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TechTime is a Neptronic publication aiming to inform you of common issues highlighted by our technical support team and the best way to deal with them. Each edition features three new topics and will be posted on neptronic.com.

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It's All About the Curves

Choosing the right temperature sensor

There are many temperature sensors on the market and it can be challenging to choose. To simplify, we can break it down into three categories: class, resistance, type or curve.

 

Class: It can be either a Resistance Temperature Detector (RTD) or a Thermistor. Neptronic uses thermistors, which have a negative temperature coefficient (NTC). This means that when temperature rises, the resistance value decreases.

 

Resistance: The thermistors are rated with the resistance value when temperature is at 25°C (77°F). Neptronic uses two resistance values: 3.3kΩ and 10kΩ (most products use the 10kΩ thermistor). So when looking at thermistors, a 20kΩ thermistor's resistance value at 25°C (77°F) will be 20,000Ω, a 10kΩ will have a resistance value of 10,000Ω

and a 3.3kΩ will have a resistance value of 3,300Ω.

 

Type or Curve: Each thermistor manufacturer has their own standard for resistance values outside the 25°C (77°F) base line. This means that for a type 3 thermistor, there can be a small difference in resistance values between manufacturers. Neptronic uses the type 3 or G, which have identical curves.

 

Where can I find the information?

Neptronic uses two remote temperature sensors; STR1 and STC8. The resistance values are available on the Specification and Installation Instructions for both products. These documents are available on our website. (Click here for: STR1 or STC8)

 

Can I use a 10kΩ type 2?

So now that we know the thermistor type required to work with Neptronic controllers (3.3kΩ and 10kΩ type 3 or G), can a 10kΩ type 2 work as well? As mentioned previously, there are some differences between manufacturers within the same type.

 

For obvious reasons, there are also differences for different types. To answer the question, the temperature range must be established. If the application is for an office, the chances are the temperature range will be between 15 and 30°C (59 to 86°F).

 

When looking at the table below, we can see that the resistance values for the selected temperature range is very similar. These differences are negligible and should not offset the room temperature reading. However, we can see that the resistance values start to differ when moving away from the 25°C (77°F) temperature baseline. Therefore, the 10kΩ type 2 would work for this application but it will not fit all applications.

 

All thermistor manufacturers should have a reference table with resistance values. If you are not sure, you can join the specification and installation instructions of the STC8 or STR1 temperature sensor along with your demand.

 

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To learn more about the changeovers on the EVCB series controllers, visit our website.

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Patrick Lafrance

Technical Support – HVAC Controls

Contact him at this email address patrick.lafrance@neptronic.com