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Use Media-Isolated Pressure Sensors to Increase Reliability and Precision of Industrial Processes

Contributed By Digi-Key's North American Editors

Designers of closed-loop industrial and commercial processes such as heating, ventilation, air conditioning, and refrigeration (HVAC/R) use electromechanical pressure transducers to enhance control and improve process performance. The problem is that the liquids and gases used in these systems, combined with the wide range of temperatures and pressures at which the systems operate, can attack the pressure transducer’s materials, causing corrosion that can lead to leaks that compromise the sensor’s integrity.

Designers need an alternative technology that can meet environmental challenges while providing the application’s required accuracy and reliability.

This article describes how strain gage-based pressure transducers work before introducing media-isolated pressure (MIP) transducers from Honeywell. These are fabricated from stainless steel and feature a hermetically welded design instead of the O-ring and adhesive seals that often prove to be weaknesses in typical sensors. The article then looks at sources of measurement errors and how they can be minimized before demonstrating how the transducers can be applied in a commercial refrigeration system to increase the efficiency of the process.

How an electromechanical pressure transducer works

Modern pressure transducers are based on electrical outputs and do away with older and fickler mechanical linkages and dials. The key advantages of today’s electromechanical devices are reliability, precision, and the ability to be remotely monitored. Their main measurement technology is based either on piezoelectric materials or strain gages. Piezoelectric pressure transducers are only suitable for dynamic pressure measurement, while strain gage units can be used for both dynamic and static pressure measurement. This article will focus on the latter.

Strain gages are electrical circuits that change in resistance when subject to strain, where strain is the ratio of the change in length of a material subject to a force compared with its unloaded length (designated “ε”). The strain gage is typically categorized according to its “gage factor” (GF) which is a measure of its sensitivity to strain. In other words, GF is the ratio of the fractional change in electrical resistance to the fractional change in length (or strain).

In use, the pressure transducer is inserted directly into the pressurized system where the system’s liquid or gas enters a port in the transducer and displaces a diaphragm. A strain gage is attached using a suitable adhesive to the upper side of this diaphragm (Figure 1).

Diagram of diaphragm-mounted strain gageFigure 1: A diaphragm-mounted strain gage suitable for use in a pressure transducer. In this example, the actual diameter of the strain gage is 6.35 millimeters (mm). (Image source: Micro Measurements)

Even under very high pressures, the change of length of the strain gage is likely to be no more than a few “millistrain” (mε), which in turn leads to a very small change in resistance. For example, suppose a test specimen undergoes a strain of 350 mε. Under this load, a strain gage with a GF of 2 will exhibit a change in electrical resistance of 2 (350 x 10-6) = 0.07 percent. For a 350 ohm (Ω) gage, the change in resistance would be just 0.245 Ω.

How to make strain gage measurements

To accurately measure such small changes in resistance while minimizing the impact of noise, the pressure transducer’s strain gage is incorporated into one leg of a Wheatstone bridge, a network of four resistive arms with an excitation voltage, E, applied across it (Figure 2).

Image of Wheatstone bridge circuit diagramFigure 2: In this Wheatstone bridge circuit diagram, the strain gage is incorporated into one arm; RG is the strain gage resistance and RL1 and RL2 are the strain gage lead wire resistances; resistors R2, R3, and R4 are fixed, known values; eo is the output voltage and E the excitation voltage. (Image source: Micro Measurements)

The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits with RG (assuming the resistance of the lead wires RL1 and RL2 is negligible) and R4 comprising one voltage divider circuit, and R2 and R3 comprising the second. The output, eo, is measured between the middle nodes of the two voltage dividers and can be calculated from:

Equation 1 Equation 1

From Equation 1, it can be seen that when RG/R4 = R3/R2, the output voltage, eo, is zero and the bridge is said to be balanced. Any change in resistance of the strain gage will then unbalance the bridge and produce a nonzero eo proportional to the strain. In a pressure transducer, the output voltage from the diaphragm-mounted strain gage is said to be “ratiometric” (linearly proportional) to the supply (excitation) voltage, E, across the complete pressure range.

Temperature compensation

A design challenge when using strain gages is their susceptibility to temperature effects. Temperature fluctuations can introduce offset and span errors and increase hysteresis.

The strain gage can heat up due to the excitation voltage, E, but this can be mitigated to a great extent by keeping E low. The downside is that this will lower the sensitivity of the system, but the output voltage from the Wheatstone bridge, eo, can be amplified if required. However, special care must be taken to avoid amplifying superimposed noise. One solution is to use “carrier frequency” amplifiers which convert the voltage variation into a frequency variation and use a narrow bandwidth output to keep the noise low and reduce out-of-band electromagnetic interference (EMI).

A second source of heat comes from the diaphragm and body of the pressure transducer itself. Hot temperatures will cause the diaphragm to expand and the strain gage to register a strain that is not directly due to liquid or gas pressure.

To mitigate these effects, modern strain gages incorporate temperature compensation measures. The strain gages are typically manufactured from a 55 percent copper/45 percent nickel alloy. The material has a very low coefficient of thermal expansion (CTE) which limits temperature-induced strain. In addition, by carefully matching the CTE of the strain gage with that of the diaphragm material to which it is attached, a degree of “self-temperature compensation” can be implemented, limiting temperature-induced strain to just a few micrometers/meters/degrees centigrade (μm/m/°C).

Another source of temperature-induced error can come from the lead wires carrying the strain gage voltage signals. In the initial discussion of the bridge characteristics in Figure 2 above, the resistance of these wires (RL1 and RL2) was assumed to be negligible; but if the lead wires are made from copper, then as little as a 10°C rise in temperature could cause a bridge offset equivalent of several hundred microstrain (µε) directly from the leads. A common technique to overcome this offset is to use a three-wire bridge (Figure 3).

Image of Wheatstone bridge circuit diagramFigure 3: In this Wheatstone bridge circuit diagram, the negative output bridge electrical node is moved from the top of R4 to the bottom of the strain gage at the end of RL2. With lead wires RL1 and RL2 forming the same resistance, the bridge will be balanced. The lead wire RL3 is a voltage-sensing wire only and has no effect on bridge balance. (Image source: Micro Measurements)

In Figure 3 it can be seen that the negative output bridge electrical node is moved from the top of R4 to the bottom of the strain gage at the end of RL2. Lead wire RL1 and the strain gage (RG) comprise one arm, with RL2 and resistor R4 forming the adjacent arm. If lead wires RL1 and RL2 have the same resistance, then the two bridge arms will be equal in resistance and the bridge is balanced. The lead wire RL3 is a voltage-sensing wire only; it is not in series with any of the bridge arms and has no effect on bridge balance.

Providing both RL1 and RL2 are subject to the same temperature fluctuations, the bridge will remain balanced. Additionally, because only one lead wire is in series with the strain gage, lead wire induced temperature sensitization is reduced by half compared to a two-wire configuration.

In addition to the effect of temperature on the pressure transducer’s output, there are other sources of error. These sources of error are often referenced to the “ideal transfer function”, which is a straight line, independent of temperature, passing through the ideal offset with a slope equal to the ideal full-scale span (FSS) over the operating pressure range. The offset is the output signal obtained when a reference pressure is applied and FSS is the difference between the output signal measured at the upper and lower limits of the operating pressure range (Figure 4).

Diagram of pressure transducer’s ideal transfer functionFigure 4: A pressure transducer’s ideal transfer function is a straight line, independent of temperature, passing through the ideal offset with a slope equal to the ideal FSS over the operating pressure range. (Image source: Honeywell)

Lower quality pressure transducers can be subject to relatively large offset and FSS errors when they leave the factory. The offset error is the maximum pressure deviation compared to the ideal offset, while FSS error is the maximum deviation in measured FSS at reference temperature relative to the ideal (or target) FSS as determined from the ideal transfer function.

Further errors come from the accuracy of the pressure transducer itself, which can be subject to pressure non-linearity, pressure hysteresis, and non-repeatability. The combination of thermally-induced errors, transducer inaccuracies, and offset and FSS errors determine the pressure transducer’s total error band (TEB). TEB is the maximum deviation in output from the ideal transfer function over the entire compensated temperature and pressure range (Figure 5).

Diagram of sources of error for a pressure transducerFigure 5: The sources of error for a pressure transducer add up to the TEB. (Image source: Honeywell)

Heavy-duty pressure transducers

Pressure transducers used in industrial applications are exposed to corrosive liquids and gases, and wide temperature fluctuations. For example, the transducers used in an HVAC/R application are exposed to refrigerants such as butane, propane, ammonia, CO2, glycol plus water, or a range of synthetic hydrofluorocarbon refrigerants such as R134A, R407C, R410A, R448A, R32, R1234ze or R1234yf. Also, temperatures in industrial HVAC/R systems span the industrial temperature range of -40 to +85°C or even greater.

Many low- to mid-range pressure transducers are manufactured from alloys such as brass and use O-rings and adhesives to seal the sensor’s electronics from the fluids and gases touching the diaphragm. When used with corrosive substances, the seals can prove a weakness and start to leak. Such leaks can go undetected at first, leading to spurious readings and poor system control. Eventually the leaks cause failure as the electronics become exposed to the corrosive fluids or gases.

To avoid these potential failure modes, designers can use Honeywell’s MIP Series of pressure transducers. These heavy-duty, media-isolated pressure transducers eliminate the internal O-ring and adhesive seals. The transducers are fabricated from stainless steel and feature a hermetically welded design instead of an O-ring seal. The design makes the MIP sensors compatible with a wide range of media including aggressive fluids, water, and gases across a temperature range of -40 to 125°C and pressures from 100 kilopascal (kPa) to 6 megapascal (mPa) (Figure 6).

Image of Honeywell’s MIP Series pressure transducersFigure 6: Honeywell’s MIP Series pressure transducers are made from stainless steel and use a hermetically welded design that eliminates the need for seals. The design makes the sensors compatible with a wide range of media including aggressive fluids, water, and gases. (Image source: Honeywell)

The MIP Series operates from a 5 volt supply and provides a ratiometric output across a 0.5 to 4.5 volt DC range. TEB across the pressure transducer’s entire temperature range is ±1.0 percent for pressures ≤1 MPa and 0.75 percent for pressures >1 MPa. The transducer’s accuracy is ±0.15 percent FSS (best fit straight line (BFSL)) (Figure 7), and it has a 1 millisecond (ms) response time and a burst rating of over 20 MPa.

Graph of Honeywell MIP Series pressure transducersFigure 7: The MIP Series pressure transducers operate from a 5 volt supply and provide a ratiometric output across a 0.5 to 4.5 volt DC range. TEB across the pressure transducer’s entire temperature range is ±1.0 percent for pressures ≤1 MPa and 0.75 percent for pressures >1 MPa. (Image source: Honeywell)

Additionally, the series features ±40 volt DC overvoltage protection and sensor output diagnostics when an electrical failure occurs (Table 1).

Table of Honeywell MIP Series pressure transducer operating characteristicsTable 1: MIP Series pressure transducer operating characteristics. (Image source: Honeywell)

Pressure transducers in HVAC applications

Pressure transducers play a key role in applications such as HVAC systems by enabling precise control to maximize efficiency while lowering energy usage. For example, consider the HVAC/R cycle used by an industrial refrigeration unit (Figure 8).

Image of diagram showing HVAC/R cycleFigure 8: Diagram showing HVAC/R cycle. Heavy-duty pressure transducers at the compressor and evaporator outlets can be used to monitor the refrigerant pressure to ensure optimum refrigerant phase changes, and in turn determine the efficiency of the cycle. (Image source: Honeywell)

At the compressor stage, low pressure vapor from the evaporator is compressed (causing heating) and pumped to the condenser. At the condenser, the high temperature vapor releases its latent heat into the air and condenses into a hot liquid. A drier then removes any water from the refrigerant. Then, at the metering device, the hot liquid from the condenser is pushed through a flow restriction which reduces its pressure, forcing the refrigerant to give up heat. Then, inside the evaporator, this cold liquid absorbs heat from the condenser’s return air flow and changes into a vapor. This vapor continues to absorb heat until it reaches the compressor where the cycle repeats. The cool air from the evaporator is used to lower the temperature of the refrigerated container.

The refrigeration cycle works because as the refrigerant changes from liquid to vapor and back again, there is a large release or gain of latent energy. To operate efficiently and effectively, the pressure in the various parts of the system must be carefully monitored and controlled. This is particularly the case when the refrigerant undergoes the liquid-to-vapor/vapor-to-liquid phase changes. For example, under low pressure, the refrigerant changes from a liquid to a gas and absorbs latent energy (heat) at a lower temperature than it otherwise would. Under high pressure, the refrigerant gas changes from a gas to a liquid at higher temperatures than it otherwise would, releasing latent energy (heat).

By monitoring the pressure at the compressor and evaporator outlet, the compressor and metering device can be set to precisely control the flow (and hence pressure) in the low and high pressure parts of the cycle, and in turn the temperature of the refrigerant phase changes in order to maximize the efficiency of the system.

Conclusion

Strain gage pressure transducers offer a good solution for pressure measurement in industrial processes, but designers of systems that are likely to be exposed to environmental extremes need to be aware of the limitations of models that use O-rings and adhesives.

Designed for applications that may experience such extremes, Honeywell’s MIP Series pressure transducers use stainless steel fabrication and a hermetically welded design. The construction makes the MIP sensors compatible with a wide range of industrial liquids and gases and ensures long life even at elevated temperatures and pressures. The Honeywell pressure transducers also offer high precision, fast response, good long-term stability, and excellent EMI immunity.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

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