· Zero Suppression – is when the transmitter is below the tapping point
· Zero Elevation – is when the transmitter is above the tapping point
· The issue with zero suppression or elevation is only during calibration. Zero is not 0 mmH20 but is calculated on the height of the elevation or suprression
· Types
| Type | Advantage | Disadvantage | When to use | Special needs |
| DP Dry leg | Cheap | Cannot measure interface, Cannot measure corrosive or plugging liquid | Clean liquid | |
| Sealed Type DP/ Remote seal/ capillary tube | Avoid plugging, avoid leaks due to corrosion | Expensive | Corrosive liquid | |
| DP wet leg | Cheaper than sealed, long length | Cannot measure interface | Process tends to condense, | Slanted tube, need to fill up tube |
| Unguided wave radar level (radio detection and range) | Cheaper for long distance | Can have false returns | Large tank | |
| Guided wave radar | False returns avoided | Expensive for long tanks, cannot work if have emulsion layer | Small to medium sized vessels | |
| Float Type | Disturbed by surface ripples | |||
| Capacitance | Error with temperature | Avoid, old technology | ||
| Ultrasonic | Error with temp change, sensitive to foaming | Avoid, old technology | ||
| radioactive | For extreme high temperatures, pressure, corrosion | troublesome | Only when nothing else works | Source need to be replaced after half life elapsed |
| displacers | Can measure interface, severe applications | Affected by density | ||
| bubbler | Cheap, for corrosive/slurry type |
· Suppressed zero is when a DP level transmitter is installed below the tapping point. The issue with suppressed zero is when setting the millibar signal at 4mA and 20mA. For suppressed zero, the 4mA is set at the length below the tapping times with gravity and density
· Elevated zero, is vice versa, when the DP level transmitter is installed above the tapping point
· Whether one elevates zero or suppresses zero, the range is always the same. The range will always be between the top and bottom tapping points
· Wet leg uses the principle of DP, where the low side of the leg is filled with product and tapped from the top of the measured vessel
· The wet leg measured DP is 0 at full and negative at empty vessel
· The wet leg manifold does not have any equalizing valve to eliminate the effect of valve parsing. A parsing equalizing valve would cause the low side filled product to drop down, reducing the low side pressure. If this occurs, we would expect a higher DP (high minus low) and would of course result in a higher level measurement
· To verify 100% level i.e. 0 dp, vent both high side and low side manifolds
· To verify 0% level i.e. lowest dp (negative), vent high side manifold first. Then, the lowside must me vented at the top tapping point. By this way you measure atmospheric pressure at high side and wet leg pressure at low side. This low dp should be equal to the LRV
· Some transmitters allow the setting of 0% level at 0 dp and 100% level at made-positive lowest dp. This is done be adding an offset or zero trimming the output with a +|lowest dp|, setting 100% level at +lowest dp and 0% level at –lowest dp + lowest dp = 0. Although this gives an error to the actual dp, one should know that this is just a transmitter setting.
· Wet Leg level transmitter relies on Specific Gravity (SG).
· Wet Leg is preferred to remote seal type due to its lesser cost.
· High accuracy
· Depends on dielectric constant
· Used for replacement of the displacer type
· Instrument has no moving parts
· Disadvantage
· Requires tuning
· Two types
· Open path
· Cannot be used in side chambers as the signal might reflect
· Guided wave
· Most common use
· Displacer Type transmitter uses the principle of buoyancy to obtain level. The measured weight of the Displacer will be converted to the signal. The weight is measured using a hall effect sensor
· For side mounted chamber Displacer, the 0-reference point is at the center of the bottom side flange. The 100% point is at the center of the top flange.
· However, the ability to measure accurately would be from the start of the bottom flange to the top of the top flange.
· An easier way to obtain the point of measure would be to hang the displacer together with its top mounted flange outside of the chamber. The point where the measured liquid starts to hit the displacer would be the start of measure. This of course requires compensation in interface measurements(liquid-liquid), where the low SG should be compensated on the measured freefall weight.
· 2 flanges are required for a displacer. The bottom side flange is used to indicate the starting point of measurement. The top side flange displacer is required to equalize the pressure between the measured vessel and the chamber. Trapped pressure would impede the level increment of the product.
