Key points
In spite of the continuing advance of electronics, mechanical temperature measuring instruments still play a significant role in the monitoring of industrial processes. They are reliable and not power dependant and enable a quick on-site verification (the question about which is the most efficient thermometer for each application, can only be answered with some clarification).
A quick look at today”s process instrumentation shows that two measuring principles are used predominantly for temperature control: bimetal and gas actuation.
The heart of a bimetal thermometer is a metal strip, deformed into a helical or spiral tube, also referred to as the helix. This consists of two sheets of metal, with different coefficients of expansion, rolled together inseparably. With any temperature change, the strip deflects approximately in proportion to it, and due to the spring form, a rotational movement results. While one end of the measuring system is clamped, the other turns the pointer shaft, usually by means of a transmission movement for a smoother pointer travel.
The measuring system of a gas actuated thermometer consists of a stem, a capillary and a Bourdon tube within a case. This sealed unit is filled under pressure with an inert gas, usually helium. With any rise or fall in temperature, the internal pressure changes and thus the shape of the measuring tube – in accordance with the “Bourdon” effect. The deflection from this is then transferred to the pointer via a movement.
Process temperature is not everything
What criteria now plays a role in selecting a measuring principle?
The first parameter is the process temperature. Bimetal thermometers work in a typical range of -70 °C … 600 °C, gas actuated thermometers, cover a range of -200 °C … 700 °C.
However, you must also take the ambient temperature into effect consideration. It has no influence on measuring results with bimetal thermometers, therefore, these instruments can be used with external conditions down to -50 °C. This is not the case with gas actuated thermometers: The lower the volume of gas, the harder it can be to maintain the EN 13190 accuracy class 1 that WIKA guarantees as a manufacturer. For an accurate measurement, the diameter of the stem must be sufficiently large (≥ 8 mm) and the active part of the thermometer (i.e. the temperature sensor filled with gas) be sufficiently long (≥ 100 mm).
This rule points us generally in the direction of the “insertion length”. For both thermometer types, the active part – with bimetal instruments this is the solid tip and helix – must be completely immersed in the medium. Otherwise, this could lead to measurement errors. Therefore, for optimal temperature measurement, the insertion length and the stem diameter should be sized as large as possible.
Differing response times
Another important parameter is the reaction or the response time of the thermometer. Gas actuated instruments are fast: Depending on the design of the measuring point, the value can be read safely after only one or two minutes. Bimetal thermometers can seem slower sluggish in comparison: They require much more time to align themselves to the medium temperature.
Depending on the medium and its temperature, working pressure and flow rate; for specific processes, measuring instruments need to be combined with thermowells. These have a significant effect on the response time of the thermometer. Where thermowells already exist, their dimensions should be given so the thermometer sensors can be designed properly. For their length, a meaningful limit should be observed. The fixed insertion length for a bimetal thermometer is a max. of 1,000 mm, while the maximum for gas actuated thermometers is 2,500 mm.
Dealing with vibration
Since thermometers must be readable under the influence of vibrations, the choice of suitable measuring instruments is significantly reduced. In such cases, manufacturers like WIKA should move to a bimetal thermometer without a transmission movement inside. As a result, the components suffering wear are kept to a minimum. A filled case and a smaller nominal size are also advantageous.
In the first phase of a new project one is often not sure of the final connection location of the probe: axial, radial, or even upside down. Anyone who wants to guard against all uncertainty from the outset should grab a thermometer with an adjustable stem and dial. Thus the display can be adjusted precisely to the desired viewing angle.
“Remote transmission” of the measured values
Gas actuated thermometers offer the opportunity to use capillaries to access difficult-to-reach locations or to bridge long distances. Via a very fine capillary with an internal diameter of only 0.2 mm, measured values can be transmitted remotely up to 60 meters to the display. With the help of a contact bulb, temperature readings can be taken on even the smallest diameter pipe – without any direct contact with the medium. Thermometers with capillaries are also excellent for being located centrally in an instrument board, a control cabinet or on a control panel.
Once the most efficient instrument has been identified for the specific requirement, the question on how to connect it to the process must be clarified. This is why the various different designs are available. Examples include having the threads directly onto the case in order to absorb vibration, and rotatable female or male threads for fitting to the thermowell. A compression fitting, on the other hand, enables the insertion length to be set to requirements, both for thermometers with a rigid stem and instruments with a capillary. In addition, there are aseptic connections for sensitive processes in the food, bio and pharmaceutical industries.
With electrical signal
As paradoxical as it sounds: In all industries, the on-going computerisation of process monitoring has given added impetus to the use of mechanical thermometers – through their combination with electronic components. These instruments offer switch contacts, which open or close an electrical circuit depending on the pointer position. They can be used for various monitoring functions, such as the control and supervision of processes, or to activate an alarm when a measured value either falls below or exceeds a pre-set value.
Switch contacts are fitted under the dial and can be set over the entire scale range using the set pointer.
The instrument pointer moves freely across the entire scale range, independent of the setting. The contacts can be freely adjusted, using a removable key, through the window. As a result of cramped installation conditions and ever-increasing cost pressures, the demand is growing for instruments with “two-in-one” functionality. These combine all the advantages of an on-site display, free from an external power supply, with the need for electrical signal transmission for the latest electronic data acquisition. Thus users save on a second measuring point. This is a major advantage since, as mentioned, many measuring points must be designed in combination with a thermowell.
Wide offering for individual solutions
The criteria outlined here for thermometer selection make it clear that off-the-shelf solutions would not lead to a satisfactory result. Almost every application ultimately results in individual instrumentation. For this reason, WIKA can reproduce, for example, about 600 different bimetal helixes for the various application-specific thermometers. And for gas actuated instruments, there is a large range of standard components available. But its core, the measuring unit, must be individually “tailored” for each operating condition. Using specialised software, all parameters (e.g. filling pressure, tube travel, bimetal compensation or dial) are calculated and harmonised with each other.
WIKA, manufactures a wide variety of mechanical temperature instruments which is essential in order to fulfil the diverse and individual market requirements. To reconcile this with high quality and reliable delivery times, are challenging targets for which WIKA has optimised its flexible production system.
