate and can drift over time.

RTD temperature sensors are often the right choice for measurements below 600 °C that require high accuracy, repeatability, and stability. Thermocouples are less expensive, can measure a wider range of temperatures and are durable. They are, however, less accurate than RTDs and may need more frequent calibration adding to the overall long-term cost.

Vs. RTDs

Thermocouples cover a wide range of temperatures and are best for higher temperatures. RTDs are designed for lower temperatures and are not generally used above +600 °C. Thermocouples can be used to measure more than 1800 °C, making them a versatile choice for many applications.

RTDs provide the highest accuracy and are used when a temperature measurement accuracy is required to be around ± 0.05 to ± 0.1 °C. This is obtained by using Class A, 1/3 DIN, 1/5DIN and 1/10DIN detectors with 3 or 4-wire construction probes.
Thermocouples have a lower accuracy. Therefore, wherever possible, our high-accuracy thermocouple probes are manufactured from Class 1 type K thermocouple wire which is chosen for improved accuracy and performance and has an accuracy of around ± 0.5 °C between 0 °C & 100 °C.

Due to their design, RTD sensors have a low risk of drift, making them stable over long periods. On the other hand, thermocouples have a higher risk of drift over time. This can be caused by the ageing of the thermocouple wire from temperature cycling or from excess heat. Due to this, regular thermocouple calibration should be considered as part of a planned maintenance program.

A thermocouple sensor usually has a faster response time due to the lower mass of the thermocouple junction, which can be made faster by offering a grounded hot junction or exposed thermocouple junction. 

RTD sensor measurements are calculated by taking the average value along the entire surface of the platinum flat film or ceramic element. This disadvantage is ever reducing as platinum element construction also reduces in size.

Thermocouples are generally less expensive than RTD sensors. However, thermocouples will require regular calibration, which adds to the long-term costs of the product.

Thermistor Probes

Thermistor sensors are manufactured using semiconductor materials which change in resistance to small changes in temperature. The resistance changes are large and can be positive temperature coefficient (PTC) or negative temperature coefficient (NTC). This simply means that the resistance of the thermistor can either get higher or lower with changes in temperature.


Here at ETI we use NTC thermistors that provide accuracy and exhibit large changes in resistance with temperature.

In comparison to RTDs that change resistance with linearity, thermistors have a non-linear change in resistance, however and advantage can be their higher resistance change per degree can provide greater resolution. 

Thermistor probes cover a limited temperature range and provide a high level of repeatability and stability and generally their small size means fast response to temperature changes.


thermistor factors to think of:

±0.4 °C between -20 & 100 °C

±0.3 °C between -10 & 0 °C

±0.2 °C between 0 & 70 °C

±0.4 °C between 70 & 100 °C

Thermistor probes are accurate probes that exhibit a change in resistance with a change of temperature. Thermistor probes have a negative temperature co-effective (NTC), the resistance decreases with the increase in temperature. Thermistor probes cover a limited temperature range and are generally slower to respond to changes in temperature than thermocouples but quicker than PT100 probes.
Thermistor probes tend to be smaller than PT100 probes but larger than thermocouple probes. Thermistor probes are not recommended for the measurement of surface temperatures.

PVC lead is a general purpose lead. FEP (fluoroethylene-propylene) lead is a low temperature lead ideal for working in sub-zero environments to a minimum temperature of -100 °C. As standard and where appropriate, each probe is supplied with a one metre straight PVC lead and is fitted with a compatible Lumberg or binder connector. Maximum temperature for both PVC and PU leads is 80 °C.

Applications quoted are typical for the specific probe, although there are many alternative uses for which the probe could be equally suitable. For advice on a specific probe for a particular application, please contact us (+44 (0)1903 202151 / Where requirements cannot be met from the existing standard range of probes, then bespoke designs can be manufactured.

The Therma Series

The Therma Series is a range of handheld Type K and type T thermocouple instruments which are robust, small and quick to respond to temperature changes. The Type K sensors have a wide range and are the most popular choice for their reasonable price. We recommend them for applications with very high or low temperatures. The Type T thermocouples have a narrower temperature range but slightly higher accuracy. We suggest these for applications where accuracy is more important.


The Therma Series thermometers feature a miniature thermocouple socket allowing users to attach thermocouple probes with miniature plugs. These plugs and sockets are the same across the world and are colour-coded for Type K and Type T. In most of the world, green is used for Type K thermometers, and brown is used for Type T (IEC colours). In the US, yellow is used for Type K, and blue is used for Type T (ANSI colours).

The Therma 20 is an extremely stable thermistor thermometer. Thermistor thermometers are best for applications where the temperature range is narrow, often within catering or hospitality, where temperatures do not go above 150 °C. The Therma 20's main benefit is that it promises assured accuracy for life. This means that ETI guarantees the accuracy will remain stable for the life of the thermometer, giving the end user confidence the thermometer and probe will perform consistently. Many thermistor probe options are available to suit a variety of applications, including waterproof probes for high steam or wet areas. As the thermistor curve is unique to ETI, you cannot use thermistor probes from another manufacturer with ETI instruments.


High system accuracy ±0.4 °C between -24.9 & 109.9 °C. Fitted with a Lumberg plug so can be used with any thermistor probe - see here.

ETI’s range of Therma Metal thermometers is designed for industrial applications where the thermometer needs to withstand harsh environments. Whether this involves salt water, high levels of steam or the potential for impact or to be dropped, the Therma Metal series will survive and continue giving accurate readings with a Type K thermocouple or thermistor probe. The extruded aluminium casing has been powder coated to give a salt-resistant finish, suitable for the most difficult of environments.

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