Pressure gauges

Does the height above sea level have any effect on the measuring result of relative pressure measuring instruments?

No, this has no effect, since it is always the pressure differential from ambient that is measured.

How does the measuring system for Model 7 differential pressure gauges with diaphragms behave outside the full scale value?

The plus or minus-sided overpressure safety, up to the maximum working pressure (PN40, PN100, PN250, PN400), is achieved through the metallic measuring element support surface arrangement.  Pressures within the permissible overload range leave no lasting damage on the measuring system.

How low is the temperature of the medium which is stored inside the cryogenic vessel when it reaches the pressure gauge?

A Cryo gas is a gas which is gaseous at ambient temperature and pressure and becomes liquid because of cooling and compression. Due to cooling and compression the volume of the gas can be reduced and thus it’s possible to store a huge quantity inside a vessel.

Typical liquid gases which are stored in closed vessels are nitrogen, oxygen, argon, carbon dioxide, liquefied natural gas (LNG) and nitrous oxide.

The following chart shows the boiling point of these gases at ambient pressure and at 20 bar inside the vessel:

  Ambient pressure  20 bar
nitrogen -196°C -157°C
oxygen -183°C -140°C
argon -186°C -143°C
carbon dioxide Not liquid -19°C
liquefied natural gas  -162°C -107°C
nitrous oxide -88°C -16°C

The lowest temperature for liquid technical gases inside a vessel is -196°C of the liquid phase of nitrogen, if there is about 1 bar inside the vessel. If the temperature of the liquid gas inside the vessel increases, then the pressure inside the vessel increases as well.

A vessel for liquid gases always consists of two vessels: one inner vessel and one outer vessel. There is a vacuum between the vessels which are connected by steel rods. The vacuum isolates the inner vessel from the outside temperature to ensure that the temperature of the medium inside the vessel is less influenced by the ambient temperature.

There are two pipes which connect the inner vessel with the pressure gauge mounted outside the vessel. One connects the upper part of the vessel (gaseous phase) with the pressure gauge and the other connects the lower part of the vessel (liquid phase) with the pressure gauge. As soon as the gas in the pipe leaves the outer vessel, that is the isolated area, it will become gaseous.

Within the pipe, the medium will reach the ambient temperature before it reaches the gauge. As a consequence, the lowest medium temperature in the pipe outside the vessel is the ambient temperature. For our Cryo Gauges the lowest permissible ambient temperature is -40°C. The medium is always gaseous when it reaches the pressure gauge.

What are the "little yellow levers" on the pressure gauge casing for?

Pressure gauges for pressure ranges under 363 psi (25 bar) generally have a filling plug with small levers. These yellow levers are closed on delivery, and must be opened on fitting, in order to prevent any possible pressure build-up in the case. This pressure in the case leads to inadmissibly high accuracy deviations for the gauge.

What can I do if the medium temperature with filled-gauges is over 212°F (100°C)?

You can use a pressure gauge syphon, a diaphragm seal or a capillary line as an additional cooling element in order to lower the medium temperature.

What happens when a model 7 differential pressure gauge with separation diaphragms made of elastomers is operated below the ambient temperature specified in the data sheet?

Below the permissible ambient temperatures, the accuracy deteriorates significantly, since the diaphragm (which is either made of FPM / FKM or NBR) stiffens at low temperatures.

What is the Accuracy class?

The Accuracy class gives the error limits in percent of the measuring span.

What is the difference between a standard pressure gauge and a safety pattern version?

A safety version (code S3 per EN837) has an additional solid baffle wall welded between the dial and the measuring system. In addition, the case has a back wall that can blow out completely. The window is typically made of laminated safety glass. If a pressure builds up in the housing (e.g. from a rupture in the Bourdon tube), this pressure will exhaust completely through the back wall, which is then ejected from the case by the pressure. A release of pressure through the window cannot occur, so there is no risk to personnel through splinters of flying glass. At WIKA these instruments are specially marked on the dial with an "S" in the circle.

What is the function of a three or five-way valve block?

These pressure-equalising valves (with integrated shut-off, purge and vent valves) enable the pressure gauge to be vented on one or both sides and the supply line to be purged.

What is the function of a single-valve block?

With upstream pressure equalising valves it is possible to achieve uniform pressure loading from the plus and minus side, to avoid single-sided overpressure loading during both start-up and operation, and also to enable zero point checks during operation.

What is the function of a three-way valve block?

With upstream pressure equalising valves it is possible to achieve uniform pressure loading from the plus and minus side, to avoid single-sided overpressure loading during both start-up and operation, and also to enable zero point checks during operation.

When must there be a CE mark on the dial?

When the measuring instrument falls under the scope of the pressure equipment directive (PED; >= 2900 psi / 200 bar), EMC directive (e.g. intelliGAUGE) or the low voltage directive (e.g. 821 or 851 contacts).

When should a restrictor be used?

For pressure spikes or sudden pressure loading and unloading.

Which instruments are suitable for liquids with small measuring ranges?

Diaphragm pressure gauges up to 0.23 psi (16 mbar) are suitable for liquids (through self-emptying of the pressure chamber).

Why are the model 736.11 and 736.51 not generally suitable for aggressive media?

The low-pressure (minus side) enters the interior of the display case and thus the dial (Al), pointer (Al), window, etc. are wetted. Only the plus side, which is made up of the capsule gauge interior, is manufactured from stainless steel and is thus resistant to aggressive media.

Why can some pressure gauges only be used up to an ambient temperature of 140°F (60°C)?

If the window of the pressure gauge is made from safety glass, then it can only be used up to an ambient temperature of 140°F (60°C). The safety glass is made from two glass discs. These glass discs are stuck together using a foil. If the temperature rises above 140°F (60°C), then the foil blisters. Consequently, the scale will no longer be able to be read reliably.

Why does one have filled pressure gauges?

The filling fluid serves as damping for the moving parts within the case. As a result, damage caused by vibration and increased wear on the moving parts can be prevented.

Why must the differential pressure in model 7 differential pressure gauges with bourdon tubes not be allowed to be less than 1/6 of the full scale value?

For model 7 differential pressure gauges, the static pressure is the same as the full scale value over 270 degrees of rotation. With an expected differential pressure of 15 psi (1 bar) at a static pressure of 145 psi (10 bar), the two hands would only be separated from each other at a distance of approximately 27 degrees. As a result of this, to ensure that readability is still acceptable, the differential pressure should not drop below 1/6 of full scale (approx. 45 degrees).

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 do the abbreviations PGT and PGS stand for?

Model PGT (Pressure Gauge Transmitter) instruments are mechatronic pressure measuring instruments which display the pressure without needing external power, and simultaneously generate an electronic output signal. Model PGS (Pressure Gauge Switch) instruments are mechatronic pressure measuring instruments which display the pressure without needing external power, and simultaneously offer an electronic switching function.

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 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 widespread synonym for the explosion protection directives of the European Union and is derived from the French abbreviation of “Atmosphere Explosible”. There are currently two directives in the field of explosion protection, namely the ATEX product directive, 2014/34/EU, and the ATEX workplace directive, 1999/92/EC.

What is meant by switching function?

Under switching function, we mean the opening or closing of an electrical circuit. Normally-closed contacts (NC, denoted by a 2) interrupt a circuit on rising pressure (clockwise); normally-open contacts (NO, denoted by a 1) close a circuit on rising pressure (clockwise).

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.

What is equipment for ventilation and air-conditioning used for?

Ventilation and air-conditioning equipment, also known as air-handling units, exchanges the used room air with fresh air from the outside. Rooms are thus adequately ventilated and the air quality is improved.

Why is it important to make V/AC equipment as energy-efficient as possible?

For reasons of environmental protection and cost, it is very important to make ventilation and air-conditioning equipment as energy-efficient as possible, since the largest energy consumption in residential, office and commercial real estate is spent on ventilation and air-conditioning. In addition, some EU standards now even regulate energy efficiency.

What is the function of ventilation and air-conditioning measuring instruments?

Measuring instruments for ventilation and air-conditioning can indicate any excessive pressure loss due to dirty filters, enable the control of the correct operating state and prevent frost damage to the heat exchangers. In addition, they can be used for monitoring since they are available with digital interfaces.

What causes pointer flutter with pressure gauges?

If vibrations or pulsations act on the pressure gauge, this can lead to “pointer flutter”. Filling the pressure gauge can prevent this and ensures the correct reading of the pressure.

Can vibrations damage the pressure gauge over time?

Vibrations can permanently damage the mechanics of the pressure gauge. This can significantly shorten the life of the pressure gauge. Fill fluid can help, as it cushions the internal mechanics and at the same time lubricates the moving parts.

How can fogging of the window of the pressure gauge be prevented?

When used in low ambient temperatures, condensation can form, which limits the readability of the pressure gauge or can even cause frosting of the window. Through a fill fluid, fogging or frosting of the window is prevented and unimpeded readability is guaranteed.

What is meant by the accuracy class of a pressure gauge?

The accuracy class of a pressure gauge is the tolerated deviation of the display in percent of the full scale value.

What information can be found on the dial of a pressure gauge?

The dial shows information about the unit of the measured values, the accuracy class, the serial number, the construction standard (standard used to make the measuring instrument) and the material designation for all components that come into direct contact with the medium. On some pressure gauges, one can also see an (S). This is the marking for a safety pressure gauge which protects employees in the event of a fault.

Are there special regulations or specifications for the scale of a pressure gauge?

There are special regulations and specifications. The scale must always be exactly 270 angular degrees. In addition, a small black triangle indicates the maximum pressure with a static load.

How is pressure measured with a Bourdon tube pressure gauge?

With Bourdon tube pressure gauges, the pressure is measured by a Bourdon tube transmitting pressure directly to the pointer.

How is pressure measured with a diaphragm pressure gauge?

In diaphragm pressure gauges, the pressure is transmitted via a wave-shaped diaphragm to a link, which transfers the pressure to the movement.

When does it make sense to use a diaphragm pressure gauge?

Diaphragm pressure gauges are used when Bourdon tube pressure gauges reach their limits. They can be used at particularly low pressures and with critical media. In addition, they feature high overload safety.

What are the smallest measuring ranges for Bourdon tube and diaphragm pressure gauges?

With Bourdon tube pressure gauges, the smallest measuring range is 600 millibars. In contrast, diaphragm pressure gauges have a large diaphragm surface, which is why measuring ranges are possible as low as 16 millibars.

How can a diaphragm pressure gauge be protected against aggressive media?

The diaphragm pressure gauge can be coated with PTFE, gold, Hastelloy and many other materials to protect the instrument.

At what pressure could the pressure measuring instrument be destroyed?

Pressure values above the overpressure limit can lead to irreversible damage of the measuring instrument. It does not matter whether this pressure is present constantly or only for a short period of time. In either case, the complete destruction of the parts exposed to the pressure and the sudden escape of the pressure medium can be expected.

Why is there some white foaming inside the case of liquid-filled pressure gauges?

With liquid-filled pressure gauges, white foam can occasionally form inside the case. This is the well-known phenomenon of an electrochemical reaction between aluminium components (e.g. pointers, dial) and the case filling (mainly with glycerine-water mixtures). For technical reasons, this medium is mainly used for lower pressures up to and including 4 bar. However, the foamy bubbles that are produced are only of an optical nature and have no effect on the correct functioning of the pressure gauge. The foaming is therefore not a reason for complaint. If you are still dissatisfied with the visual appearance of the instrument, WIKA can offer you possible alternatives (e.g. silicone-oil case filling).

What does LNG mean?

LNG is the abbreviation for Liquefied Natural Gas, also Natural Gas (main constituent Methane), that is liquefied through low temperatures and relatively low pressures and stored and transported in cryotanks.