Thermometers with switch contacts

What are mechatronic measuring instruments?

Where electronic components or assemblies are integrated into mechanical measuring instruments. Thus, there is a purely mechanical on-site display giving either an additional electrical output signal or offering a switch function. The advantage of instruments like this is that, should the power supply or the measuring signal be disrupted or interrupted, the measured value can nevertheless still be read reliably on site.

What is a change-over contact?

When the set point is exceeded, simultaneously one circuit is opened (NC) and one circuit is closed (NO). Changeover contacts are denoted by the number 3 (SPDT).

What is a magnetic snap-action contact (model 821)?

The magnetic snap-action contact is a mechanical physical contact for switching loads of up to 30W 50VA. The signal output is achieved either ahead of or behind the movement of the actual value pointer. To close the circuit, the contact pin of the movable contact arm, just before reaching the set point, is attracted by the action of a permanent magnet mounted on the carrier arm. Through the holding force of the magnet, magnetic snap-action contacts are immune to vibration. To open the circuit, the magnet keeps the contact arm attracted until the restoring force of the measuring element exceeds the magnet's strength and the contact springs open.

What is a reed contact (model 851)?

Reed contacts are often used for switching small voltages and currents, since their hermetically-sealed design, in combination with contacts in inert gas, cannot become corroded on the contact surfaces. Their high reliability and low contact resistance make them suitable for a large number of applications.They include, for example, PLC applications, signal conversion in measuring instruments, signal lights, acoustic signal transmitters and many more.

What is a sliding contact (model 811)?

The sliding contact is a mechanical physical contact for switching loads of up to 10W 18VA.

What is an electronic contact (model 830.E)?

Electronic contacts are fitted with non-contact slot sensors. They are especially suitable for oil-filled gauges and should be preferred for low voltages and small DC loads, such as for the signal input for a PLC (programmable logic controller).

What is an inductive contact (model 831)?

Inductive limit switches in pointer gauges are fitted with non-contact electrical distance sensors (proximity sensors) in accordance with EN 50227. The output signal is determined by the presence or absence of a flag, moved by the actual value pointer within the range of the electromagnetic field of the proximity switch. They are mainly used in hazardous areas.

What is an isolating amplifier?

The isolating amplifier transmits digital signals from the hazardous area. The signal transmitters can be either sensors per DIN 19234 (NAMUR) or mechanical contacts. The input is securely isolated from the output and the power supply in accordance with DIN EN 50020. The output and the power supply are securely isolate from each other in accordance with DIN EN 50178.

What is ATEX?

ATEX is a widely-used synonym for the European Union ATEX directives. The name comes form the abbreviation of the French 'Atmosphere explosible'. The directive currently includes two directives in the field of explosion protection, namely the ATEX equipment directive, 94/9/EC, and the ATEX workplace directive, 99/92/EC.

Which standard regulates contact gauges?

Sliding and magnetic snap-action contacts fitted in pressure gauges and thermometers with housing diameters of 100 and 160 mm are governed by DIN 16085.

Can mechanical dial thermometers with a CE marking be placed on the market?

With CE marking, the manufacturer, the company or person placing the product on the market or the EU authorised representative declares that the product complies with all relevant European guidelines.
Since there is no binding instruction (directive) for mechanical thermometers in accordance with DIN EN 13190, such instruments can also be placed on the market without a CE marking.

Do gas-actuated thermometers fall within the Pressure Equipment Directive and may an appropriate CE marking be applied?


Gas-actuated thermometers fall within the scope of the 2014/68/EU directive (previously PED 97/23/EC), without additional adapted thermowell as “pressure accessories” and also in the non-fitted state or fitted within a thermowell as “pressure vessels”. The classification is based on Article 4, Paragraph 3 of the applicable 2014/68/EU directive (Design, manufacturing and testing carried out in accordance with sound engineering practise). The products should not be marked with CE or their conformity confirmed in respect of the above-mentioned directive. A manufacturer's declaration can be requested if required.

Does the ambient temperature have any influence on the measuring tube within the case of the gas-actuated instrument?

Yes, that is why a bimetal compensation is installed between the movement and the measuring tube. This compensation is technically limited within a specific range. For WIKA gas-actuated instruments the standard ambient temperature is approx. 23 ± 10 °C. Deviating ambient temperatures are possible, but have to be specified.

How does a bimetal thermometer work?

A strip made from permanently-laminated rolled sheets, made from metals having different coefficients of expansion ("bimetal"), will bend as a result of any temperature changes. The bending is roughly proportional to the change in temperature. For bimetallic strips, two different measurement systems have been developed: helically wound or spirally wound. Through the mechanical deformation of the bimetal spring in either of these spring forms, on any change in temperature a rotational movement occurs. If one end of the bimetal measuring system is fixed securely, the other will rotate a pointer shaft. The scale ranges are between -70 and +600 °C with an accuracy class of 1 and 2 in accordance with EN 13190.

How does a gas actuated thermometer operate?

The measuring system of the dial thermometer consists of a stem, capillary and Bourdon tube in the casing. These parts are combined to form a single unit. The entire measuring system is filled with an inert gas under pressure. A change in temperature will cause a change in internal pressure in the stem. The pressure deforms the measuring spring and the deflection is transferred to the pointer via a dial movement. Fluctuations in ambient temperature affecting the casing can be neglected, as a bimetal compensation element is fitted between the dial movement and the measuring spring. The scale ranges are between -200 and +700 °C with an accuracy class 1 in accordance with EN 13190.

How does an expansion thermometer work?

The measured value registration is made through the liquid-filled measuring system which consists of a temperature probe, capillary and Bourdon tube. All three components combine to form a closed tube system. The internal pressure in the system changes with the adjacent temperature. This causes the pointer axis connected to the spring to turn and the temperature value to be displayed on the scale. The capillary, with lengths between 500 and 10,000 mm also enables measurements to be made on remote measuring points. The scale ranges are between -40 and +400 °C with accuracy classes 1 and 2 in accordance with EN 13190.

How long does it take for an expansion thermometer to display the true temperature of the medium?

Rule of thumb: after 90 sec adjustment time, approx. 99% of the measured value has been achieved.

To which standard are gas actuated thermometers and bimetal thermometers manufactured?

Gas actuated thermometers and bimetal thermometers are manufactured to EN 13190. If electrical connectors are built in, DIN 16196 applies.

What does one mean by the active length of a thermometer?

The active length of a thermometer is the length over which the thermometer effectively averages the temperature, through in-and outflows of the heat.

What influence does the ambient temperature have on the measuring result?

This depends on the following parameters:
1. Ratio of the volume of the probe (tube) to the measuring line and Bourdon tube (rule-of-thumb: 99:1)
2. Length of the measuring line (capillary) - the longer it is, the more the influence
3. Magnitude of the ambient temperature (value), high temperatures cause an offset in the display, low temperatures cause a reduction
Possibilities for compensation:
1. via a bimetal compensation spring (opposed to the pointer direction)
2. via error adjustment (only possible if the ambient temperature is known and constant)

What is the maximum length of a capillary for gas actuated thermometers?

Theoretically, the capillary of a gas-actuated thermometer can be up to 100 m long. However, a large probe volume is needed so that the gas-actuated thermometer will operate within class 1. With the expansion thermometer, the maximum length is limited to 15 metres, otherwise the required filling volume would be too great.

What mechanical influences, other than the actuation of switch contacts, can cause measuring errors in bimetal thermometers?

In helical bimetal spring designs, a stroke movement of the pointer may occur, which may lead to the pointer touching on the dial or the window.  With the assistance of modern design and manufacturing techniques, such errors are avoided nowadays.

When does one use a gas actuated thermometer with capillary?

Gas-actuated or expansion thermometers with capillaries are used in locations which are not easily accessible and where long distances have to be bridged. As a protective coating for capillaries, a flexible spiral protective sleeve or PVC coating is available.

Which gas is used as the fill fluid for gas actuated thermometers?


Why can one not use a magnetic snap-action contact with bimetal measuring systems?

Since bimetal measuring systems only offer very low actuating forces.

Why can't the shaft of the bimetal thermometers be manufactured longer than 1m?

Since the weight of the pointer shaft would be greater than the turning bimetal coil (i.e. the pointer would no longer be able to move).

Why is liquid damping an advantage with high vibration?

Since the pointer is damped and the temperature can be read better.

Why is liquid damping an advantage with negative ambient temperatures?

Since with unfilled instruments, there can be a possible build-up of condensing water, the window can become steamed up. This is not possible in filled instruments.

Why must the clearance between the thermowell stem wall and the bimetal coil be as low as possible?

Since the heat transfer from the wetted thermowell stem to the bimetallic spring is improved.

What are mechatronic measuring instruments?

Where electronic components or assemblies are integrated into mechanical measuring instruments. Thus, there is a purely mechanical on-site display giving either an additional electrical output signal or offering a switch function. The advantage of instruments like this is that, should the power supply or the measuring signal be disrupted or interrupted, the measured value can nevertheless still be read reliably on site.

What is a change-over contact?

When the set point is exceeded, simultaneously one circuit is opened (NC) and one circuit is closed (NO).

What is a micro switch?

By a micro switch, one refers to an electrical switch whose contacts, when open, have a maximum clearance of 3 mm.

What is meant by switching function?

Under switching function, we mean the opening or closing of an electrical circuit. Normally-closed contacts interrupt a circuit on rising temperature (clockwise); normally-open contacts close a circuit on rising temperature (clockwise).

Are there any GOST certificates for thermowells?

No. GOST certificates only exist for measuring instruments and a thermowell is only considered a component part of a thermometer.

Do thermowells need to be CE marked?

Thermowells must not be CE marked, in principle. An exception as a result of its special design is the model TW61 thermowell with DN>25, suitable for orbital welding. This must be CE marked in accordance with the Pressure Equipment Directive (2014/68/EU).

How high is the permissible pressure loading for thermowells?

In the Appendix to DIN 43772 are loading diagrams, from which, depending on temperature and medium, can be taken, the maximum allowable pressure load for the different geometries. If the conduit geometry does not correspond to DIN 43772, individual calculations can be performed in accordance with ASME PTC 19.3 or Dittrich / Klotter, which as static results include the max. pressure loading.

What are suitable materials for thermowells for negative temperatures?

The first choice for high-temperature applications should always be stainless steel, such as 1.4404 or 316L. (Approval per AD2000 W10 down to -270 °C) Carbon steels should be considered carefully in detail, through the effect of the drop-off effect.

What are the factors influencing the response times of thermowells?

Put simply, one can say that the more stable a thermowell is constructed, the slower it reacts to temperature changes. In order to optimise the response time, there are thin wall thicknesses and low air space between sensor and the bore's interior walls. Further optimisations in design are pocket-drilled bottoms and effective insertion lengths of more than 100 mm.

What is the difference between one-piece and fabricated thermowells?

Fabricated thermowells are manufactured from tubes which are sealed by a solid welded tip (for example) at the process. Single piece thermowells are manufactured from a complete element of bar stock (round or hexagonal).

What is the maximum insertion length for a thermowell?

For multi-piece thermowells, the maximum length is limited by the manufactured lengths of the tubes, which is about 5-6 meters.  One piece thermowells are made of solid material and limited by the production length of the drill hole, which, for each product is between 1,000 mm and 2,000 mm. . Longer one-piece thermowells have to be made by welding individual elements together.

What is the maximum permissible temperature for thermowells?

The maximum temperature depends on the materials used and the standards which must be met.  So, for example, a standard stainless steel can be used in air up to about +900 °C, the maximum operating temperature is approximately +600 °C and an approval can be made up to +450 °C.

What is the minimum insertion length of a thermowell?

The insertion length of a thermowell will be specified through the thermometer  used. In general one can assume a length of  60-100 mm for mechanical thermometers from a minimum total length. Electrical thermometers need an insertion length of at least 35 - 50 mm. Each individual case should be checked, though.

What tests and inspections are stipulated for thermowells?

In accordance with DIN 43772 Point 4.6, all tests and certifications should be agreed between the manufacturer and operator.

When are one-piece or fabricated thermowells typically used?

Fabricated thermowells are generally recommended for low to medium process loads. One-piece  thermowells are suited to the highest process loads, depending on their design. Thus internationally or in the petrochemical industry, one-piece thermocouples are now used almost exclusively.