How to Solve the Problem when Using the Low Differential Pressure Sensor?

cost effective pressure transducer

In the process of using the low differential pressure sensor, various bad experiences may occur due to inattention to some details. Today some experience of using the low differential pressure sensor is summarized to share with you, hoping to help you solve problems quickly.

1. The low differential pressure sensor has a large deviation compared with the pointer pressure gauge.

The appearance of deviation is a normal phenomenon, just confirm the normal deviation range. Due to the small measuring range of the low differential pressure sensor, the sensing element will affect its output. The pressure sensitive part of the low differential pressure sensor should be axially perpendicular to the direction of gravity during installation and adjust the zero position of the low differential pressure sensor to the standard value after the installation and fixation.

 

2. When the pressure goes up, while the output of the low differential pressure sensor can not go up.

 

In this case, check whether the pressure interface is leaking or blocked firstly, if it is confirmed not, check the mode of connection and the power supply, if the power supply is normal, simply pressurize to see whether the output changes or check whether the sensor zero position has output if there is no change, the sensor is damaged, which may be a problem caused by the instrument damage or other links of the entire system.

3. The output signal of the low differential pressure sensor is unstable

This failure may be a problem with the pressure source. The pressure source itself is an unstable pressure, which is most likely due to the weak anti-interference ability of the instrument or low differential pressure sensor, and the severe vibration of the sensor itself as well as the sensor failure.

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The Importance of Using Low Differential Pressure Sensors in Critical Environments

electrical pressure transducer

The low differential pressure sensor divides the pressure into two types: positive pressure and negative pressure. The former is used for protective isolation, and the latter is used for isolation and control of infectious diseases transmitted through the air. For example, hospital wards, operating rooms, pharmacies, clean rooms for pharmaceuticals and semiconductor production, life science laboratories, and animal and botanical gardens use positive pressure protection, while the hospital wards for tuberculosis, measles, chickenpox, or SARS patients and biosafety laboratories are isolated by negative pressure to form a protective space for the public.

The low differential pressure sensor, also called negative pressure sensor / positive pressure sensor, is used to accurately measure the small pressure difference between the key area and the adjacent area (usually the adjacent corridor or lobby). These pressure gradients can prevent airborne infectious diseases or pollutants from spreading from the protected area to the contaminated area, and vice versa.

The critical medical environment has the characteristics of extremely low differential pressure measurement, with a full scale of 25Pa (0.0036 PSI). With the increasing importance of building energy consumption, the environmental differential pressure requirements are continuously decreased under the premise of ensuring safety.

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The Applications of Low Differential Pressure Sensor in Data Center

electrical pressure transducer

There are intensive equipment in data center, where the heat is concentrated，at the same time, these precision equipment have high requirements for the temperature, humidity and cleanliness of the environment, therefore, it is necessary to have a reasonable airflow organization and distribution to effectively remove the heat in the machine room, so as to ensure that the machine room meet the environmental requirements of equipment such as temperature, humidity, cleanliness and air supply speed.

The low differential pressure sensor is mainly used to monitor the pressure difference between the two ends of the filter in the air conditioning system, participating in the variable air volume control system (VAVBox) and monitoring the pressure difference between the machine room, other rooms and corridors.

The filter in the air conditioning system is used to filter dust and impurities in the ventilation duct. As time accumulates, the dust on the filter will accumulate, which will gradually affect the air circulation of the filter. Due to poor air circulation, the pressure difference between the two ends of the filter increases continuously. The differential pressure transducer detects the pressure change, and the upper computer receives the signal from the low differential pressure sensor, when the signal value reaches the alarm point, it will start an alarm to remind of replacing or cleaning the filter.

The VAV controller adjusts the air supply volume and the temperature within the set range according to the actual temperature detected by the temperature sensor and the difference with the set temperature, the low differential pressure sensor detects the pressure change in the air duct to determine the air supply volume.

In order to ensure the air quality in the computer room, the primary room should maintain positive pressure. The pressure difference between the primary room, other rooms, and corridors should not be less than 5Pa, and the static pressure difference between the primary room and the outdoor should not be less than 10Pa. Using the low differential pressure sensor to monitor it can prevent the infiltration of outdoor air from destroying the environmental parameters in the machine room.

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The Types of Low Differential Pressure Sensor

pressure sensor material

The low differential pressure sensor is generally divided into three categories: piezoresistive sensor, capacitive transducer and microelectromechanical (MEMS) sensor.

1. Piezoresistive sensor

Piezoresistive sensor is usually made of silicon and is bonded to a steel substrate by the adhesive effect between metals. When pressure is applied, this single-crystal diaphragm will deform, so that the resistance in the Wheatstone bridge will detect the pressure and then output the voltage proportionally. Piezoresistive sensor has good resolution and bandwidth, which is suitable for the applications with insufficient budget.

But this type of sensor also has its limitations. It is very sensitive to temperature changes and tend to drift. What is worse, this type of sensor cannot provide a large enough area to induce pressure to produce effective diaphragm deformation. It is no doubt that the diaphragm can be made thinner, but this will reduce its strength and integrity as well as also increase the cost. When used for low pressure measurement, its measurement results are noisy and the long-term stability is poor. Therefore this type of sensor is more suitable for high pressure measurement.

2. Capacitive sensor

Capacitance-based pressure sensor has become the mainstream of pressure measurement applications. Compared with piezoresistive pressure sensor, capacitive type pressure transducer has higher pressure sensitivity and lower temperature sensitivity.

Other advantages also include: simple design, no need to use special materials, low energy consumption, high resolution and low cost.

The capacitive sensor consists of a compact housing and two insulated metal plates placed in parallel inside. One of the metal plates adopts a metal diaphragm, which can be slightly deformed under pressure, and the other one is an insulated stainless steel electrode. When a pressure difference is applied to the metal diaphragm, the metal diaphragm will be displaced relative to the fixed electrode, which will change the capacitance. The linear comparison circuit detects that the capacitance change generated is proportionally amplified and outputs a high-level voltage signal. The deformation of the metal diaphragm is extremely small, which is beneficial to reduce the hysteresis and repetitive errors as well as speed up the response.

3. Microelectromechanical (MEMS) sensor

MEMS (Microelectro Mechanical Systems) sensor is a common capacitive sensor. It etches actuators, mechanical parts and electronic devices on the silicon substrate by using microlithography technology, which can be manufactured in large quantities at low cost. In addition, its volume is constantly shrinking, and its diaphragm area has been reduced to 13mm².

However, this miniaturization trend also limits performance, such as insensitivity, inaccuracy and signal drift. Because it is made of brittle materials, MEMS sensor is prone to fracture under high stress and cannot withstand excessive force. The biggest limitation of capacitive MEMS sensors is that miniaturization limits its pressure sensitivity in high-pressure applications.

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The Working Principle of Low Differential Pressure Sensor

ultra low pressure transducer

The low differential pressure sensor is a typical sensor, which has certain applications in various industries. It is also a commonly used sensor. The following is an introduction to the working principle of the low differential pressure sensor.

The two pressures of the measured medium of the low differential pressure sensor enter into the high and low pressure chambers, and act on the isolation diaphragms on both sides of the δ element (that is the sensitive element). Then they are transmitted to the both sides of measuring diaphragms through the isolation diaphragms and the filling liquid in the elements. The electrodes on the measuring diaphragms and the isolation diaphragms on both sides constitute a capacitor respectively.

When the pressures on both sides are inconsistent, the measuring diaphragms will be displaced, and the displacement is proportional to the pressure difference, therefore, the capacitance on both sides is not equal, which is converted into a signal proportional to the pressure through the oscillation and demodulation link. The working principle of pressure transmitter and absolute pressure transmitter is the same as that of differential pressure sensor, the difference is that the pressure in the low pressure chamber is atmospheric pressure or vacuum.

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The Errors in Differential Pressure Transmitter

low cost pressure sensor

No matter what the transmitter is, there will always be some unavoidable problems in the process of using, such as the inevitable errors of the differential pressure transmitter. And we are able to consider comprehensively when choosing the differential pressure transmitter, such as the accuracy and linearity of the differential pressure transmitter, so as to avoid the errors of the differential pressure transmitter as much as possible. Generally, there are four unavoidable errors in differential pressure transmitters, and a brief introduction is as follows.

1. Linear error: This is a factor that has less influence on the initial error of the dp pressure transmitter. The cause of the error is the physical non-linearity of the silicon chips. However, for the transmitter with amplifier, it should also include the non-linearity of the amplifier.

2. Offset error: Since the vertical offset of the differential pressure transmitter remains constant throughout the pressure range, the variations of the transmitter diffusion and laser adjustment correction will produce offset errors.

3. Hysteresis error: In most cases, the hysteresis error of the differential pressure transmitter can be completely negligible, because the silicon chips have a very high mechanical rigidity. In general, only the hysteresis error needs to be considered in the case when the pressure changes greatly.

The above three errors are unavoidable. Now can only try to choose a differential pressure transmitter, of which the precision that is suitable for oneself. Using good production equipment and technology to reduce these errors and it is also possible to perform error calibration before leaving the factory to minimize the impact of errors.

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How to choose a right pressure transducer/transmitter?

cost effective pressure transducer

There are multiple types of pressure transducers for a variety of applications. Each pressure transducer has different aspects that will impact how it works and the applications the pressure transducer works best for. When selecting a pressure transducer, keep these five criteria in mind:

 

1.Pressure range:

First of all, it is important to choose a sensor with a measuring range that corresponds with the pressure range you need to measure, taking into account normal conditions and possible occasional pressure variations.Alpha provide Unidirectional minimum range is 0-25Pa(0-0.1 in.WC)  and maximum range is 0-25000Pa(0-100 in.WC) ;Bidirectional minimum range is -10Pa to 10Pa(-0.05 to 0.05 in. WC) and maximum range is -10000Pa to10000Pa(-50 to 50 in. WC)

2.Stability

Long-term stability is one of the most important parameters that determine the overall measurement accuracy of a pressure sensor. Long-term stability in pressure sensors is defined by the maximum acceptable change in zero and span values during the course of the calibration interval of the sensor. In short, for a sensor to be considered stable, it should maintain a very low drift in its measurements over the calibration interval. Alpha stability is ±0.5% FS/YR which higher than most of the others.

3.Over pressure

Overpressure refers to the maximum pressure value a transducer can withstand without affecting performance; it can also refer to a situation where the pressure value exceeds the limits. Although it is not ideal, pressure transducers can withstand occasional pressures up to their overpressure rating, eventually returning to their natural state. However, pressures that reach burst pressure, the maximum pressure that may be applied to the positive pressure port without physically damaging the internal sensing component, can rupture the diaphragm and cause leakage. Pressures above overpressure but below burst pressure can result in permanent diaphragm deformation, causing an output shift affecting operational performance or accuracy readings.

  1. Accuracy

Pressure gauges come in many different accuracies. Accuracy of common pressure transducers could range from 0.5% to 0.05 % of the full-scale output. While Alpha focus on high accuracy from 0.6%-0.25% for very low pressures for critically demanding applications.

 

5.Compensation range

The compensated temperature range defines the limits of operation for specified measurement accuracy.  e.g. A pressure sensor has an accuracy of 0.25% full scale over a compensated temperature range of -20 to +80 degC.

Since temperature errors are often significant for many measurement devices, a manufacturer will incorporate digital or analogue temperature compensation.  If the measurement device is used outside the compensated temperature range, the accuracy statement is no longer valid, because the device has not been tested or compensated for that temperature.

6.Why choose a capacitive pressure sensor

In a  capacitive pressure sensor, the membrane is one of the capacitor plates. Under pressure, the membrane deforms, varying the capacitance of the capacitor. These sensors provide a good level of accuracy and sensitivity, they can measure low pressures, in the mbar range.

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What is the difference between Capacitance and Piezoresistance

cost effective pressure sensor

Now there are a variety of pressure sensors on the market for equipment engineers to choose from. Among them, capacitance and Piezoresistance pressure sensors are the most widely used.

Capacitance pressure transducer is a kind of pressure sensor which uses capacitance as sensitive element to change the measured pressure into capacitance value. Capacitive transducer for pressure measurement generally uses a circular metal film as an electrode of the capacitor. When the film is deformed due to pressure, the capacitance formed between the film and the fixed electrode will change, and the electrical signal related to the voltage can be output through the measuring circuit.

Piezoresistance pressure sensor is mainly based on piezoresistive effect. Piezoresistive effect is used to describe the resistance change of materials under mechanical stress. The pressure of the measured medium acts on the diaphragm of the sensor directly, which makes the diaphragm produce a micro displacement proportional to the pressure of the medium, and changes the resistance value of the sensor. The change is detected by an electronic circuit, and a standard measurement signal corresponding to the pressure is converted and output.

Piezoresistance Sensor Capacitance Transducer
Performance

·The earliest and most widely used type of pressure sensor.

·simple construction, low cost and good durability.

·Sensors must be powered, which makes them unsuitable for low-power or battery operating systems.

· When measuring the micro differential pressure within 10KPA, the sensitivity and accuracy are not high enough, and it is easy to drift。

 

 Performance

· Because there is no direct current flowing through the capacitors, they are essentially low-power.

·The capacitor elements are mechanically simple and robust and are very adaptable to overpressure conditions.

·Ideal for low voltage applications and quite harsh environments.

·Capacitance Transducer has low hysteresis and good repeatability. The temperature sensitivity is also very low.
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Intrinsically Safe – what it means

168

Intrinsically Safe refers to the function of making the production equipment or production system itself safe by means of design, which will not cause accidents even in the case of misoperation or failure. Specifically, it includes failure-safety (misoperation will not cause accidents or prevent misoperation automatically), failure-safety function (equipment and process can work normally temporarily or change safety state automatically in case of failure).

Intrinsically safe electrical equipment is characterized by the fact that all its circuits are intrinsically safe, that is, in normal operation or under the specified fault state the electric spark and thermal effect can not ignite the specified explosive mixture of the circuit. In other words, this kind of electric appliance is not explosion-proof by outer shell and filling, but the energy of electric spark or thermal effect produced by its circuit during normal use or failure is less than 0.28mJ (B-class explosion-proof), that is, the gas concentration is 8.5% (the most explosion-prone concentration), the minimum ignition energy.

The theoretical basis of Intrinsically Safe is to ensure that the electrical and thermal energy in the system is low enough not to cause the combustion of explosive gases, so only low voltages and small currents are allowed to flow through the hazardous area, and there are strict limits on energy storage.

Alpha offer Intrinsically Safe Pressure Transducer

The Model 168 intrinsically safe pressure transducer USES a metallic aluminum enclosure and complies with NEMA Level 4 (IP-65) standards

At the same time the product has the intrinsically safe explosion-proof function, very solid and reliable.

168

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Keeping Isoltation Rooms safe from Alpha

surface mount pressure sensor

Keeping Isolation rooms Safe During Disease Outbreaks

The outbreak of an infectious disease can be overwhelming if a hospital is not prepared. Hospitals employ a variety of strategies to prevent the spread of such illnesses. One of the most prevalent approaches is to treat these patients in isolation rooms. For those working in and around isolation rooms, their safety depends on proper control of the differential pressure of the room. As the coronavirus becomes ever more a pressing concern, it is critical for hospitals and staff to take the right precautions for keeping isolation rooms safe.

What are Isolation Rooms?

Isolation rooms are designed to prevent contaminated particles from spreading beyond a quarantined area. To prevent the contaminated air from escaping, isolation rooms require negative air pressure relative to the outside reference area, a difference of at least -0.01” WC. This ensures clean air is drawn into the room and contaminated particles, such as cough droplets, inside the room are not able to escape.
A relevant example of an infectious disease currently being treated in isolation rooms is the coronavirus. Since coronavirus can be easily spread through the air, treating patients in an isolation room keeps staff and other hospital patients safe from contracting the disease.
Although an isolation room might have the equipment necessary to meet minimal safety standards, such technology is often outdated and has disadvantages that can put patients and staff at risk. In an outbreak such as coronavirus, upgrading old instruments to eliminate said drawbacks can help ensure the safety of an isolation room. Improper or unbalanced room pressurization can put patients and staff at risk of exposure if dangerous contaminants escape the space.

How to Better Maintain Safety of Isolation Rooms

Hospitals are often fiscally constrained from making facility upgrades; yet, now more than ever, the healthcare system needs to demonstrate it can handle a potential epidemic, especially as the risk of coronavirus looms.
The most resourceful way to implement an isolation room strategy is to utilize the real-time monitoring and alarm capabilities of a room pressure monitor (RPM). RPMs are the safest and most reliable monitoring solution for isolation rooms. A transducer-based pressure sensor–DP transducer makes it more accurate and dependable than a ball pressure indicator and requires zero manual testing.
Some devices also have the ability to monitor temperature and humidity.

Alpha Can Help Keep Hospitals Safe

Alpha has engineered the best devices to protect patients and staff, especially those in isolation rooms. Model 178A can help prevent airborne infections like the coronavirus from spreading.

Model 178A can replace outdated and unreliable technology like a ball pressure indicator or pressure gauge. Using Alpha’s dead-ended variable capacitance pressure sensor, Model 178A allows staff to see at a glance if a room is properly pressurized, even from down the hall.
Unlike the visual-only function of a ball pressure indicator, Model 178A analog output can be directly connected to a BMS to indicate when a room goes into alarm.

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