Taking measurements generally involves a tradeoff between measurement range and precision. Traditionally, pressure sensors are no exception to this rule: researchers and product developers must decide whether high-precision measurements are required at either the high- or low-pressure end of the range in any given pressure application. This presents significant challenges for accurately measuring pressures that vary over a large range. This is all about to change, thanks to a new flow-based differential pressure sensor capable of making measurements across a ±5,000 Pa range with sub-pascal precision at low pressure regions.
Pressure sensors have a wide breadth of application in many industries – from leak detection to medical equipment. While highly accurate ultra-low-pressure sensors are widely available, being able to accurately measure pressures across a very wide range has long proven to be difficult.
Now, a new breed of pressure sensor, developed by First Sensor, offers a way to do just this. These high dynamic range sensors are made possible thanks to calorimetric flow sensing technology. Let’s start by taking a look at how this technology works.
Measuring Differential Pressure Using Calorimetric Principles
In this approach to pressure sensing, a differential pressure is measured by allowing gas to flow through the sensor, from an area of high relative pressure to an area of low relative pressure. An embedded heating element supplies a known heat flux to the gas, and its flow rate is determined using two temperature sensors. By measuring the flow rate, the differential pressure can be determined.
First Sensor uses this calorimetry-based approach in their existing LMI, LME and LDE series of pressure sensors with a few important modifications.1,2 In these sensors, a microscale (60μm ) flow channel and thermal sensing elements are integrated into a single silicon chip using MEMS techniques. This offers significant advantages over comparable sensors which use molded channels: First Sensor’s smaller flow channels offer high flow impedance, intrinsic immunity to dust and humidity, and very low leakage rates.3,4 By making the channel very narrow, gas behaves more predictably and artefacts such as turbulence and chimney effects are nonexistant.5 All of this results in a small, robust and high-sensitivity pressure sensor.
Connecting such a sensor in a “bypass configuration” is advantageous when measuring flow rate or gas velocity, where the high impedance of the sensor ensures that there is minimal interference with the main flow.6
The performance and versatility of this sensor architecture lends the LDE/LME/LMI series of sensors to a range of applications in HVAC, medicine (respirators, anesthesia devices etc.) and leakage measurement.
High Dynamic Range Measurement Problem
Despite the performance of these sensors, measuring pressure variation with high accuracy across a wide range remains a problem. This can be described in terms of the “dynamic range” of a system: dynamic range describes the ratio of the smallest value of some quantity (in this case, pressure) to the largest value. In systems where the pressure or flow rate varies drastically, we say that this has a high dynamic range. But typically, pressure measurements can only be taken with high accuracy within a certain narrow region of that range.
This problem can be especially pronounced in certain flow applications. Flow rate or air velocity measurements typically rely on pressure measurements as described above. But functional elements of these flowmeters are essentially nonlinear, and generated pressure varies approximately with the square of flow rate. This means that if a given system’s flow rate has a dynamic range of 1:100, then the pressure generated at the sensor will have a dynamic range of around 1:10,000.
Accommodating this kind of dynamic range with a single accurate sensor device has proven to be problematic – until now.
LHD ULTRA Series Pressure Sensors
First Sensor’s new LHD ULTRA line of pressure sensors builds on their existing calorimetric microflow technology to deliver a versatile pressure sensor capable of incredibly accurate measurements over a wide range. LHD ULTRA pressure sensors feature two microflow channels and two sensing elements on a single chip. With one sensing element designed to accurately measure low differential pressures with high resolution, and the other designed to expand the measurement range over a much wider region; the device as a whole enables accurate pressure measurements at low pressures with an extremely high dynamic range.
An on-board microcontroller with accurate 24-bit analogue-digital conversion provides signal processing, which includes stitching of the responses of the two elements, linearization, and temperature compensation. The result is a sensor which is more than the sum of its parts.
First Sensor’s LHD ULTRA sensors offer pressure ranges between ±1,250 and ±5,000Pa (±5 to ±20 inH2O), with a resolution of around 50mPa at low differential pressure values. They offer all the performance advantages of the existing LMI and LDE lines of pressure sensors including high flow impedance, immunity to dust and humidity and low leakage rates. The sensors offer no loss of sensitivity when using long tubing, and have outstanding long-term stability and precision thanks to patented real-time offset compensation and linearization techniques which make pressure measurements both accurate and simple to obtain.
The LHD ULTRA series sensors also feature an on-board barometer for optional pressure output compensation, and an operating compensated temperature range of -20 to +85°C.
Learn more about the LHD ULTRA series here.
References and Further Reading
1. LMI series-digital low differential pressure sensors. Available at: www.first-sensor.com.
2. LDE series-digital low differential pressure sensors. Available at: www.first-sensor.com.
3. LDE/LME/LMI Series-superior immunity to humidity. Available at: www.first-sensor.com.
4. LDE/LME/LMI Series-dust test. Available at: www.first-sensor.com. (Accessed: 28th April 2020)
5. Whitesides, G. M. The origins and the future of microfluidics. Nature 442, 368–373 (2006).
6. LDE/LME/LMI pressure sensors in bypass configuration for gas flowmeters. Available at: www.first-sensor.com.