What is a pressure gauge?
A manometer is a measuring instrument that allows knowing the manometric pressure of a fluid (whether gas or liquid) that is enclosed in a container; When you have a fluid enclosed in a container, in an industrial application, it is vital to measure and know the pressure and temperature.
What is a thermometer?
A thermometer is an instrument whose usefulness is to measure temperature, through various mechanisms and scales. Temperature measurement can be carried out using different technologies that are based on the variation of certain physical properties as a function of temperature: expansion, pressure, electrical resistance, infrared radiation, etc.
It is important to keep in mind that most thermometers actually indicate their own temperature, or at least the temperature of their sensing element. For this to correspond to the temperature of the fluid to be measured, the thermometer and the fluid must reach thermal equilibrium.
Manometers and thermometers are used in a wide variety of applications in industry, ensuring that production systems and processes operate within determined parameters. In numerous industrial operations, highly aggressive process media are handled and must be carefully treated to prevent safety risks and environmental contamination.
This is how the design of manometers and thermometers is largely determined by the process media and application conditions, and these different types of fluids and conditions can affect the instrument both physically and in its operation.
Selecting an unsuitable manometer and/or thermometer can have very serious consequences. Firstly, the useful life of an instrument can be significantly reduced, resulting in higher maintenance costs. And secondly, inaccurate measurement can lead to process deviations, accidents and costly downtime.
In addition, a defective pressure gauge or thermometer can cause the process fluid to escape, which, if dangerous, can put the safety of operators and facilities at risk.
So, how do you know what type of manometer or thermometer to choose when you need to measure pressure and/or temperature in a certain process? The necessary time must be dedicated to carefully study and consider the different factors that could affect the meters.
Factors to consider to ensure proper selection based on application
1. Compatibility with the fluid: It must be carefully taken into account that it is going to be measured in the pipe or tank so that you can select the materials compatible with the process fluid. All parts in contact with the medium of the instrument must be made of materials resistant to corrosion to a greater or lesser extent depending on the fluid. 316L stainless steel is quite resistant, but for specific applications there is a wide variety of materials such as Hastelloy, Monel, etc.
For thermometers, thermowells can be used to protect the stem of the instrument.
2. The temperature of the fluid: Depending on this factor, the appropriate thermometer will be chosen, the minimum and maximum temperature must be taken into account.
A diaphragm separator has a flexible membrane to separate the measuring fluid from the sensitive element of the pressure gauge and provides temperature dissipation.
3. Environmental conditions: For very aggressive environments, instruments with special materials such as Monel are recommended. In these cases it is advisable to select instrumentation approved by NACE (National Association of Corrosion Engineers).
4. Service pressure ranges: Stainless steel pressure gauges from diameter 100 can only occasionally withstand an overpressure of 30%. Best practice is to select a range that is approximately twice the normal service pressure. If, for example, the normal service pressure is 8 bar, the pressure gauge must have a final value scale of 16 bar or more.
5. Conditions that affect wear: The relevant conditions that cause the greatest wear are mechanical vibrations, extreme temperatures, pulsations and pressure peaks. In case of vibrations, a liquid-filled box is recommended. To counteract strong pulsations, a pressure limiter or valve must be mounted.
For these cases, choosing a non-contact thermometer is a good choice since they allow you to measure the temperature from a distance, without direct contact with the fluid. This type of thermometer works by infrared.
6. Selection of gauges designed for a particular service: Process gauges are designed with Heavy Duty type tubes and other components. Avoid applying standard types in extreme applications to avoid tube breaks.
7. Select robust instruments with appropriate safety features: Some applications require precautions against internal burdon tube breakage, even if the probability is very low. An explosion of the instrument towards the viewfinder side could cause serious risks to operators due to proximity. In this case, safety visors and safety boxes that channel the shock wave toward the dorsal side are recommended.
8. Select a manometer or thermometer with the required accuracy: Industrial processes require exact conditions for optimal results. Most processes require accuracy within 1%. This accuracy must be periodically verified to maintain process safety.
9. Select a meter with a readable dial: It seems obvious, but in practice instrumentation with an indication that does not correspond to current regulations is often seen. Only instruments that comply with international design regulations should be selected, this is the only way to ensure correct visibility.
10. Install the instrument correctly: Last but not least is the assembly. The pressure gauge and/or thermometer must be oriented to ensure optimal reading and guarantee the tightness of the connection. Sealing gaskets often have to be installed with the appropriate material.
