Characterization of On-Board Vehicular Hydrogen Sensors
|Publication Date:||1 October 2018|
This SAE Technical Information Report (TIR) provides test methods for evaluating hydrogen sensors when the hydrogen system integrator and/or vehicle manufacturer elect to use such devices on board their hydrogen vehicles, including hydrogen fuel cell electric vehicles (FCEV).
The tests described in 5.1 of this document are performance-based and were developed to assess hydrogen sensor metrological parameters. These tests were designed to accommodate a wide range of environmental and operating conditions based on different possible situations and sensor implementations within the vehicle. Section 5.2 covers supplemental electrical safety and physical stress tests. These are based upon standard tests developed for qualifying electrical and other components for use on vehicles and do not explicitly pertain to gas sensor metrological performance assessment. Since the use of on-board hydrogen sensors is not standardized or mandated, their implementation can vary greatly from vehicle to vehicle and among potential applications or functions. For example, an on-board sensor could be located in a relatively dry environment such as in the passenger compartment or in a "highly humidifed" environment, such as within the process exhaust from the fuel cell system. As this is a guidance document and not a standard, no specific application will be identified. Also, as a guidance document, no performance specification or pass/fail criteria will be defined. For this reason, the hydrogen system integrator and/or vehicle manufacturer need to determine which tests and associated test conditions are relevant for their application(s). Thus, it is the prerogative of the hydrogen system integrator and/or vehicle manufacturer to define specific test acceptance criteria necessary to achieve the required performance of their process control and protective systems within the vehicle. The sensor manufacturer or testing laboratory is to present results of each test to the hydrogen system integrator and/or vehicle manufacturer, who will then use the results to ascertain the suitability of a sensor technology for their application.
The test methods provided in this TIR were derived from methods originally developed by researchers at the NREL Hydrogen Safety Sensor Test Laboratory . Many are analogous to the test procedures presented in ISO 26142 . As part of the development of this TIR, the original NREL test methods were modified to be more compatible with the on-board vehicular environmental and operating conditions and accordingly are consistent with SAE component standards. The sensor test protocols in 5.1 were adapted to allow test procedures that can be performed within a more practical time period than that afforded by the methods specified in either ISO 26142 or the standard NREL sensor test protocols while maintaining rigorous data quality. Practical guidance on the implementation of the test procedures is presented in Appendices B and C.
Hydrogen Concentration Units
The amount of a minor constituent (MC) in a background matrix (e.g., hydrogen in air, carbon monoxide in hydrogen, salt dissolved in water) can be expressed in different ways. In liquids, the minor component is called the solute, while the background matrix is the solvent. Liquid solutions (solvent + solute) are often defined in concentration units (e.g., gsolute/Lsolution or molessolute/Lsolution, which is the definition of molarity), or as a mass fraction or mass ratio (e.g., gsolute/[gsolvent + gsolute]). Ratios are unitless. Also, depending on the amount of solute, ratios are often presented as parts per million (ppm), which is generated by multipling the mass fraction by 106 be 1 ppm) or percent which is generated by multiplying the mass fraction by 100 (e.g., a mass fraction of 0.04 would be 4%). The symbols ppmwt and wt% are sometimes used to indicate that the expressed quantity is based upon a mass fraction.
Alternatively, in gas mixtures, the amount of a minor component (MC) is commonly presented as a volume ratio, VMC/(VMATRIX + VMC), which from the ideal gas law is equivalent to the mole ratio (molesMC/[molesMATRIX + molesMC]). Volume ratios are also often represented as percent or parts per million (ppm). To differentiate between ratios based on mass, volume ratios will sometimes be indicated as vol% or ppmv 1. An equivalent definiton of ppmv is μmole/mole. The amount of a flammable gas is also indicated by the fraction of the lower flammable limit (LFL), which is the lowest concentration that a flammable mixture of gas or vapor in air that can be ignited. For example, the LFL of hydrogen is 4 vol%; thus, 2 vol% H2 may be indicated as 50% LFL. This is useful because LFL varies among different gases, but regulations are often dictated by the concentration of a specific gas relative to its LFL. Table 1 indicates the preferred unit for expressing hydrogen concentrations as specified in a variety of sources. The term lower explosion unit (LEL) is often used as synonymous with the LFL, although this is not correct, since there are distinctions between flamability and an explosion and thus the corresponding LEL and LFL for a specific gas  .
In this document, unless noted otherwise, the amount of hydrogen or other minor components in a gas mixture will be indicated as either vol% or ppmv. The background matrix will be air unless otherwise indicated. Also, the testing laboratory is to report sensor test results in a numerical format equivalent to vol% H2. See Appendix A for details on data analysis and reporting of test results.
Description of a Hydrogen Sensor
There are numerous definitions for chemical or physical sensors. One definition is as follows: A sensor is a small device that, as the result of an interaction or process between a chemical or physical stimulus and the sensor device, transforms chemical or physical information of a quantitative or qualitative type into an analytically useful signal . A hydrogen sensor is the special case where the target stimulus is hydrogen. There is, however, no universally accepted definition of a chemical sensor, and the concept of what constitutes a chemcial sensor varies considerably in the literature. Accordingly, some clarification will be provided as to the meaning of "hydrogen sensor" as it pertains to this TIR and the needs of the hydrogen system integrator and/or vehicle manufacturer.
Figure 1 illustrates the functional design of a "sensor" as the term is used within this TIR. A sensor consists of several components. The sensing element is the component that performs the basic sensing operation and provides a measurable, continuously changing electrical signal in correlation to the presence or magnitude of the stimulus, such as the amount of hydrogen. The sensing element can include a receptor and one or more transducer elements. The receptor is the site of interaction with the stimulus. For chemical sensing elements, the receptor is the site of the chemical interaction, which is then converted by the transducer into an analytical quantity that is usually electrical in nature. Thus, two distinct functions are associated with the sensing element; it is the site for the interaction of the stimulus at the receptor and the generation of electrical signal by the (electrical) transducer. The transducer and receptor may be the same physical component within the sensing element. Sensing elements based on different transduction mechanisms are commercially available for the detection of hydrogen.
Although related to the magnitude of the stimulus, the sensing element's electrical signal is often of an arbitrary magnitude with a low-output impedance to render it easily corrupted. Thus, additional circuity is typically required to produce a usable electrical signal. As illustrated in Figure 1, a sensor is the integration of the sensing element and appropriate control circuitry to transduce a raw electrical response into a useful signal via electronic buffering, amplification, or transduction into more conveniently measurable electrical responses (e.g., a current-to-voltage transduction). This supplemental circuitry can also control sensor operation such as to maintain the operating temperature for heated sensors or the operating potential for electrochemical sensors. Multiple sensing elements that respond to different target stimuli can be incorporated into a sensor. For example, there may be sensing elements that can be used to independently measure environmental parameters such as the temperature (T), pressure (P), and relative humidity (RH) within a chemical sensor.
Although some international standards (e.g., ISO 26142 ) explicitly distinguish between the sensing element and sensor, the term "sensing element" is not commonly used in the scientific, engineering, and commercial literature. Instead, the term sensor is often used to refer to either a sensing element or a sensor, or even a detection apparatus (as defined above). Sensing elements and sensors as illustrated in Figure 1 are commercially available for the detection and quantitation of hydrogen. However, the use of sensing elements will require development of electrical circuitry by the end-user to get a useful signal, while a sensor will only need electrical power. Table 2 lists the major sensing element types that are commonly available for the detection of hydrogen along with a brief description of the transduction mechanism and the nature of the electrical signal. Sometimes the term "sensor platform" is used to indicate the sensing element type. More details on the different sensor platforms developed for the detection of hydrogen can be found in Hübert et al .
The electrical output of a sensor can be integrated into a detector apparatus. Other terms used for the detection apparatus are "instrument" or "instrument system," "control unit," and "analyzer." The detector apparatus can also be the integration of the sensor with an on-board microprocessor. A detection apparatus utilizes the sensor response to perform functions such as warning and alarm activation, implement corrective measures such as increased ventilation, shutoff or isolation of the fuel supply or even system shutdown, and display of prevailing conditions such as concentration of hydrogen in the surrounding environment. However, there is no formal definition as to what distinguishes a sensing element, sensor, or control system from each other. The working definition for a "hydrogen sensor" within the context of this document will be a device that outputs a signal that is either directly related to the amount of hydrogen in a test gas or is easily transformed to the amount of hydrogen through the use of a calibration expression.
Sensor Output in Response to Hydrogen
The response of a hydrogen sensor is typically a change in electrical signal (current, voltage, or frequency change) in response to a change in the concentration of hydrogen in the environment surrounding the sensor. The electrical signal of a hydrogen sensor can be expressed in a variety of formats. Many sensors have an analog output that is pre-calibrated to hydrogen. Common types of calibrated analog outputs are 4 to 20 mA, or 0 to 5 V that are assumed to be linear over a specified range (e.g., 0 to 2 vol% H2, or 0 to 4 vol% H2). Figure 2(A) illustrates the response to 1 vol% H2 for a sensor with a 4 to 20 mA output with a measuring range of 0 to 4 vol% H2, while Figure 2(B) shows the conversion of the sensor output to vol% H2 using the linear transformation derived from this relationship. There are also hydrogen sensors whose output signal is in units of vol% or other common concentration units (e.g., ppmv or % LFL). Typically, these will be sensors with a digital output, such as CAN (Controller Area Network). Such sensors typically cannot be recalibrated by the end-user, but end-users should confirm sensor accuracy, such as through performing the measuring range test described in 5.1.2.
A sensor signal depends on the magnitude and time profile of the stimulus. A standard method to evaluate a hydrogen sensor performance is to measure the sensor signal in response to what is often represented as a step-change in the hydrogen concentration. Typically, this is performed with other parameters (e.g., T, P, and RH) defined. In this idealized exposure profile, the normalized input stimulus (e.g., hydrogen) is given by:
[H2] = 0 for t < t H2-ON
[H2] = 1 for t > t H2-ON and t < tH2-OFF
[H2] = 0 for t > t H2-OFF
In the above expressions, t refers to the test time, starting at time zero, while t H2-ON is the time at which the test gas is applied or injected into the test fixture and t H2-OFF is the time at which the test is removed from the test fixture. This test protocol is illustrated in Figure 2. The sensor response profile consists of several empirically observed regions  associated with the hydrogen exposure. There is a dead time (tdead), which is associated with the time required for the hydrogen to reach the sensing element following the establishment of the hydrogen concentration setpoint in the test apparatus and is controlled by the pneumatic design of the test fixture. The dead time does not contribute to the fundamental sensor response time. However, in a properly designed test apparatus, tdead can be negligible. There is also the delay time (tdelay) which is the time following actual test gas exposure to the sensor and the observation of a sensor response. Following the delay time, the signal rises in response to the hydrogen (the adjustment time, ta) and then levels off to a time-invariant response. As illustrated in Figure 2, the response for a well-behaved sensor will reach or approach a time-invariant (steadystate) output for a fixed hydrogen concentration, the magnitude of which has been termed the "final indication" (F.I.) of the sensor. The F.I. is the analytical response of the sensor that is most often correlated hydrogen concentration.
Signal noise and drift are associated with every real sensor response. Thus, criteria for verifying that a sensor signal has reached an appropriate time invariant state have to be defined. It is typically based upon defining an acceptable change in the sensor response over a prescribed period. For those applications where the hydrogen level is normally zero, the timeinvariant response can be defined as when the sensor response to a constant hydrogen concentration does not change more than 5% of reading over one minute, which is consistent with the definition provided in international standards (e.g., ISO 26142 ); a more stringent requirement may be imposed by the hydrogen system integrator and/or vehicle manufacturer for certain applications where real-time accuracy is necessary.
The imposition of a change in the test gas composition is often depicted as a step change, as described above and illustrated in Figure 2 (the dashed lines). Such an ideal step change in the test gas composition is not possible, although it can be approximated with some test apparatus designs (e.g., a flow-through apparatus, such as that described in Appendix C). However, an instantaneous change in test gas composition is not required to evaluate the sensor performance as per the protocols presented in 5.1. There will always be a lag time between the establishment of the apparatus control parameters to a new test gas set point and actually supplying the desired composition at the sensor. This effect is illustrated in Figure 3. The lag time will vary with apparatus design. In 5.1, the test protocols are presented as a series of discrete steps in which test conditions (e.g., test gas composition, temperature, humidity, pressure, orientation, or other parameters) are defined. The step changes in the protocol figures in 5.1 are not meant to imply an instantaneous adjustment of the test gas composition at the sensor, but rather to define the adjustment to a new gas composition set point, and, although not explicitly indicated in the protocol figures, there will be a stabilization time for the test gas composition at the sensor to reach the new set point. The stabilization time to achieve the gas composition set point at the sensor is not explicitly defined in the protocol because it will vary with apparatus design and operational parameters. It is not necessary to explicitly define this stabilization time. Rather, the protocols require that each step be sufficiently long to allow the test sensor to reach a stable final indication, which can only occur when the test gas exposed to the sensor is invariant in composition.
A step-change in gas composition can be approximated, however, if the pneumatic system of the test apparatus can be quickly purged. This can be achieved with a flow-through apparatus (see C.2.2), providing the apparatus is designed with fast valves or mass flow controllers to quickly adjust the incoming test gas composition to the desired set point and small internal gas volume, coupled with a high gas flow rate to minimize the purge time of the pneumatic system. Only the response time determination requires that the test gas composition be changed quickly (see 5.1.1 and 5.1.10). The actual apparent (observed) sensor response time will be a convolution of the fundamental sensor response kinetics and the purge time of the gas that is exposed to the sensor. A good approximation for the fundamental sensor response time may be achieved in a flow-through system with a fast purge time. Alternatively, in a chamber test apparatus, the kinetics of the sensor response will often be dominated by gas purge time of the chamber. As a result, chamber methods should not be used for the determination of the sensor response time but are very useful for measuring the sensor concentration dependence and stability, as well as to quantify the impact of environmental parameters (e.g., T, P, RH, and orientation) on sensor performance.
The output of most hydrogen sensors is an electrical signal that continuously changes over the sensor measurement range in response to changes in the magnitude of the hydrogen. In other words, the F.I. of the sensor is a function of the hydrogen concentration. With appropriate electronics and with calibration, the electrical signal can be used to determine the hydrogen concentration; the calibration is typically performed by the manufacturer. Test protocols for performance assessment of sensors with an electric signal that varies with hydrogen concentration are presented in 5.1. Alternatively, the response of some sensors may only provide information pertaining to whether the hydrogen is present above some threshold value. The threshold value is typically set by the manufacturer. This sensor type shall be referred to as an "indicator" or "indicator sensor." The response of an indicator sensor can be either visual (e.g., a colorimetric indicator) or electrical. Indicator sensors typically have only two output states: corresponding to low when the hydrogen level is below a threshold value, and high for when the hydrogen level is above a threshold value. While colorimetric indicators are commercially available for hydrogen, reliable electronic indicator sensors are currently not very common. Indicator sensors require different test protocols for evaluation than sensors with a continuously varying output; the TIR will be updated should commercial indicator sensors become commonly available.
The purpose of this document is to align the terminology, to define a common set of tests that can be used to evaluate hydrogen sensors, and to encourage the development of sensors suitable for on-board vehicular applications.
Field of Application
This document is applicable to sensors for the detection of hydrogen in hydrogen-powered fuel cell electric vehicles or vehicles powered by hydrogen internal combustion engines (ICEs).
1 Mass ratios are not directly converted into volume ratios. For example, if the solute in 1 L of a 1 ppmwt solution of dissolved hydrogen was volatilized into 1 L of air, the room temperature hydrogen composition would be about 12000 ppmv.