Month: January 2023

A thermistor is an element that senses temperature and is made up of sintered semiconductor material which displays a big change in resistance in proportion to a small alteration in temperature. They normally have negative temperature coefficients meaning the resistance goes down as the temperature goes up.

Thermistors are constructed using a combination of metals and metal oxide materials. Once they are mixed, the materials are formed and pushed into the required shape. They can then be used naturally as disk-style thermistors without any changes needed. Alternatively, they can be further shaped and put together with lead wires and coatings to create bead-style elements.

How do thermistors compare to RTDs?

Whilst RTDs change resistance in an almost linear way, thermistors have a very non-linear change in resistance and reduce their resistance as temperature increases. They continue to be a popular way of measuring temperature for several reasons including:

• Their increased resistance change per degree of temperature offers better resolution
• There is a high level of repeatability and stability
• Impressive interchangeability
• Their small size means they can respond faster to changes in temperature

Thermistors typically have two types of coatings: epoxy for use in lower temperatures (-50 to 150C) and glass coatings for higher temperature applications (-50 to 300C). These coatings are used mechanically to protect the bead and wire connections whilst offering a degree of protection from corrosion and/or corrosion too. A very small diameter and solid copper or copper alloy wires are normally supplied with thermistors. In most cases, these wires are tinned to allow for easy soldering.

Base resistance

NTC thermistors decrease in resistance with increased temperatures. This is also the case for how much resistance change per degree it will provide. For quite low temperature applications (-55 to around 70C) lower resistance thermistors are generally used. Higher resistance thermistors are used for higher temperature applications in order to optimise the resistance change per degree at the temperature that is needed. These elements are available in a range of resistances and “curves” and resistances are typically specified at 25C.

General purpose

General purpose designs can be work in a broad range of applications. From electronic equipment to structures, processes and design, and reliability testing uses, sensors of this style are easy to fit and monitor over time.

Liquid immersion measurement

Thermistors need to be protected from corrosion when they are exposed to liquids as well be carefully positioned in the fluid so it will arrive at the required temperature. This is normally done using closed ended tubes and housing that are specifically designed for the job. You need to take care and ensure that there is a clear thermal path to the thermistor and that thermal mass is as small as it can be.

How can TRM help?

At TRM, our team provide engineered solutions that perform at the highest temperatures and in the harshest environments. We are specialists in all thing’s temperature measurement, trace heating, and fireproof wiring, offering a range of useful products like MI thermocouple cable to ensure our customers get the solutions they need in their business operations. Contact us today to discuss your specific requirements and what we can do for you.

 

A resistance thermometer or resistance temperature detector (RTD) is a device which measures temperature through the resistance of a conductor. Resistance of the conductor can vary with time. It is this property of the conductor that is used for industrial temperature measurement. The RTD’s primary purpose is to produce a resistance alteration in response to temperature.

Metals usually exhibit high temperature coefficients, indicating resistance increases as temperature rises. On the other hand, carbon and germanium typically demonstrate low temperature coefficients, thereby showing a resistance decrease with increasing temperature.

The material used in a resistance thermometer

The resistance thermometer has a sensitive element that is made from the purest metals such as platinum, copper, or nickel. There is a direct connection between the resistance of the metal and temperature. In most cases, platinum is used in resistance thermometers due to its high accuracy, stability, and its ability to withstand extremely high temperatures.

Metals like gold and silver are not used for RTD heat trace because they don’t exhibit the high resistance that is needed, they have low resistivity. Whereas a material like tungsten has high resistance but is very brittle. Copper is often used to make the RTD element as it has low resistivity and is a cheaper metal. The only downside to copper is that is has less linearity.

The maximum temperature of the copper is around 120C. An RTD material is made from either platinum, nickel, or nickel alloys. Nickel wires work well within a certain temperature range but are not linear. The RTD sensor requires a conductor with a high resistivity so a small amount of conductor volume can be used. The resistance should vary as much as possible with temperature.

How can TRM help?

If your business manufacturing processes need heat management then temperature monitoring and control will be extremely important to you. Our high-temperature cable and measurement sensors can suit practically any environment in any industry.

The team at TRM are experts in heat management and industrial temperature measurement, offering services relating to design, supply, and installation as well as providing a wide selection of temperature-related products. Contact us today to discuss the specific requirements of your operations and find out how we can help you.

A J type thermocouple is one of the most common and applicable thermocouple types. The most important thing to know about this thermocouple is that is has a small temperature range and a reduced lifespan if used at higher temperatures. It has positive leg which is made from an iron wire and negative leg consisting of a constantan (copper 55% and nickel 45%) alloy wire. It is the Curie Point of the iron at 770C that gives type J its restricted temperature parameters of -40C-750C. 

If this thermocouple is used in an oxidising environment at a high temperature the iron will change on a molecular level and no longer have its normal voltage output versus temperature. It won’t return when the iron is cooled down either. The cost and dependability of this thermocouple mirrors the type K. However, in order for the J type to work properly reduction atmosphere would be best and it shouldn’t be used at particularly low temperatures either.  

The linearity of a J type thermocouple can vary by -70C across its full range from -210C to 1200C. It has a particularly straight section from 100C to 500C which veers off at around -0.5C. Both the lower and higher ranges can be lengthened with linearity loss.  

What are the downsides to type J thermocouples? 

• Can’t be used for temperatures that are over 760C 
• They have an iron wire in one leg which means it will form rust in humid conditions and the rust can lead to incorrect readings or at worst an open circuit 
• They can become oxidised, so it is not the right option for damp areas or low temperatures 
• If used at a higher temperature than 760C there will be a quick magnetic alteration that will cause irreversible recalibrations 

Summary 

J type thermocouples are great for a lot of industry applications, but they have their limitations and in specific cases they won’t be the right fit for the job. If you need help understanding thermocouples and what your business operations need in terms of temperature measurement and control, TRM are here to support you. Contact us today to discuss your needs and our team of specialists will be on hand to implement the right solutions. 

 

Sensors that measure temperature can come in a wide variety with different features, but they all have one thing in common: they all measure temperature by checking for some change in a physical characteristic. In this article, we will be going through each type of industrial temperature sensor and how it works.

How to measure temperature

Thermocouples

Thermocouples are an essential part of high temperature measurement. They are voltage devices that measure temperature with a shift in voltage. As temperature increases, the output voltage of the thermocouple rises – not always linearly. Typically, a thermocouple is placed inside a metal or ceramic shield to guard against various conditions. Metal-sheathed thermocouples can include coatings such as Teflon, which permit use in acidic and caustic solutions.

Bimetallic devices utilise the heat-induced expansion of metals. Combining two metals and connecting them to a pointer, one side of the strip will grow more than the other when heated. When it is geared correctly to a pointer, the temperature measurement is shown.

Bimetallic devices have the advantages of being easy to transport and independent of a power supply, but they are less accurate than electrical devices. You can’t easily record temperature values with bimetallic devices, but portability can be a useful advantage.

Thermometers

Thermometers are liquid expansion devices used for temperature measurement, and there are two main types: mercury and organic. Mercury devices have specific limitations in how they can be safely transported. For instance, mercury is thought to be an environmental contaminant, so breakage can be dangerous.  Before shipping mercury products, check to see if there are any current restrictions on air transportation.

Change of state sensors

These temperature sensors measure exactly what the name suggests, a change in the state of a material brought about by a shift in temperature, like a change from ice to water and then to steam. Devices like these that are available on the market include labels, pellets, crayons, or lacquers.

For instance, labels might be used on steam traps and when the trap needs altering it gets hot, then the white dot on the label will highlight the rise in temperature by turning black. The dot will stay black, even if the temperature goes back to normal.

Change of state labels measure temperature in both Fahrenheit and Celsius. The white dot changes to black when it surpasses the temperature indicated, and it won’t change back. Temperature labels are useful to prove that the temperature didn’t exceed a specific point, which is beneficial for engineering or legal reasons during delivery.