Process temperature measurement instruments are governed by the ASME B40.200-2013 standard. This standard is the one that all thermometer manufacturers must comply with and must be adopted by the country’s industries.
The standard as such is divided into four sections:
• Bimetallic thermometers
• Thermometers with filling liquid, includes remote reading ones
• Column thermometers
• Thermowells
Bimetal thermometers
This type of thermometer works using a metallic bi as a sensing element; two thin tapes, each made of different metals, with different expansion coefficients. This set of ribbons are united and supportive, they deform forming a helix. When the bimetallic metal detects a change in temperature, it deforms or untwists. It is attached to the axis of the needle and generates a rotation that is observed on the dial. It is a simple and irreparable system in case of damage, the positive thing is that there is no risk of contamination to the measured fluid.
The elements that the user should know in this type of thermometer are:
Cover: It is the display of the instrument, it is incorporated into the case, which in this case is always round. The diameters are: 1″, 1 ¾”, 2″, 3″ and 5″; the first three diameters are not used in industry, they are commonly used in portable or pocket thermometers; the diameters of 3″ and 5″ are those used in industrial plants. The boxes are normally made of stainless steel, they are Hermetic and can comply, if the user requires it, with any of the NEMA protections. The covers come in the boxes and are protected by glass, which is normally safety glass.
- Bulb: It is a tube, normally made of AISI 304 stainless steel, this tube is immersed in the fluid to be measured. This material is generally used because the bulb, in most cases, has a thermowell, which is what really should be made of a material that is chemically and/or mechanically compatible with the fluid. The bulb has a diameter of ¼”, for bulbs longer than 24″, the diameter must be 3/8″. The length of the bulb is one of the most important criteria, a good selection will generate good results in the measurement. The most used bulb lengths are: 4″, 6″, 9″, 12″, 15″, 18″ and 24″. There are cases in which bulbs larger than 24″ are required, such as 30″, 32″, 36″, 48″. A good practice for selecting the length of the bulb, if you need to measure in a pipe, is that the bulb should be greater than half the nominal diameter of the pipe. When installed in an elbow, vertically, it is recommended that the bulb be greater than half the advance of the elbow.
- Range: The range of a thermometer should be one of those presented in the table below. If a special scale is required, the manufacturer will increase its price. The common ranges are expressed in Celsius or Fahrenheit, in many cases it is common for them to have a double scale face.
- Precision: It is defined as the maximum deviation of the instrument reading with respect to a standard value. The precision classes are Grade AA, A, B, C and D, to understand these grades you must consult the table illustrated in Figure 5.
Connection: Bimetallic thermometers have two types of connection to the process, they can be vertical or rear, it all depends on the place where it is going to be installed; Some brands come with an equalizable connector, that is, it has both positions. The most used connector is male NPT or BSP threads, ¼” ½”.
Bimetallic thermometers are the best mechanical solution for measuring temperature in an industrial process, being also the most economical.
Column Thermometers
This thermometer was developed in a simple way, the sensor can be an organic liquid, an alcohol or in the past mercury was used; Any of these are encapsulated in a glass tube that is mounted on a scale, which can be in one or two units, the most common is degrees Celsius or degrees Fahrenheit.
Due to its construction, this thermometer can be used in healthcare industries. A very common use of these thermometers is in industrial air conditioning equipment.
The elements that the user should know in this type of thermometer are:
- Case and face: In these instruments it is an elongated rectangular panel; The box is normally plastic, the cover or scale is printed on an aluminum plate and the whole is protected by glass that may be safety glass. This box is airtight and can comply, in some cases, with NEMA protections. The most common covers are 7”, 9” and 12”, this dimension refers to the height of the rectangle.
- Bulb: It is a glass tube. This type of thermometer is really nothing more than a traditional glass thermometer, it is simply very protected to be used in the industry. The glass bulb is encapsulated in a plastic thermowell and this assembly, in turn, is inserted into the thermometer bulb itself, which is normally made of bronze or AISI 304 stainless steel. As it is not the most suitable thermometer for heavy industry, it is not common to use thermowells or bulbs made of special materials.
- Range: Since Mercury stopped being used, the thermometer is filled with different organic liquids with a high temperature coefficient. The ranges in which it is convenient to use column thermometers are -40°F to 1000°F or -40°C to 204°C; manufacturers normally supply these thermometers with a double scale.
- Accuracy: For these thermometers it can be classes A, B or C. See Figure 5.
- Connection: Column thermometers have two types of connection to the process, it can be vertical or rear, it all depends on the place where it is going to be installed; Some brands come with an equalizable connector, that is, it has both positions. The most used connector is male NPT or BSP threads, ¼” ½”.
In general terms, the bimetallic thermometer is the most used in the industry, as it is more robust and has a greater field of application, in ranges, in protection and use. The column thermometer is less robust, which makes it suitable for use in clean areas and for less demanding applications.