Differential Pressure Sensors
Among the most broadly applied field devices, differential pressure instruments have a huge range of capabilities and uses. Here are some basic concepts and tips for successful installations.
David Gunn, Honeywell Process Solutions
Differential pressure (DP) measuring devices are the most versatile of all the pressure sensors, with the broadest measuring capability and application exposure. Battery-powered wireless differential pressure transmitters add the dimension of increased flexibility for installations where wiring costs or the absence of available power have limited the use of traditional wired transmitters.
Pressure devices are typically categorized into three types of pressure measurement: gage pressure, absolute pressure, and differential pressure. But fundamentally, they are all differential pressure measurements.
A gage pressure device is one that measures process pressure relative to surrounding atmospheric pressure. The process side of the sensor is exposed to process pressure while the non-process side is left exposed to atmosphere. The gage measurement is the difference between the two. If the non-process side of the sensor was sealed off from the atmosphere, any trapped volume of gas would expand and contract with temperature changes and impart a pressure change on the non-process side of the sensor. This would create a gross error to the gage pressure signal. As a rule, all gage pressure sensors must have an internal passage from the non-process side of the sensor to atmospheric pressure.
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An absolute pressure measurement device is similar to a gage pressure sensor in that only one side of the sensor diaphragm is exposed to process pressure. The non-process side is designed to contain a near perfect vacuum. With the non-process side of the sensing diaphragm at full vacuum, the absolute pressure reading is obtained by measuring the differential pressure force between it and the process side of the sensor. Any applied pressure to the process side would be a positive differential and be referred to as an absolute pressure reading.
This shows that all pressure readings are relative to something, so gage and absolute pressure types are inherently differential measurements. That gives differential pressure transmitters the ability to be more broadly applied than any other type.
A differential pressure measurement is typically performed by subjecting a diaphragm-type sensor to two independent pressure inputs. The pressures are applied to opposite sides of the sensor with the forces counteracting each other to result in a sensor response that is equal to the net difference of the two pressure inputs (see graphic). Both the high side (blue) and low-side (yellow) pressures are independently transmitted to the sensor in order to determine the differential pressure.
Leaving the low side pressure port open to atmosphere allows the transmitter to be used for a gage pressure measurement as the applied process pressure will be a positive differential from atmosphere. Applying a full vacuum to the low pressure port will create an effect similar to an absolute pressure transmitter.
To measure a vacuum where atmospheric pressure is to be the zero starting point, vacuum is applied to the low pressure port while the high pressure port is left vented to atmosphere. The increasing differential value (increase in vacuum) is shown as a positive output change.
Differential pressure devices can measure or assist in measuring other process variables, including flow, liquid level, and liquid density.
The most popular application for differential pressure transmitters is to measure the pressure drop across an orifice plate for flow measurement (see graphic). Here we’ll just look at basic concepts, but there are many books written on this subject, so more detailed studies are available.
Orifice plates, pitot tubes, venturis, V-cones, and so forth are examples of primary flow elements that create a pressure loss as fluid flows past the restriction. The amount of restriction is tailored to meet the process considerations of piping, pressures, and flow. The pressure loss is proportional to the size of the restriction and fluid properties, including density and flow velocity. By measuring the pressure drop created across the primary element, it is possible to calculate the fluid flow rate. Pressure taps located upstream and downstream of the flow element are connected to the high and low input ports of the differential pressure transmitter. The pressure in the line may be upwards of 1,000 psi, but the pressure drop is usually very small. Using two separate gage pressure devices and subtracting the readings would result in the combined error of two devices having wide measuring spans. The result would likely be a very poor measurement seldom suitable for control. A differential pressure transmitter allows the measurement to be made with one device over a much smaller span, resulting in greatly improved precision. It also serves to reduce the amount of field wiring by eliminating the need for a second transmitter.
Operational tip—Make sure you’ve bled your impulse lines to remove any slugs of air or other gas. Manifolds often have bleeding ports.
Differential pressure values can be expressed in many units, but inches of water column (in. H2O) is very common. This value becomes a variable in an equation to convert the differential pressure reading to a flow value. A simplified approach to convert from pressure to flow is to take the square root of the pressure reading, express it as percent of flow, and calculate the flow value from the percentage.
For example, a given flow element has been engineered to represent full flow being 1,000 gpm (gallons per minute) at a pressure drop of 100 in. H2O. The square root of 100 is 10, which is expressed as 100% of flow. Under operating conditions if the pressure reading was at 49 in. H2O, the square root is 7, which represents a 70% flow reading. 70% of full flow equals 700 gpm.
Given the sophisticated capabilities of current field transmitters, it’s simple for a device to perform math functions internally so that the local display can be configured to show the flow value instead of the pressure reading.
Operational tip—To improve flow measurement accuracy when starting up, it is a good practice to momentarily equalize the pressure between the high side and low side of the DP cell and perform a zero correction. Some manifolds make this a very quick and simple procedure. This is done to remove any zero effects that may have occurred as a result of mechanical strain from the line pressure. This is much more significant when working with high pressures typical of super-heated steam applications.
Filter screen quality
Many processes require fluid filtering to ensure that particles beyond a given size are separated out and not allowed downstream. Filters come in many shapes and sizes depending upon the needs of the process. Filter types range from a wide-mesh screen across the water intake at a power plant to a micron-sized filter at a paint pigment plant. Regardless of the application, the approach to filter cleanliness quality is very similar. As a filter traps particles it reduces the free space for fluid to pass through. The restriction in free space causes pressure on the upstream side of the filter to increase. By measuring the upstream and downstream differential pressure, a determination of filter cleanliness can be obtained: an increasing differential pressure indicates clogging. As with the flow measurement application described earlier, upstream pressures will often be much larger than the amount of pressure drop. So here too, a differential pressure measurement provides much greater precision and reduces the number of devices required to perform the measurement.
Operational tip—Even a clean filter should generate a measurable differential pressure across its element. A zero differential reading while fluid is flowing likely indicates a filter element failure where fluid is passing through unrestricted, allowing unfiltered fluid to travel downstream.
Liquid level in vessels is a very popular application for differential pressure transmitters, where they are often the most cost-effective solution. Fluid within a tank develops a head pressure that can be measured at the bottom of the tank. Head pressure is a function of the fluid’s height and density.
If the tank is an open or vented vessel, the level measurement could be performed with a gage pressure device, but as is typical with most tanks, the amount of pressure produced is often very low. Head pressures of less than 400 in. H2O are very common. Gage pressure transmitter spans are usually much larger. To improve measurement precision, it is often better to use a more sensitive DP transmitter and vent the low pressure port to atmosphere.
If the tank is a closed vessel, a DP transmitter is an even more cost-effective solution. Volatile fluids can release vapors when left vented to atmosphere, so it is important to contain those to avoid harming the environment. When kept in a sealed tank, liquids release vapor until pressure equilibrium is achieved. The vapor pressure rests on top of the fluid and adds to the head pressure reading that is measured at the bottom of the tank. To calculate a true fluid level, vapor pressure needs to subtracted from the total head pressure. Here again, rather than using two independent gage pressure transmitters, a single DP transmitter can be applied to receive both the total head pressure and vapor pressure. The differential pressure is representative of the net head pressure, which indicates liquid level.
When considering an actual application, there are more complex considerations than the simple description provided here. Use of isolation diaphragms, filled capillaries, physical mounting requirements, and so forth, has to be taken into consideration, so research your needs thoroughly.
Operational tip—When installing a DP transmitter with remote seals on a closed tank, it is a good practice to route the capillaries so they are subjected to the same temperature conditions. Sunlight hitting one capillary and not the other will cause an error due to the capillaries having unequal fluid thermal expansion.
With tank level applications that contain expensive materials, it is usually desirable to measure level as accurately as possible for inventory purposes. Changes in temperature typically cause fluid density changes, which in turn affect the relationship of head pressure at the tank bottom and fluid level. One method that can measure density changes uses a fixture on the side of the tank with taps that are a fixed distance apart. By mounting a differential pressure transmitter off of the two taps, a measurement of the fluid density can be calculated. If the tank contents have a density of 1.0 and the transmitter taps are separated by 10 in., the transmitter will read 10 in. H2O all the time, regardless of the level, so long as both taps are submerged. But if the fluid temperature drops, there will be a corresponding increase in density. For example, if the density increases by 3%, then the reading of the transmitter will increase by 3% to 10.3 in. H2O. The value of 0.3 in. of level can easily equate to thousands of gallons of miscounted product in large diameter tanks. Having the density reading available allows for corrections to be applied to the level value.
At various times during startups or maintenance, it is necessary to perform leak testing on piping and vessels. There are several approaches for leak testing, some involving sniffing for the presence of specific gases such as helium, or watching for migration of dyes or fluids viewable under ultraviolet light. Such methods can get expensive. A simple and low-cost alternative is to perform a pressure decay test.
Pressure decay involves applying test pressures to a vessel and monitoring for a pressure drop over a period of time. Larger vessels are usually tested at lower pressures than smaller ones due to the greater force that pressure applies over a wider surface area. Small changes at low pressure levels are usually more readily observable with a differential pressure transmitter, as the span of the differential transmitter tends to be lower than that offered on gage pressure instruments.
One widely-used approach to detect pressure changes from leaks more quickly is to have a reference chamber of a volume similar to the chamber being tested. The reference chamber must be one that has been previously qualified as leak free. Pressure or vacuum is applied to both chambers simultaneously with a differential pressure transmitter connected between them. When the same test pressure value is reached in both vessels, they are then isolated from each other. The transmitter’s narrow span range will quickly detect any differential pressure change occurring between the two tanks, which would indicate a leak if one were to exist.
Since this kind of leak detection on tanks and vessels is often a temporary test and performed where there is no easy access to power, a battery-powered wireless DP transmitter provides local display for immediate feedback while the test is in progress, but also offers the means to capture and document the test results without having to pull wires to data logging equipment.
Coriolis meters are becoming more common and are widely accepted for their ability to measure many fluid attributes beyond mass flow. Coriolis units can provide data on density, percent solids, specific gravity (Brix, Baume), and temperature measurements. A less frequently performed measurement is fluid viscosity. Adding this capability typically requires installing a differential pressure transmitter with taps located upstream and downstream of the Coriolis sensor. By using flow, density, and temperature data from the flowmeter, combined with the differential pressure reading, a calculation can be performed by the host controller to derive fluid viscosity.
Differential pressure transmitters account for roughly two-thirds of all pressure transmitters sold. This should be no surprise given their versatility and ability to provide such broad application coverage. Units are available from a variety of manufacturers in many construction materials with a broad selection of installation and process mounting configurations. Honeywell’s ST 3000 smart pressure transmitter family is an example of the product category.
David Gunn is a wireless product specialist for Honeywell Field Solutions.