Sikora, J.
On April 20, 2012, a complaint issued from the Boston Municipal Court charging the defendant, Pauric Houri-can, with operating a motor vehicle while under the influence of alcohol (OUI) in violation of G. L. c. 90, § 24(l)(a)(l). Before trial, the defendant moved to suppress evidence of a postarrest breathalyzer test. A Boston Municipal Court judge denied the motion, and a single justice of the Supreme Judicial Court allowed the defendant’s application for leave to pursue an interlocutory appeal. See Mass.R.Crim.P. 15(a)(2), as appearing in 422 Mass. 1501 (1996). For the following reasons, we reverse.
Factual background.1 At approximately 2:45 a.m. on April 20, 2012, Officer David Mammone of the Boston police department heard a “loud bang” and turned to see that the defendant had driven his black pickup truck into a police patrol wagon. The officer observed that the defendant had “glassy eyes” and smelled of alcohol. After he failed two of three field sobriety tests, police placed him under arrest. The ensuing voluntary breathalyzer test produced two breath samples measuring .121 and .143 percent blood alcohol content by weight (a difference of .022). The breath testing instrument was the Alcotest 9510; it employed a gas calibration standard. A result of .08 percent, if credited by the factfinder, establishes guilt of OUI. G. L. c. 90, § 24(l)(a)(l), first par.
Motion to suppress. Before trial, the defendant moved to suppress evidence of the breathalyzer test. He argued that the test was invalid because .022 percent blood alcohol content units separated his two breath samples. Title 501 Code Mass. Regs. § 2.14(4) (2010) provides that, if the administration of a prescribed sequence “does not result in breath samples that are within +/- 0.02% blood alcohol content units, a new breath test sequence shall begin”2 (emphasis added). The judge denied the [410]*410defendant’s motion in a ruling from the bench. She reasoned that the regulation required rounding the differential of .022 percent to .02 percent because it “simply says 0.02. It doesn’t take it out to the third decimal point.” She explained, “If [the differential] were [for example] 0.026, then I’d say we have an issue here because we can round it off to 0.03. But you’re 0.022.”
Discussion. 1. Standard of review. The interpretation of a regulation is a question of law which we review de nova. See Commonwealth v. B & M Fitzgerald Builders, Inc., 71 Mass. App. Ct. 486, 491 (2008), citing GPT-Acton, LLC v. Department of Envtl. Protection, 64 Mass. App. Ct. 103, 106 (2005). See also Corsetti v. Stone Co., 396 Mass. 1, 12 (1985). “We interpret a regulation in the same manner as a statute, and according to traditional rules of construction.” Warcewicz v. Department of Envtl. Protection, 410 Mass. 548, 550 (1991). “When the language of a regulation is ‘plain it must be given its ordinary meaning, and the language used constitutes the principal source of insight into regulatory purpose.’ ” Young v. Patukonis, 24 Mass. App. Ct. 907, 908 (1987), quoting from Morin v. Commissioner of Pub. Welfare, 16 Mass. App. Ct. 20, 24 (1983). See Mullally v. Waste Mgmt. of Mass., Inc., 452 Mass. 526, 535-536 (2008). “Where reasonably possible, no portion of the language of a regulation should be treated as surplusage.” Warcewicz v. Department of Envtl. Protection, 410 Mass, at 551, quoting from Bottomley v. Division of Administrative Law Appeals, 22 Mass. App. Ct. 652, 657 (1986). “Language in a regulation, like language in a statute, must be considered in [411]*411light of the other words surrounding it, and its scope and meaning must be determined by reference to context.” Knapp Shoes, Inc. v. Sylvania Shoe Mfg. Corp., 418 Mass. 737, 744 (1994) (quotation, citations, and brackets omitted). These canons are advisory, not mandatory; we do not apply them mechanically or dogmatically; and we inspect the results of their application for rationality and practicality. See Commonwealth v. Buccella, 434 Mass. 473, 481 (2001) (refusing to interpret a regulation in a manner that “would be utterly absurd and . . . clearly not what the [authoring agency] intended”).
2. Evolution of the regulations. The Legislature has charged the Secretary of the Executive Office of Public Safety and Security (Secretary) “with promulgating rules and regulations ‘regarding satisfactory methods, techniques and criteria for the conduct of breathalyzer tests.’ ” Commonwealth v. Steele, 455 Mass. 209, 212 (2009) (brackets omitted), quoting from G. L. c. 90, § 24K.3 For a breathalyzer test result to be valid and admissible in evidence, the Commonwealth must demonstrate compliance with those regulations that “go to the accuracy of the [breath testing] device or manner in which the [breathalyzer] test was performed.” Commonwealth v. Kelley, 39 Mass. App. Ct. 448, 453 (1995). See Commonwealth v. Barbeau, 411 Mass. 782, 786 (1992); Commonwealth v. Zeininger, 459 Mass. 775, 778-780 (2011).
In 2006, the Secretary promulgated regulations requiring that “[a]ll breath sample and calibration sample results shall be recorded in the two decimal mode. Any third or subsequent decimal place is to be truncated prior to the comparison of the results.” 501 Code Mass. Regs. § 2.56(5) (2006). The regulations provided further that, “[u]pan completion of a valid test, the test results shall be made available to the individual under arrest. If the two adequate breath samples, expressed in the two decimal mode, differ within +/- 0.02 blood alcohol content units, the lower of the two adequate breath samples shall be taken as the individual under arrest’s blood alcohol level.” 501 Code Mass. Regs. § 2.57 (2006).
[412]*412In 2010, the Secretary promulgated new regulations “to accommodate advancements/improvements in applicable technology.” See 1155 Mass. Reg. 78 (April 30, 2010); 501 Code Mass. Regs. §§ 2.00 et seq. (2010). Those regulations, which’ govern here, no longer require the recording of all breath samples in the “two decimal mode.” Instead, “[t]he results of the analysis of each breath sample and calibration standard shall be reported to at least two decimal places if the test was administered using a liquid calibration standard. The results of the analysis of each breath sample and calibration standard shall be reported in three decimal places, if the calibration standard is gas [as here]” (emphasis supplied). 501 Code Mass. Regs. § 2.15(1) (2010).
Despite this change, the 2010 regulations still provide that, if administration of a prescribed breath test sequence “does not result in breath samples that are within +/- 0.02% blood alcohol content units, a new breath test sequence shall begin.” 501 Code Mass. Regs. § 2.14(4) (2010). The regulations also retain the requirement that “the lower of the two breath sample results shall be truncated to two decimal places and reported as the arrestee’s [percentage of blood alcohol content by weight].” 501 Code Mass. Regs. § 2.15(2)(b) (2010).
3. Interpretation of 501 Code Mass. Regs. § 2.14(4) (2010). The decisive issue on appeal is the correct interpretation of 501 Code Mass. Regs. § 2.14(4) (2010).
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Sikora, J.
On April 20, 2012, a complaint issued from the Boston Municipal Court charging the defendant, Pauric Houri-can, with operating a motor vehicle while under the influence of alcohol (OUI) in violation of G. L. c. 90, § 24(l)(a)(l). Before trial, the defendant moved to suppress evidence of a postarrest breathalyzer test. A Boston Municipal Court judge denied the motion, and a single justice of the Supreme Judicial Court allowed the defendant’s application for leave to pursue an interlocutory appeal. See Mass.R.Crim.P. 15(a)(2), as appearing in 422 Mass. 1501 (1996). For the following reasons, we reverse.
Factual background.1 At approximately 2:45 a.m. on April 20, 2012, Officer David Mammone of the Boston police department heard a “loud bang” and turned to see that the defendant had driven his black pickup truck into a police patrol wagon. The officer observed that the defendant had “glassy eyes” and smelled of alcohol. After he failed two of three field sobriety tests, police placed him under arrest. The ensuing voluntary breathalyzer test produced two breath samples measuring .121 and .143 percent blood alcohol content by weight (a difference of .022). The breath testing instrument was the Alcotest 9510; it employed a gas calibration standard. A result of .08 percent, if credited by the factfinder, establishes guilt of OUI. G. L. c. 90, § 24(l)(a)(l), first par.
Motion to suppress. Before trial, the defendant moved to suppress evidence of the breathalyzer test. He argued that the test was invalid because .022 percent blood alcohol content units separated his two breath samples. Title 501 Code Mass. Regs. § 2.14(4) (2010) provides that, if the administration of a prescribed sequence “does not result in breath samples that are within +/- 0.02% blood alcohol content units, a new breath test sequence shall begin”2 (emphasis added). The judge denied the [410]*410defendant’s motion in a ruling from the bench. She reasoned that the regulation required rounding the differential of .022 percent to .02 percent because it “simply says 0.02. It doesn’t take it out to the third decimal point.” She explained, “If [the differential] were [for example] 0.026, then I’d say we have an issue here because we can round it off to 0.03. But you’re 0.022.”
Discussion. 1. Standard of review. The interpretation of a regulation is a question of law which we review de nova. See Commonwealth v. B & M Fitzgerald Builders, Inc., 71 Mass. App. Ct. 486, 491 (2008), citing GPT-Acton, LLC v. Department of Envtl. Protection, 64 Mass. App. Ct. 103, 106 (2005). See also Corsetti v. Stone Co., 396 Mass. 1, 12 (1985). “We interpret a regulation in the same manner as a statute, and according to traditional rules of construction.” Warcewicz v. Department of Envtl. Protection, 410 Mass. 548, 550 (1991). “When the language of a regulation is ‘plain it must be given its ordinary meaning, and the language used constitutes the principal source of insight into regulatory purpose.’ ” Young v. Patukonis, 24 Mass. App. Ct. 907, 908 (1987), quoting from Morin v. Commissioner of Pub. Welfare, 16 Mass. App. Ct. 20, 24 (1983). See Mullally v. Waste Mgmt. of Mass., Inc., 452 Mass. 526, 535-536 (2008). “Where reasonably possible, no portion of the language of a regulation should be treated as surplusage.” Warcewicz v. Department of Envtl. Protection, 410 Mass, at 551, quoting from Bottomley v. Division of Administrative Law Appeals, 22 Mass. App. Ct. 652, 657 (1986). “Language in a regulation, like language in a statute, must be considered in [411]*411light of the other words surrounding it, and its scope and meaning must be determined by reference to context.” Knapp Shoes, Inc. v. Sylvania Shoe Mfg. Corp., 418 Mass. 737, 744 (1994) (quotation, citations, and brackets omitted). These canons are advisory, not mandatory; we do not apply them mechanically or dogmatically; and we inspect the results of their application for rationality and practicality. See Commonwealth v. Buccella, 434 Mass. 473, 481 (2001) (refusing to interpret a regulation in a manner that “would be utterly absurd and . . . clearly not what the [authoring agency] intended”).
2. Evolution of the regulations. The Legislature has charged the Secretary of the Executive Office of Public Safety and Security (Secretary) “with promulgating rules and regulations ‘regarding satisfactory methods, techniques and criteria for the conduct of breathalyzer tests.’ ” Commonwealth v. Steele, 455 Mass. 209, 212 (2009) (brackets omitted), quoting from G. L. c. 90, § 24K.3 For a breathalyzer test result to be valid and admissible in evidence, the Commonwealth must demonstrate compliance with those regulations that “go to the accuracy of the [breath testing] device or manner in which the [breathalyzer] test was performed.” Commonwealth v. Kelley, 39 Mass. App. Ct. 448, 453 (1995). See Commonwealth v. Barbeau, 411 Mass. 782, 786 (1992); Commonwealth v. Zeininger, 459 Mass. 775, 778-780 (2011).
In 2006, the Secretary promulgated regulations requiring that “[a]ll breath sample and calibration sample results shall be recorded in the two decimal mode. Any third or subsequent decimal place is to be truncated prior to the comparison of the results.” 501 Code Mass. Regs. § 2.56(5) (2006). The regulations provided further that, “[u]pan completion of a valid test, the test results shall be made available to the individual under arrest. If the two adequate breath samples, expressed in the two decimal mode, differ within +/- 0.02 blood alcohol content units, the lower of the two adequate breath samples shall be taken as the individual under arrest’s blood alcohol level.” 501 Code Mass. Regs. § 2.57 (2006).
[412]*412In 2010, the Secretary promulgated new regulations “to accommodate advancements/improvements in applicable technology.” See 1155 Mass. Reg. 78 (April 30, 2010); 501 Code Mass. Regs. §§ 2.00 et seq. (2010). Those regulations, which’ govern here, no longer require the recording of all breath samples in the “two decimal mode.” Instead, “[t]he results of the analysis of each breath sample and calibration standard shall be reported to at least two decimal places if the test was administered using a liquid calibration standard. The results of the analysis of each breath sample and calibration standard shall be reported in three decimal places, if the calibration standard is gas [as here]” (emphasis supplied). 501 Code Mass. Regs. § 2.15(1) (2010).
Despite this change, the 2010 regulations still provide that, if administration of a prescribed breath test sequence “does not result in breath samples that are within +/- 0.02% blood alcohol content units, a new breath test sequence shall begin.” 501 Code Mass. Regs. § 2.14(4) (2010). The regulations also retain the requirement that “the lower of the two breath sample results shall be truncated to two decimal places and reported as the arrestee’s [percentage of blood alcohol content by weight].” 501 Code Mass. Regs. § 2.15(2)(b) (2010).
3. Interpretation of 501 Code Mass. Regs. § 2.14(4) (2010). The decisive issue on appeal is the correct interpretation of 501 Code Mass. Regs. § 2.14(4) (2010). The defendant argues that the regulation establishes that a breathalyzer test is invalid unless breath samples agree within .020 percent. See 501 Code Mass. Regs. § 2.13(4) (2010) (a “breath test shall be valid and the results admissible in a court of law if it complies with 501 [Code Mass. Regs. §] 2.14”). The Commonwealth, by contrast, argues that the regulation requires truncation to two decimal places of any differential between breath samples. Truncation means a cutoff of the measurement at two decimal places without regard to the value of any number in the third decimal place. It differs from the colloquial meaning of “rounding off” the second decimal place downward or upward by consideration of the third decimal place number.4 Thus, according to the Com[413]*413monwealth, the regulation would permit a differential of up to .029 percent to be truncated to .02 percent.
Each rationale has some support. On the one hand, .02 percent and .020 percent are mathematically equivalent. Accordingly, the defendant’s contention that any percentage above .020 is not “within .02%” has force. In addition, the theory of truncation advanced by the Commonwealth would appear to render meaningless the requirement that “the results of the analysis of each breath sample and calibration standard shall be reported in three decimal places, if the calibration standard is gas.” 501 Code Mass. Regs. § 2.15(1) (2010). See Bottomley v. Division of Administrative Law Appeals, 22 Mass. App. Ct. at 657 (“Where reasonably possible, no portion of the language of a regulation should be treated as surplusage”). Further, the Commonwealth’s view fails to acknowledge that the 2010 promulgation omits any requirement of truncation to two decimal places for comparison of the two predicate readings. Finally, as noted by the defendant, the record contains the “Breath Test Report Form” reporting his test results. It reads, “Test Results: Test Not Completed: TESTS OUTSIDE TOL[ERANCE].” The training manual for the Alcotest 9510 provides: “If first two [breath] samples DO NOT agree within +/- 0.02% ‘TEST OUTSIDE +/- TOE’ will appear on display briefly.” See Jones & Geraghty, Drunk Driving Defense App. B-2, at 795 (2013-2014 ed.).5
On the other hand, the Commonwealth correctly observes, as did the motion judge, that the “plain text” of 501 Code Mass. Regs. § 2.14(4) (2010) “allows for only two decimal places.”
[414]*414The Commonwealth points out that, if the Secretary had “intended to [impose] the level of precision that the defendant claims is required under the regulation, [she] would have expanded the permissible range of disparity [between breath samples] to include three decimal places, that is, no greater disparity than .020%.”6 Also, the Secretary left in place the requirement that, for the purpose of determining an arrestee’s percentage of blood alcohol content, “the lower of the two breath sample results shall be truncated to two decimal places.” 501 Code Mass. Regs. § 2.15(2)(b) (2010). Retention of that two-digit standard suggests that the Secretary has determined that any differential between breath samples may be truncated to two decimal places as a tolerable approximation.7
[415]*415On balance, we conclude that 501 Code Mass. Regs. § 2.14(4) (2010) is ambiguous and that we must reverse the denial of the defendant’s motion to suppress. See Falmouth v. Civil Serv. Commn., 447 Mass. 814, 818 (2006), quoting from AT&T v. Automatic Sprinkler Appeals Bd., 52 Mass. App. Ct. 11, 14 (2001) (language “capable of being understood by reasonably well-informed persons in two or more different senses” is ambiguous). Our primary reasoning is literal. The differential in the defendant’s test results, .022, is not “within +/- 0.02% blood alcohol content units,” as required by 501 Code Mass. Regs. § 2.14(4) (2010). Nothing in the regulations or record pertaining to testing procedures and comparison of results to determine their validity directly supports truncation of the third decimal place of a differential between breath samples obtained from a breath testing instrument employing a gas calibration standard. Standards which “touch upon criminal acts” demand greater precision than those which “do not define or relate to criminal conduct.” Caswell v. Licensing Commn. for Brockton, 387 Mass. 864, 873 (1983). A regulation “penal in character” should be unambiguous. See Druzik v. Board of Health of Haverhill, 324 Mass. 129, 134 (1949) (regulation “penal in character . . . should be so clearly expressed that those who may be subject thereto should not have to guess at its meaning”); Aristocratic Restaurant of Mass., Inc. v. Alcoholic Bevs. Control Commn. (No. 1), 374 Mass. 547, 552-553, appeal dismissed, 439 U.S. 803 (1978); Grocery Mfrs. of Am., Inc. v. Department of Pub. Health, 379 Mass. 70, 83 (1979) (assuming, without deciding, “that a regulation whose violation is a criminal act is tested by a higher standard of definiteness than a noncriminal regulation”).
Definiteness will assist not only the person subject to a regulation, but also the officials charged with its enforcement. Here, adherence to a plainly termed regulation and to the monitor warning against the validity of the first test sequence as beyond the permitted differential would have led the technician to conduct a second test sequence. In short, a duty of clarity rests upon an agency authoring a regulation defining criminal conduct [416]*416or defining a standard of proof of that conduct. The present regulations do not satisfy that duty.8
Beyond this literal ground, practical considerations of purpose and fairness support the same outcome. First, the .022 differential between the two measurements, each well above the .08 threshold, does not cause an illogical result, as suggested by the Commonwealth at oral argument: i.e., an election between two breathalyzer results substantially across the actionable .08 threshold. Rather, the differential determines the underlying reliability of the instrumentation and process creating all test results. An excessive differential casts doubt upon the reliability of all the readings. See Commonwealth v. Steele, 455 Mass, at 213 (“[I]mposition of the applicable margin, within .02 percent, see 501 Code Mass. Regs. § 2.57 [2006], can be said to safeguard further the reliability of the process. If the results fall outside this margin, the test is altogether invalid”) (interpreting former regulation).
Second, we must be concerned with the gravity of a measure [417]*417that, if admitted in evidence, will be a powerful determinant of guilt or innocence. The law provides the prosecution with two modes of proof of the OUI offense. It may offer evidence of impaired operation by the testimony of percipient witnesses. Concomitantly or independently, it may offer into evidence a properly qualified breathalyzer reading of .08 or higher. If the factfinder accepts the reading, it establishes per se the crime. See Commonwealth v. Colturi, 448 Mass. 809, 815-818 (2007); Commonwealth v. Hubert, 71 Mass. App. Ct. 661, 662 n.2 (2008), S.C., 453 Mass. 1009 (2009). The reading therefore requires the assurance of accuracy.
Third, strict enforcement of the limited differential does not operate unfairly or impractically against the prosecution of OUI charges. Independently of the breathalyzer results, the Commonwealth here and in typical cases can pursue proof of impaired operation by police testimony about the accused’s driving performance, appearance, demeanor, execution of field sobriety tests, and conduct at booking. See Commonwealth v. Colturi, supra at 811, 817. The requirement of precise standards and reliable breathalyzer readings does not deprive the Commonwealth of a sole source of evidence.
Finally, the Secretary has the ready ability to cure the present ambiguity by a remedial amendment of the regulation to define the permissible differential between two breathalyzer readings reported to the third decimal place. That improvement would demonstrate the competence of the government as the guardian of public safety and as the author of clear standards of criminal liability for violation of the public safety.
Conclusion. We therefore reverse the order denying the motion to suppress.
So ordered.