By measuring differential pressure, users are able to easily and accurately monitor filter conditions, liquid levels in closed tanks, liquid flow rates inside a pipe, and even the output torque of hydraulic motors.

There are three methods of measuring pressure. The most common type of pressure measurement is gauge pressure, with reference to atmospheric pressure. This is any pressure applied to the system on top of atmospheric pressure, also known as ambient pressure. A prime example of gauge pressure measurement is a car’s tire pressure.

Absolute pressure, with reference to a full vacuum, measures pressure independently of changes in atmospheric pressure. Absolute pressure measurement is used in applications where it is critical to monitor the peak of a vacuum, and is needed in laboratories, meteorology, aviation, and other fields.

Basics of Differential Pressure and DP Gauges

Differential pressure – the third method of measuring pressure – is simply the difference between two applied pressures, often referred to as delta p (Δp). In the example, Δp = p1 – p2.

But why even use a differential pressure (DP) gauge? Why not just place a standard pressure gauge at the p1 and p2 measurement locations, and then have a technician work out the difference? Besides the extra time and effort required for manual calculations, a DP gauge is superior for several reasons:

• Sensitivity. Differential pressure gauges are designed to detect minute differences that the human eye cannot see. As an example, let’s put two standard gauges on either side of a filter. Both pointers might indicate 100 psi, but a DP gauge would be sensitive enough to pick up a difference of as low as 10″ H2O (inches water column), or 0.36 psi. A differential pressure gauge indicates only the Δp; it basically eliminates all the unnecessary “noise.”
• Range. The range of a differential pressure gauge can go as low as 0.2″ H2O for air handling systems and as high as 15,000 psi with a Bourdon tube DP gauge. Even at very low differential pressure ranges, the DP gauge must be rugged enough to withstand very high working pressures.
• Working pressure. Besides the differential pressure range, the maximum working pressure is very critical. Without knowing the working pressure, we cannot determine the correct DP gauge for the application. The working pressure in almost every DP application is significantly higher than the actual DP range.
• Options. Differential applications often require different pressure port positions, additional pressure ports, and different process connections than the typical ¼” or ½” NPT male thread used on standard gauges. For liquid level measurements, a combined top/bottom connection (total of four pressure ports) in combination with a ¼” NPT female thread can be the norm. For filter applications, in-line connections (also known as end connections) are typical, and to measure the low pressure in air handling systems, a hose barb connection is the one most often used.

There are also many options for units of measurement other than just psi, bar, and inches of water column. If measuring content in liquid level applications, users can choose among scales that read in pounds, kilograms, or gallons. In flow applications, differential pressure gauges often read in SCFM (standard cubic feet per minute), GPM (gallons per minute), m³/s (cubic meters per second), etc. And in aviation, using pitot tubes, a differential pressure gauge measures airspeed in knots or miles.

Four Applications for Differential Pressure Measurement

Differential pressure measurement goes beyond regular pressure measuring. Indeed, this type of pressure measurement is the means by which many industries monitor filter conditions, liquid level, liquid flow rate, and torque output.

1. Filter monitoring

This is the most common application for differential pressure measurement, used in industrial oil filter applications, air filter monitoring in gas turbines, and filter monitoring – such as membrane sensing – in water/wastewater facilities. DP gauges for these industries include models 700.04, 732.25, and 732.51. To detect very low pressure in commercial and industrial HVAC systems, products in WIKA’s air2guide series, such as the A2G-10, are excellent options, as is the 716.11.

As the filter becomes clogged, the differential pressure increases. For extra convenience and performance, choose a DP gauge with an output signal, like the A2G-15, to remotely monitor the status of a filter.

2. Liquid level measurement

In an open vessel where nothing is pressurized, a simple pressure gauge is sufficient for calculating the liquid level. But in a sealed tank with liquid and gas phases, the only way to monitor that liquid level is to deduct the low-pressure side (gas or vapor) from the high-pressure side (liquid).

WIKA has about a dozen technologies for measuring tank level. The choices include the Cryo Gauge for liquid gas tanks, which can be accessorized with liquid-level and working-pressure transmitters for using the output signal in a telemetry system. A telemetry device is used to remotely monitor your customer’s tank level and to deploy a refill as needed.

3. Flow measurement

A primary flow element, such as an orifice plate, flow nozzle, Venturi tube, Venturi nozzle, or our high-accuracy FlowPak (FLC-HHR-FP), creates a constriction from a larger upstream diameter (point 1) to a smaller downstream diameter (point 2). This constriction in a pipe causes a pressure drop that is proportional to the square of the flow rate. Using Bernoulli’s equation, one can relate the differential pressure of the fluid with its flow velocity. Thus, the combination of a differential pressure gauge and a primary flow element creates a reliable flow meter.

4. Drill head monitoring

In hydraulic systems, a Bourdon tube-type DP gauge can be used for measuring the output torque of positive displacement motors. The gauge measures the pressure drop of the motor powering the gearbox by simultaneously measuring the pressure on the pressure and return sides of equipment during operation. By measuring the pressure drop, the DP gauge calculates the amount of torque that the hydraulic motor generates.

How to Select a Differential Pressure Gauge

Similar to selecting a standard pressure gauge, several criteria go into the selection of a DP gauge. Here are some questions to ask when choosing a differential pressure gauge:

1. What differential pressure range does the application call for? This is the pressure difference that you want the scale to read.
2. What is the maximum working pressure of the process? This is the maximum pressure at which the system is capable of operating for a sustained period.
3. What media will the wetted parts come into contact with? The wetted parts of a regular gauge are basically just the Bourdon tube and process connection. A differential pressure gauge sometimes has two chambers; in one chamber, more parts – the movement, pointer, dial, window, and gasket – might come into contact with the media. Corrosive media might require stainless steel or a special material.
4. What is the application? The choice of DP gauge often depends on whether it is used for filter monitoring, liquid level measurement, flow measurement, or drill head monitoring.
5. Are there any special requirements? WIKA manufactures DP gauges for specific industries, such as NACE-compliant gauges for sour gas (hydrogen sulfide) service or gauges cleaned for oxygen service.
6. What type of mounting will the gauge need? Mounting can be very specific to the industry. Furthermore, some DP gauges can be very bulky and heavy, weighing up to 30 lbs (13.6 kg). A customer can choose from different types of brackets – not just front or rear flange, but also pipe-mount brackets, Barton brackets, “H” or “C” brackets for liquid level measurement, etc.
7. How about other options? WIKA’s differential pressure gauges can come with switches for automating processes, output signals for remote monitoring, and manifolds that include shut-off valves and a bypass for pressure equalizing.

The WIKA Difference for Differential Pressure Measurement

WIKA is a global leader when it comes to pressure solutions, and differential pressure measurement is no exception. What separates us from our competitors is the breadth of our products. Many of our competitors specialize in only one or two technologies. We have eight different types of DP technologies:

• Piston type
• Piston type with diaphragm
• Frictionless magnetic movement
• Bourdon tube
• Single diaphragm
• Dual diaphragm
• Capsule
• Compression spring with diaphragm

WIKA offers all DP technologies except for bellows because of their susceptibility to pressure spikes, especially in liquid level measurement applications. A better choice would be any of the diaphragm technologies.

Talk to one of our experts today at (855) 737-4714, or fill out our online form to learn more. 

When installing thermowells in a pipeline, a user must first answer several questions regarding their location, quantity, stem length, distance from each another, and effects on the process media.

Thermowells are highly effective devices for protecting temperature sensors, such as resistance thermometers (RTDs), from process media in a pipeline. They are usually inserted perpendicular to the flow, using a flange connection. However, there’s an art to thermowell placement and installation. Some of the most common questions I’ve received:

• What is the ideal insertion length for a thermowell?
• How far apart should thermowells be from each other?
• Where in relation to an elbow should the thermowell be installed?
   

Here are some answers about thermowell selection, placement, and installation.

1. Insertion length for a thermowell
   The right length for a thermowell largely depends on the diameter of the pipe or tube. One rule of thumb is to insert a thermowell anywhere from one-third to two-thirds of the way into the fluid stream. Other guidelines recommend that the insertion length be 10 times the thermowell tip diameter or a minimum of 2 inches (50mm) into the process.
   
   The goal is to balance the potential for mechanical failure and the potential for sensing error. On the one hand, the longer the insertion length, the greater the chances that the thermowell will bend or suffer mechanical fatigue due to the process media’s velocity. On the other hand, the shorter the insertion length, the greater the chances that users will see unreliable results due to poorer heat transfer. In summary, there is not one perfect stem length for a thermowell, but a goal of balancing outcomes.

   One way to reduce vibration and mechanical fatigue is to use a thermowell with a ScrutonWell® design, which has helical strakes to suppress vortex-induced vibrations. Rigorous endurance tests have prove the effectiveness of the ScrutonWell® Design as a vortex breaker.

2. Multiple thermowell installations
   Most of the time, one thermowell with a temperature sensor is sufficient for a section of pipe. However, some processes call for multiple thermowells in an area. The key when installing several thermowells is to minimize their influence on one another while providing a consistent flow character in the process. There are two ways to do this:
   
   Offset angles – In this scenario, both thermowells are installed at the same location but at angled offsets from each other. By having them at the same location, they are not influenced upstream or downstream from an inline installation. They should be installed at a minimum angular offset to allow for easy installation and removal. Also, the thermowell tips should be far enough away from each other so as to not influence each other’s readings.
   
   Inline – To ensure laminar flow in the process, the distance between thermowells can vary from 10 to 100 times the pipe diameter, a wide range indeed! Several factors go into how far apart inline thermowells should be placed, but a conservative estimate is 25 times the pipe size. For example, in a pipeline with a 4-inch (100mm) diameter, the distance between thermowell installations is about 8 feet (2.5m): 4″ x 25 = 100″ = 8.33′.
    

3. Elbow installations
   The installation in an elbow allows the sensing area of the thermowell to be placed in the centerline of the pipe, ensuring an optimal location for measuring the process’s temperature. There are two different sites of thermowell installations in an elbow:

   Facing upstream – The thermowell tip (temperature sensing area) is upstream of any influence, such as mixing or swirling, of the elbow. Many users prefer this elbow installation over “facing downstream” (see next bullet), although the bending moment calculations to ASME PTC 19.3 TW-2016 are outside the scope of this standard.

   Facing downstream – The thermowell tip is downstream of the elbow, which means that it can be influenced by any mixing or swirling that the elbow causes. The advantage when performing thermowell wake frequency calculations is that facing downstream takes a conservative approach and assumes it is a perpendicular installation.

Other considerations for thermowell installation

Thermowell length, distance apart, and location are the main considerations when installing these protective fittings, but they are not the only ones. Users should also keep in mind these other factors:

• Pipe size – ranging from small (2″ to 4″) to large (> 60″)
• Process media – whether it’s gas or liquid
• Two-phase flow – such as gas and liquid, two different liquids, a liquid and solid particles, or a gas and solid particles
• Type of flow – steady or pulsating
• Distance from other measuring instruments or fittings

Talk to one of our experts today at (855) 737-4714 or fill out our online form to learn more.

Selecting a pressure gauge is a lot like buying a car. The marketplace is filled with manufacturers, each offering various makes and models with different features. When deciding on a vehicle, buyers look at factors such as the seats and storage space needed (two-seater, sedan, station wagon, minivan), primary driving conditions (city, highway, racing, towing), transmission type (manual, semi-automatic, automatic), and fuel (gasoline, hybrid, electric, hydrogen fuel cell). Cost, of course, is another important consideration.

When choosing a pressure gauge, buyers go through a similar process but with different priorities. Here’s a quick tutorial on how to select a pressure gauge.
 

Digital or Mechanical Pressure Gauge?

In the world of pressure measurement, the equivalent of a supercar is a digital gauge. With an accuracy of up to ±0.025% of span, this instrument is so precise and high-performance that it can be used for calibration. Top-of-the-line digital gauges like the CPG1500 also communicate wirelessly, a necessity for remote monitoring and industrial IoT (Internet of Things). Understandably, digital gauges are expensive.

Most industrial processes do not require that level of accuracy or number of features. A mechanical, or analog, pressure gauge is sufficient.

Steps for Selecting a Mechanical Gauge

There’s a simple mnemonic for remembering the seven factors for gauge selection: STAMPED.

1. Size
   Mechanical pressure gauges come in a variety of nominal sizes, and the one you choose depends on your requirements for readability, space, and precision. The larger the dial face, the more gradations it will have for more exact readings, and the easier it can be seen from a distance – an important consideration if technicians cannot get close to the gauge. However, some applications don’t have room for a large pressure gauge. WIKA gauges range from 1.5″ (40 mm) to 10″ (250 mm).
   
   Another factor to keep in mind is that the size of the end connection will determine what sizes of gauge are available. For example, a 1.5″ gauge is too small to accommodate a ½ inch size connection, based on the wrench flat area in proportion to the case profile.
   
   Regardless of the gauge size, low-light situations make it difficult to read a dial. At WIKA, many of our gauge dial faces come with the option of InSight™, a retro-reflective material, or InSight Glow™, which is InSight™ with the addition of photo luminescence for visibility during power outages.
    

2. Temperature
   Both the ambient temperature and media temperature will determine the material of the wetted parts (brass, stainless steel, nickel alloy, etc.) and whether it will have a dry case or be liquid-filled. The lower the ambient temperature, the more likely it is that a liquid-filled gauge is the right choice. Gauges in extremely cold environments, like the oil fields around the Arctic Circle, are filled with a special low-temperature silicone oil to prevent the internal parts from icing.

   If the media temperature will reach 140°F (60°C) or higher, use a stainless steel gauge. This is because brass gauges are soldered, and solder begins to break down at that temperature. We’ve seen customers who used brass gauges for steam applications based on price, and those gauges failed since steam exceeds the 140°F threshold for solder. SS gauges can withstand temperatures up to 392°F (200°C), depending on the configuration.

3. Application
   Basically, in what industry will the gauge be used? Here are some examples: Gauges for drinking water applications need to be lead-free, while process industries like refineries and pharmaceuticals require industrial process gauges. Cryogenic gas tanks call for a pressure solution that measures both differential pressure and working pressure, and is cleaned for oxygen service. Gauges used in sanitary processes must have a hygienic design. The highly aggressive gases used in the semiconductor industry means these applications need gauges with an ultra-high purity (UHP) design. What‘s more, some applications require special approvals. For example, gauges for use with fire sprinklers must have UL (Underwriter Laboratories) and FM (Factory Mutual) approvals.

   For reliability and long service life in high-vibration applications, use a liquid-filled gauge to dampen movement and protect the instrument’s internal mechanism. Note that in high-pressure cycles (pulsation), liquid fill should be used in conjunction with a restrictor or a snubber.

   Some common questions we hear have to do with these accessories. What’s the difference between a restrictor and snubber? Besides dimensional restraints, when would a snubber be the better choice? Restrictors are a less expensive option for gauges in applications with dynamic pulsation. However, they are limited based on the orifice size, and they are prone to clogging in debris-filled media such as wastewater. Snubbers mitigate dynamic pulsations and pressure spikes much like restrictors, but they come in a wider range of sizes and are not as prone to clogging. Snubbers are also more adjustable in the field with the use of interchangeable pistons or external adjustment screws, and this flexibility reduces downtime.

4.  Media
   The media that the pressure gauge, especially its wetted parts, comes in contact with will determine the gauge material. In other words, what’s in the pipeline? A brass (copper alloy) gauge is suitable for water, air, or other non-aggressive liquids or gases. But sour gas (hydrogen sulfide), ammonia, creosote, and other harsh chemicals require corrosion-resistant materials such as stainless steel or a nickel-copper alloy like Monel®. For media that can clog gauge mechanisms, opt for the addition of a diaphragm seal, which provides a physical barrier between the fluid and the pressure instrument.

   The media also affects the type of case filling used. Glycerin is the standard fill fluid for non-oxidizing environments, while highly reactive media call for an inert oil like Halocarbon or Fluorolube®.

5. Pressure
   This question encompasses several aspects. First, what type of pressure do you need to measure – gauge pressure (working pressure), absolute pressure, or differential pressure?

   Second, what is the operating range of the application? In general, select a gauge whose range is 2X the optimal operating pressure, as this ensures the best performance. Standard pressure gauges can handle up to 20,000 psi (1,600 bar), with specialty products like the PG23HP-P going as high as 87,000 psi (6,000 bar). For low pressure measurements, use a capsule gauge to detect small pressure differences in units such as millibar (mbar), inches of water column (inH2O), or ounces per square inch (oz/in2).

   Finally, what is the desired pressure scale? Gauges come in a variety of measurement units – e.g., psi, bar, kPa, inH2O. All WIKA gauges can be customized, such as dual scale, triple scale, or custom scales, based on your application needs.

6. “Ends” (process connections)
   What “ends,” or process connections, do you need? The most common type in the U.S. and Canada is NPT, while other countries tend to use G (metric) connections. Then for each type there’s the question of connection size, such as ⅛, ¼, and ½. And finally, the location of the process connection; the two most common connection locations to choose from are lower (bottom) mount or back (rear) mount.
    
7. Delivery time
   Most buyers don’t consider this last factor, but the issue of delivery time is very relevant. If you need a large quantity by tomorrow, your choices will be standard gauges in popular nominal sizes that are already on the shelf. But if you can wait a few weeks, you’ll be able to get the exact pressure gauge you want with all the desired options.

WIKA’s System for Model Numbering

With a few exceptions, WIKA’s mechanical gauges have a five-digit model number. The system may look complicated, but it’s really quite simple. Let‘s take the model 213.40 Bourdon tube pressure gauge as an example.

Using this chart, we can see that the 213.40 is an industrial gauge (200 series) made of brass, is liquid-fillable/liquid-filled, and has a forged brass case. This is WIKA’s hydraulic gauge, as it is designed to withstand extreme shock, vibration, and pulsation.

Talk to one of our experts today at (855) 737-4714 or fill out our online form to learn more.

One of the most challenging elements to motion control is the integration of robots.  However, the benefits that can be achieved through this type of integration cannot be understated. They will inevitably enhance the quality of the system and boost productivity.  Thus, the exercise is well worth the investment.  However, it can be a very daunting task if you aren’t as familiar with your options as you’d like to be.

In fact, there are four questions that you must address before walking through the decision tree of what robots will best fit your application:

1. What level of speed do you need?
2. How precise does the motion need to be?
3. How adaptable does the robot need to be?
4. How much is in your budget?

Once you have a solid foundation to work from by understanding the answers to these questions, you’re ready to work with an experienced professional to make the important decisions about which robots to integrate.

Cartesian robots are the simplest type to integrate. They are very flexible models, utilizing various technologies like ball screws and linear motors. This allows for configurations with more than two axes.  These robots do tend to be a bit on the slower side, but the integration is very straightforward, making it the easiest to integrate. Cartesian robots are not very versatile, but they are also one of the more affordable options.

Selective Compliance Articulated Robot Arm (SCARA) models are very attractive to operators.  They are very easy to integrate, have a very high speed and degree of precision and boast a moderate versatility.  The price also is very middle-of-the-road, although a bit more expensive than the Cartesian variety.  

Articulated robots can achieve high precision and is the most flexible of the models discussed here.  It’s highly versatile but also requires a high level of programming expertise. And comes at a higher cost.  

Collaborative robots or “cobots” are designed to collaborate with humans, so they allow for minimal integration.  They typically operate at a lower speed and lower precision, but they rank high in terms of versatility. This is reflected in their higher price tag.  If an activity may require on-the-fly adjustments by the operator, cobots may be the ideal choice.

The last element for consideration is maintenance.  Operators are always trying to minimize downtime, so understanding the preventative maintenance requirements on these robots is critical.  This requires a proactive effort to gather data for monitoring purposes. 

I did a much deeper dive on the pros and cons of the different robot types for integrating into a motion control system last year in Control Design Magazine.  For a little more in-depth discussion, be sure to check it out. 

Talk to one of our experts today at (855) 737-4716 or fill out our online form to learn more.