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Bi-Metal Thermometers & Measuring Process Manufacturing

The United States holds a record of 12.69 million [1] jobs in manufacturing.

Needless to say, technology and engineering play important roles in the manufacturing process. Making sure your process is performing well and staying safe is even more crucial. How do we make sure everything is working properly? We do so by measuring the process variables of temperature, pressure, level, and flow in critical areas.

In this article, we’re going to explain the importance of measurement in the process of manufacturing, what bi-metal thermometers are, and why they’re so useful in the manufacturing industries.

The Importance of Measuring

Measurement of what is going on inside of a process is the only way to truly control that process. Measurement is a tangible way to retrieve critical process data and connect all the different moving parts that make up the manufacturing process.

Most importantly, the measurement of process variables makes it easier to find the area or part of a process that is not performing optimally or safely.

At any given time in the manufacturing process, dozens of different “steps” of the process can be happening. Looking at the data produced by the process of measuring equipment allows an operator to determine where a problem may exist, then take proper corrective action based on what they see occurring.

The keyword in all this is data. Measurements give operations personnel data that translates into useful information about the quality of the process or processes taking place. Measurements are a company’s way of making sure everything runs well from beginning to end.

But how do we measure manufacturing processes?

How To Measure Variables in the Manufacturing Process

The main process variables measured in any given manufacturing process are temperature, pressure, level, and flow. The data generated from measuring these 4 variables are the key to controlling any manufacturing process. The number 1 measured process variable is temperature because this variable can be used to determine the efficiency and safety of many manufacturing processes. Chemical manufacturing, production of Hydrocarbon based fuels like gasoline, and air separation are all good examples of processes heavily dependent on temperature measurement and control.

There are 2 ways to measure temperature in a process. 1) Mechanical measurement with a thermometer and 2) Electronic methods such as an RTD or Thermocouple [2]. In most cases even if an electronic device is installed to give an electronic signal back to a control room, operators will prefer to have a mechanical thermometer as a visual check.

Bi-Metal Thermometers

Bi-metal thermometers were first developed in the 18th century as a simple method used to predict and ultimately measure temperature. But, how does it work?

How Does It Work

The bi-metal thermometer works by having a bi-metallic strip wound tightly into a helical coil or helix. Then the coil is placed inside a metal tube. When the temperature around the thermometer rises, the helical coil expands and tries to unwind. Thermal expansion [3] of the helix continues throughout a fixed predictable rate given the metallurgy of the helix. When we connect the helix to a pointer mechanism, we can translate this expansion into a temperature measurement.

Bi-metal thermometers are limited in range because of the limited area available to the helix in the expansion tube, and because the metal used to make up the helix is only linear in a certain range. For this reason, most thermometers have a fixed range, usually around 400 degrees F. By changing the metallurgy of the helix, we can expand this range to a maximum range of 700 degrees F.

Bi-metal Specifications

There are a few specific features of the construction of bi-metal thermometers we can change and specify to configure the device for a given application. Let’s explore those features in detail.

Bi-metal thermometers have a dial. This dial can range from 1 inch to 6 inches in diameter, although the most common sizes range from 3 inches to 5 inches. Like most mechanical gauges, larger size can translate into higher accuracy, but check the specifications to be sure.

The dial is located in a protected housing usually covered by glass or safety glass (or even polycarbonate or acrylic plastic). Dials can be mounted to the stem known as a fixed dial or can be mounted to an adjustable arm known as an any-angle dial which
allows for the angle adjustment of the dial to facilitate better visibility.

The dial face can vary in features as well. Some dial faces can be painted with luminescent paint to aid in low light visibility, and have a read switch attached offering a switch closure as a rudimentary “alarm” at a given setpoint. Tattle Tale Pointers or secondary pointers designed to indicate the maximum range the thermometer has seen are another option. Variations change from manufacturer to manufacturer, and not all features mentioned are available in a single device.

The final consideration when specifying a thermometer is the length of the stem or insertion depth of the stem. This is typically dictated by the length of the thermowell or protection tube the thermometer will be inserted into. The thermowell is a whole other topic of conversation, but in short, the length of the thermowell will depend on the pipe size of the process and the velocity of the fluid moving in the process. Consult our blog on thermowells, view our video on thermowells [4], and certainly adhere to the industry standard ASME 19.3TW for sizing your thermowell.

The Correct Thermometer for your Manufacturing Process

Whether generating power, manufacturing plastics, blending chemicals, processing food, or any number of other manufacturing processes, measurement tools like the bi-metal thermometers are invaluable devices. Mechanical measurements like a bi-metal thermometer ensure operators of any process will have a quick visual indication of what is occurring at the point of measurement.

Thank you for taking the time to read this short piece on these invaluable devices. We hope this gives you some foundation for purchasing these devices in the future. As always, if you have questions, please reach out to us [5], check out our blog [6], or watch some of our informational videos on this topic on our YouTube Channel [7]. We are here to help.

 

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