General information on temperature measuring devices

Area of application

Different device versions are available depending on the requirements and operating conditions. Below we provide some information on the assessment and commissioning of our devices.

Selection criteria

The user must ensure that the correct temperature measuring device is used with regard to

• Display range and

• Design (e.g. resistance of the materials to measuring substances, measuring range, etc.)

is selected.

The regulations applicable to the application must be observed.

Measuring principles

Mechanical temperature measuring devices are simple, robust and reliable. They do not require any external energy to function. Bimetal systems or gas pressure or liquid measuring systems are generally used for dial thermometers.

Bimetal thermometer

The bimetal thermometer essentially consists of the temperature sensor with a fixed bimetal coil and a display unit with dial and pointer that is permanently connected to it. The 270° rotation of the bimetallic coil is transmitted directly to the pointer via a shaft. Display ranges between -100°C and +600°C can be selected using various combinations of bimetal materials. Designs and dimensions are specified in EN 13190.

The following versions can be offered with bimetal:

• Bimetal thermometer with axial immersion tube (bimetal coil is directly connected to the pointer via a guide shaft)

• Bimetal thermometer with vertical immersion tube (bimetal coil and pointer are connected to each other via a 90° deflection spring or, in industrial versions, with an angle mechanism)

• Bimetal thermometer with adjustable joint shaft (bimetal coil and pointer are connected by a deflection spring and can be adjusted up to 90° by a joint)

Function of the bimetal

The bimetallic coil consists of two firmly connected metal strips with different temperature expansions. These strips are wound in a spiral. A suitable choice of winding length, diameter and material pairing ensures that a rotary movement of 270° is achieved at the nominal temperature range. Bimetal coils are available for continuous operating temperatures up to 500°C. Short-term loads of up to 600°C are permissible for devices with corresponding display ranges.

Spring thermometer

Spring thermometers essentially consist of the immersion tube with a pressure vessel and the display unit. Both parts are connected to each other via a capillary. The vessel is the active part of the immersion tube and is filled with a gas or liquid as an expansion medium. A temperature change in the boiler causes a deflection of the worm-spring measuring mechanism and is thus displayed. Different measuring ranges can be achieved by selecting the appropriate boiler:

• Systems with gas filling: -200°C to +400°C

• Systems with liquid filling: -50°C to +600°C

Designs and dimensions are based on EN 13190. The following versions can be designed as spring thermometers:

• Spring thermometer with vertical immersion tube

• Spring thermometer with axial immersion tube

• Spring thermometer with adjustable articulated shaft (immersion tube and indicator can be adjusted up to an angle of 90° to each other)

• Spring thermometer with capillary (immersion tube and indicator are connected to each other via a capillary)

Worm-spring measuring unit

This consists of the worm spring, a spirally wound flat tube made of special steel and the pointer mechanism. A change in pressure or volume of the expansion material in the boiler causes a deformation of the worm spring, which is converted into a rotary movement of 270° by the pointer mechanism and displayed. The volume of the worm spring is extremely small in order to minimize the influence of the ambient temperature on the display.

Immersion tube

The immersion tube consists of a closed tube body made of steel or stainless steel. The connection fitting, which is available in various threaded versions, is located in the upper section of the immersion tube. The area of the pressure vessel is considered the active part of the immersion tube, which must reach the process temperature completely.

Transmission line

The capillary made of steel, copper or stainless steel connects the immersion tube to the display unit. In order to minimize the display error caused by fluctuating capillary temperatures, gas thermometers should have the largest possible boiler volume. In the case of liquid thermometers, a capillary with a double capillary and a special compensation mechanism can eliminate the display error caused by external temperature effects. This method is called full compensation.

Display and measuring ranges

The display range and the measuring range differ for temperature measuring devices. The error limits are maintained within the measuring range. The temperature measuring device should therefore be selected so that the measuring range is not exceeded during normal operation. The corresponding display and measuring ranges can be found in the table under Display ranges, accuracy classes and error limits (extract) in accordance with EN 13190.

Error limits

The error limits for thermometers are divided into 2 classes (1 and 2) and are defined in standard EN 13 190. See also display ranges, accuracy classes and error limits (extract) in accordance with EN 13190.

Ambient conditions

Vibrations

If vibrations of the temperature measuring device cannot be avoided by suitable installation, devices with pointer mechanism damping or liquid filling must be used.

Corrosive atmosphere

In corrosive atmospheres, suitable housings and components made of resistant materials must be provided. Special surface treatments are also used for external protection.

Thermowells

A particular problem with the installation of temperature measuring points is the transport of heat into or out of the process via the measuring device. As a result, the active part of the immersion tube does not reach the full process temperature, leading to incorrect readings.

If the minimum immersion depth is not specified on the immersion tube, the full process temperature must be present up to the screw connection of the device. For devices with capillaries, the temperature influence on the capillaries must be taken into account. If these are fully or partially exposed to the process temperature, they can be calibrated at the factory. If required, the temperature effect on the capillaries can be calculated.

Various types of thermowells can be found under Thermowell design for universal bimetal thermometers, Thermowell design for bimetal thermometers and Thermowell design for gas pressure thermometers.

Assembly

The load capacity of the thermometer depends on the following parameters:

• Measuring substance

• Medium pressure

• Medium temperature

• Flow velocity

• Installation length and

• Material

Before installation, make sure that the device is correctly installed with regard to

• Process connection

• Media compatibility

• Temperature resistance and

• Measuring range

is suitable. Seals must be suitable for the process connection and resistant to the medium.

To minimize the measuring error due to heat dissipation, the sensor should be immersed as deeply as possible in the medium to be measured.

Vibrations and pulsating media can have a significant impact on the service life of movements. It is recommended to use housings with liquid damping.

When operating with a capillary, it must be laid firmly and with low vibration. The temperature influence on the capillary must be taken into account. If the capillary is exposed to the process temperature, it can be calibrated at the factory.

When installing with thermowells, the thermal resistance between the medium, thermowell and immersion tube must also be taken into account for the heat flow and associated measurement errors. The thermal resistance between the thermowell and immersion tube can be significantly improved by using heat-conducting pastes or oils.

Maintenance and repair

Temperature measuring devices are generally maintenance-free, maintenance of the devices is generally not required.

Repairs may only be carried out by the manufacturer. Before returning a device for repair, the parts in contact with the medium must be carefully cleaned of the medium, especially in the case of hazardous media. The return of goods must be handled using the goods return form.

Additional electrical equipment

Installation and electrical connection may only be carried out by trained specialist personnel. Appliances with additional electrical equipment are marked with a type plate indicating how the electrical connection must be made.

The load limits must be observed. Exceeding them could result in damage. The national and international safety regulations (e.g. VDE 0100) must be observed during installation, commissioning and operation of the devices. Ensure that the cable diameters match the nominal widths of the sealing inserts. Screw connections must be tightened securely. Only then are confirmed degrees of protection given. For versions with angled plugs, plug connectors or cable junction boxes, the central fixing screws must be hand-tightened.

For devices with magnetic snap-action contacts, please note that the CE marking in accordance with the EMC Directive only applies if the switching frequency does not exceed 5 switching cycles per minute.

If specified, suitable isolating switching amplifiers or multifunction relays must be used (e.g. for devices with inductive contacts). The applicable operating instructions must be observed.

Storage

Store and transport the thermometers in dry and clean conditions and, if possible, in their original packaging. Permissible storage temperature: -20°C to +60°C. Avoid shocks and vibrations.

In case of doubt or uncertainty, please contact the manufacturer.

Installation instructions

The manufacturer’s installation guidelines should be observed. When installing the thermometers, it is important to select a suitable installation location, taking into account unrestricted access during installation/disassembly/maintenance and when taking readings. Thermal dead spots should be avoided. The influence of the ambient temperature must be limited by suitable measures so that they correspond to the nominal operating range of the ambient temperature. Ambient temperatures outside this range influence the measurement result and lead to deviations in the display. As far as possible, thermometers should be installed where the influence of vibrations is minimal or non-existent. In the case of thermometers with capillaries, their lines must not be kinked. The manufacturer’s specifications for the smallest bending radius must be observed.