Remote seals connected with capillaries do not lose the fluid like a wet leg can.
But,
1) A filled system is a thermometer
A remote seal connected with a filled capillary is a "closed, filled system" and according the laws of chemistry (Boyles law, Charles law) and physics, a filled system is a thermometer, as well as a pressure transfer system.
As the temperature of the capillary changes, the fill fluid expands or contracts with the temperature change, adding or subtracting a pressure component to the sensed pressure. Increasing temperature increases the fill fluid contribution to the total pressure.
The trend of an outdoor tank level for a tank with a constant level will show obvious cycling as the capillaries heat up during the day and cool off at night, when one would expect a flat, straight line. It is the nature of a filled system to reflect temperature as well as pressure. There is no way around this, it a law of physics.
The rule of thumb is to have capillaries of the same length in the hope that the ambient temperatures that the capillaries are exposed to will create pressure changes equal in each capillary leg, thereby nullifying the resulting temperature pressure changes, because a DP subtracts the high side port reading from its low side port reading. Field experience shows that this a more of a forlorn hope than a reality.
2) Capillaries 'filter' the pressure changes
Capillaries, by nature, filter the pressure changes. Low temperatures that increase the viscosity of the fill fluid create more of a filter than do high temperatures.
Sometimes a filter works to one's advantage, other times to a disadvantage.
3) DP range
The fill fluid exerts a pressure on the DP's sensing diaphragms.
This results in LRV and URV ranges might not seem to make sense at first glance, but are nonetheless valid.
A typical LRV for capillary seals might be -4.354m (that's negative), even though the datum, the elevation at which the lower port on the tank is connected, is at the same level as the transmitter, where one might assume the LRV = 0.000m.
This has fooled some uninformed users who have bought a remote seal DP with a range suitable for the elevation of the level in a tank if the DP were directly connected, but have discovered, to their dismay, that the fill fluid pressure exceeds that range, and they've had to replace the transmitter with one with a wider range to accomodate the pressure component of the fill fluid.
My suggestion is to make sure the vendor understands that it is a level application and that they be provided with exact details of the tank so that they can provide you with a range value that they've calculated (befor you buy it). If the LRV is not a large negative value, find another vendor, because this vendor does not understand remote seal DP configuration for level. The vendor should guarantee that the range is suitable for information you supply. This is not news, this is 40 year old technology, but not all vendors have people trained in level.
You might even consider having the vendor configure/range the DP unit for you, because that puts the entire ball into their court.
4) Cost
The cost (purchase price and delivery) of a remote seal capillary DP transmitter is much more than a stand-alone DP transmitter.
The process of filling capillaries (evacuating with a vacuum pump, back filling without bubble inclusion, sealing) is a learned skill. Be wary of the price cutter who uses street urchin labor to fill capillaries. Caveat Emptor, you get what you pay for.
