Shift in zero point

When the exact mounting details of the transmitter and the Diaphragm Seal are not known at calibration, the span is set from zero to the required value (e.g. 0-400 mbar). When mounting the Diaphragm Seal System in the field, the zero can shift depending on how it is mounted. This is caused by the gravity effect after installation. This shift can be zero suppression or zero elevation. 

Zero Suppression:
the factory calibrated zero is shifted above zero after installation (e.g. 150 + 550 mbar)  

Zero Elevation:
the factory calibrated zero is shifted below zero after installation (e.g. -390 + 10 mbar)  

The shift in zero point can be calculated with the density of the Badotherm fill fluid, the height difference of the installed application, and the gravity. The shift in zero point is important to know prior to installation because the shift must remain within the limits of the transmitter range. If the zero shift is outside the limits of the transmitter range, the transmitter will fail to measure.  

Differential Pressure Applications

Differential pressure applications can be used for various purposes: differential pressure, level, flow, density, and interface measurements. There are three different mounting styles to be recognised, depending on the position of the instrument compared to the Diaphragm Seal.

• DP Style 1: instrument between the Diaphragm Seals
• DP Style 2: instrument below the Diaphragm Seals
• DP Style 3: instrument above the Diaphragm Seals

Each style has specific advantages, limitations, and considerations that should be taken into account. DP Styles 1 and 3 only allows for limited vacuum and DP Style 2 allows for full vacuum (1mbara), when ‘H’ is at least 50 cm. The selection of one of the above indicated mounting styles is often driven by:

• Ease of installation
• Accessibility of the instrument: reading of local display and maintenance
• Obstacles that do not allow to place the instrument elsewhere

However, the process conditions should be considered first whilst selecting one of the mounting styles. The following example illustrates this. 

Suppose DP Style 3 has been selected because the vessel is positioned below the walking grid (H1=360cm). The fill fluid (density 1,020 kg/m³) column in the capillary that connects the lowest seal (P2) with the DP instrument, represents a pressure of (p=h*ρ*g): 0,360*1,020*9.81 = 360,2 mbar. This hydrostatic pressure is sensed at the DP instrument as a negative pressure, -360,2 mbar. When in this application the minimum operating pressure is as low as -800 mbar (200 mbara), the selected mounting by style 3 will damage the Diaphragm Seal System: -360,2 -800 mbar =          –1160,2 mbar. This implies a pressure below absolute zero which is impossible. This will either damage the diaphragm of the seal or the diaphragm of the DP instrument. Also, the fill fluid can be damaged because it is pushed over the limits of its vapour pressure.  

In this example DP Style 2 would have been the better option. The transmitter is then placed below the vessel (H=160CM) and the hydrostatic pressure is then:  0,160*1,020*9.81 = 160,1 mbar. This is sensed at the DP instrument as a positive pressure +160,1 mbar. With the minimum operating pressure of -800 mbar, it would result in a pressure at the instrument of -800 + 160,1= – 639,9 mbar. This is well above absolute zero, and thus the instrument and Diaphragm Seal System will function properly

Gauge Pressure Applications

Gauge pressure applications can be used for pressure and density measurement. Also for this application there are three different styles to be recognised depending on the position of the instrument compared to the Diaphragm Seal. 

• GP Style 1: instrument equal to the Diaphragm Seal 
• GP Style 2: instrument below the Diaphragm Seal 
• GP Style 3: instrument above the Diaphragm Seal 

The effect of the mounting on a GP instrument is similar as described for the DP instrument. When an instrument is placed above the seal, it will sense this as a negative pressure and that pressure, in combination with the minimum operating pressure, should not exceed a value below absolute zero. The Diaphragm Seal System would be damaged in a similar way as described for DP applications. 

Absolute Pressure Applications

Absolute pressure applications are only used to measure pressure. For this application there can be also three different styles recognised depending on the position of the instrument compared to the Diaphragm Seal.  

• AP Style 1: instrument equal to the Diaphragm Seal 
• AP Style 2: instrument below the Diaphragm Seal 
• AP Style 3: instrument above the Diaphragm Seal 

For absolute pressure measurement the instrument should be mounted below the diaphragm seal in order to protect the instrument at all possible conditions. This is presented in AP Style 2. If for example H=50cm the pressure on the instrument is already +50 mbar. With this mounting style the instrument has additional protection before it reaches absolute zero. AP Style 1 is also possible, but not preferred as there is no additional protection so it is possible to reach the absolute zero and damage the application. 

Mounting the instrument above the Diaphragm Seal (AP Style 3) will damage the Diaphragm Seal System in a similar way as described under DP applications. Care should also be taken, when an instrument is direct mounted on the Diaphragm Seal and installed as shown in AP Style 3. As a standard, Badotherm uses a distance tube of 80 mm. With a silicon BSO fill fluid, this will created a negative pressure of -8 mbar at the pressure sensor. When the absolute pressure runs below 8 mbara, the Diaphragm Seal or instrument will be damage

Diaphragm Seals and Wet Legs

A wet leg is made with tubing directly to the transmitter and is filled with fluid. Diaphragm Seals Systems offer significant installation flexibility and maintenance advantages over wet leg systems. Diaphragm Seals make it easier to maintain the fluid between the tap and the transmitter, especially on the reference or low pressure side. In vacuum systems, a closed seal system, rather than an open wet leg, maintains a constant height for the low side reference. The Diaphragm Seal System does not need to be refilled or drained. They are also not vulnerable to plugging or freezing and they are easier to control than wet leg systems.

Balanced System

A balanced system means that the volume of the Diaphragm Seal System is equal at the HP and LP side of the DP measurement. This can be obtained by ensuring that the fill volume in the Diaphragm Seal and the capillary lengths are similar at both sides. An unbalanced system can be equipped with two different sized Diaphragm Seals and/or with using two different lengths of capillary.  This result in a larger volume of fill fluid on one side compared to the other side.  

A balanced Diaphragm Seal System is preferred above an unbalanced system because it reduces or avoids the effect mentioned below: 

• Ambient temperature effects: balanced systems will have equal expansion and/or contraction of fill fluid due to ambient temperature changes. As both sides are equal the effect on the DP transmitter will be zero or close to zero. Unbalanced systems have a different expansion and/or contraction of fill fluid on either side due to ambient temperature changes. This results in a different pressure build up (expansion) or drop (contraction) which negatively influences the accuracy of the overall measurement. 
• Process temperatures effects: For applications with an  elevated process temperatures (>65°C) the effect of an unbalanced system can be considerable. For example there is an unbalanced system with a direct mounted Diaphragm Seal to the HP side and a capillary mounted to the LP side. The heat transfer from the process temperature to the direct mount on the HP side of the instrument will be higher then to capillary mount on the LP side of instrument. This result in a different pressure build up (expansion) which negatively influences the accuracy of the overall measurement.
• Static pressure effects: In case of fluctuating static pressures unbalanced systems can become unstable because the static pressure will be transferred faster to the side with the lowest volume.  

Reduction of Measurement Errors

There are different ways to minimise the measurement errors of the Diaphragm Seal System: 

• Keep capillary lengths as short as possible  
• Use capillaries of the same length on both taps when measuring differential pressure 
• Never mount seals and capillaries in direct sunlight 
• Be sure that both capillaries experience the same temperature. For instance, avoid installing one capillary in a shady area and the other in the sun