Ask Deandra: What’s the Difference Between an Ignition Interlock and a SCRAM Device?

There is one difference nobody tells clients, and it shapes everything else. The interlock is a regulated device and SCRAM is not. The interlock has its own bond statute, its own probation statute, a statutory definition, a state device-approval process, and a DPS rule chapter governing vendors. Texas has no statute that so much as names transdermal alcohol monitoring, no device approval, no calibration standard and no vendor certification. SCRAM rides on a judge’s general power to set reasonable conditions, and the science behind it is a private company’s proprietary algorithm.

Here is the longer answer: how each device works, what the research actually shows about the transdermal one, when each is imposed and under what authority, what the violations look like, and the practical realities of living under one or both. For the full map of court-ordered monitoring in Texas, including Soberlink and remote breath testing, see Alcohol Monitoring Devices. For SCRAM on its own, see What Is a SCRAM Device?

How the Ignition Interlock Works

An ignition interlock device is a small breath-testing instrument wired into a vehicle’s ignition system. Before the vehicle can start, the driver must blow into the device. A built-in fuel cell measures alcohol in the breath sample. If the reading is below the device’s programmed setpoint, the device permits ignition. If the reading is at or above it, the device locks out the ignition and the vehicle will not start.

Transportation Code §521.247 tells DPS to adopt rules approving devices, and the rule DPS wrote, 37 Texas Administrative Code §10.31, does not specify a threshold at all. It instead requires that a device be tested by an independent accredited laboratory against the model specifications published by the National Highway Traffic Safety Administration, whichever version is current when approval is requested. NHTSA’s model specifications set the testing setpoint at 0.02 g/dL, and have since 2013. The 0.025 figure that circulates in Texas materials is the pre-2013 federal number. What the device in your car is actually programmed to is a function of the court’s order and the vendor’s programming, not of any Texas rule, and NHTSA says expressly that the choice of operating setpoint remains each state’s to make. If it matters to your case, ask the vendor what yours is set at rather than assuming.

Devices installed in Texas commonly include several additional features. Only some of these are actually required:

• Rolling retests. After the vehicle is started, the device prompts for additional breath samples during operation. The retests exist to stop a sober person from starting the car for an intoxicated driver. Neither Texas rule nor the federal specification fixes how often they come. The federal conformance test uses a window of five to seven minutes after a successful start; the actual in-service schedule is left to the state and the program. A failed rolling retest does not stop the vehicle, for obvious safety reasons. What it does is flag the data logger as an alert condition and prompt a service call, and if the engine is then shut off the device will not permit a restart without one. A missed retest is flagged too.
• Camera verification. Common in Texas, but not required by any Texas rule or by the federal specification. NHTSA considered mandating an anti-circumvention method and declined, on the ground that naming a specific technology would be a design standard rather than a performance standard. Cameras appear on most Texas-installed devices because vendors and courts want them, not because the law demands them.
• Data logging. The device logs every sample, every retest, every start and every lockout. Data is downloaded at service appointments and reported to whoever supervises the order, whether that is the court, the probation department or DPS. Under Transportation Code §521.2476, a vendor must report a violation of a court order to the issuing court and the supervising officer within 48 hours of discovering it, must repair or replace a malfunctioning device within 48 hours, and must keep records for five years.
• Service appointments. Practice in Texas runs 30 to 60 days, but again that is practice, not a rule. 37 TAC §10.12 requires a calibration confirmation test at installation and on each occasion of service without setting any interval. What the federal specification requires is that a device hold calibration for at least 37 days, which is 30 days plus a 7-day lockout countdown, and manufacturers may qualify devices for 60, 90 or 180 days. In practice the device warns you, counts down for seven days, and then will not start the car.

37 TAC §10.16 allows the minimum breath sample size to be reduced to 1.2 liters for a device user with diminished lung capacity, on documentation from the judicial authority. If you have COPD, asthma, a history of chest surgery or any other condition that makes a full sample hard to produce, that is a real accommodation and it has to be asked for. DPS keeps its program materials and its list of approved devices on its ignition interlock page.

How SCRAM Works

A SCRAM device is an ankle bracelet worn 24 hours a day. It samples the wearer’s perspiration, what the literature calls transdermal alcohol, about every 30 minutes, using the same kind of electrochemical fuel cell an interlock uses. Roughly one percent of consumed alcohol leaves the body through the skin, and that is what the sensor reads. SCRAM Systems publishes the product details.

Key features:

• Continuous monitoring. Unlike an interlock, which tests only when the wearer tries to drive, SCRAM samples around the clock. There is no off-duty time.
• Transdermal, not breath or blood. The device measures alcohol that has crossed from the bloodstream through the skin. This is the single most important thing to understand about it, because transdermal readings do not track blood alcohol in real time. See the next section.
• Tamper detection. The bracelet detects attempted removal and attempted insertion of material between the device and the skin to block sampling. Tamper alerts are logged and reported, and in Texas practice a tamper record is treated far more seriously than a single elevated reading.
• Regular data transmission. Current systems transmit to a monitoring center on a schedule, usually daily, through a base station in the wearer’s home.
• Removal restrictions. SCRAM cannot be removed by the wearer. Removal requires authorized service.

The device does not report every elevated number. According to SCRAM’s own non-compliance reports, as those have been recited in court records, a confirmed consumption event requires the transdermal reading to start at zero, rise at a rate below a set hourly figure to a peak, decline at or below a set hourly figure back to zero, and then pass an environmental contaminant test. The threshold most often described in the reported cases is three consecutive readings at or above 0.02. An analyst then reviews the alert, and a second reviewer checks it.

What the Research Actually Says About SCRAM

This section is here because the vendor’s description of transdermal monitoring and the peer-reviewed description of it are not the same description. Three findings are well established and all three matter in a real case.

One: the lag is much longer than most descriptions admit

Transdermal alcohol does not rise and fall with blood alcohol. It rises later, peaks later, peaks lower, and stays up after breath alcohol has returned to zero. A 2022 meta-analysis in Addiction pooled the validation studies and put the average lag behind blood or breath alcohol at roughly 96 minutes, with a confidence interval running from about 55 to 136 minutes, and found that ankle-worn devices lag roughly twice as long as wrist-worn ones. Individual studies of peak-to-peak delay for SCRAM specifically have reported about two hours in one laboratory and four and a half hours, plus or minus nearly three, in another. Published estimates of the lag range from 30 minutes to five hours. The exact number is genuinely contested in the literature, which is itself the point: a timeline built on the assumption that a transdermal peak marks the drinking hour is built on sand.

Two: SCRAM is not sensitive to light drinking, and that cuts both ways

A controlled dosing study published in Alcoholism: Clinical and Experimental Research gave volunteers measured numbers of beers and watched what SCRAM did. Using the 0.02 threshold, it detected nobody who drank one beer, missed more than 68 percent of people who drank two, and missed 23 percent of people who drank three. Under the manufacturer’s own confirmation criteria, drinking was confirmed in only 55.7 percent of participants who drank three beers and 90.2 percent of those who drank four, and confirmation did not reach 100 percent even at five. A 2020 systematic review reports overall sensitivity under manufacturer criteria at about 40 percent against self-reported drinking days. The devices are deliberately engineered to be conservative, and the design target is reliable detection of roughly five drinks over two to three hours.

Three: the environmental-contamination question is real and is not settled

Everyone agrees that alcohol-containing products on or near the bracelet can produce an elevated reading, and that the manufacturer’s answer is curve shape: contamination is said to rise and fall too fast to look like metabolism. The independent literature supports the general proposition that environmental alcohol produces steeper absorption, higher peaks and steeper elimination, and also says that a systematic protocol for identifying interferents is still needed.

The contrary view is on the record too. In State v. Lemler, a South Dakota case that remains the fullest published examination of SCRAM methodology, a defense expert testified that fuel cells are not specific to ethyl alcohol, that methyl and isopropyl alcohol, butyl alcohol and hydroxyl-group compounds found in glycols and cleaning solutions can produce a fuel cell reaction, and that if such a compound entered the body and diffused through the skin the resulting curve may not look different from a drinking curve. The court admitted the SCRAM evidence. It did not resolve the science.

The algorithm is proprietary, so it cannot be independently audited. And there is no independent peer-reviewed field study establishing a false-positive rate against a chemical gold standard: the specificity numbers in circulation come from comparison to self-report, from participants’ denials, or from the manufacturer’s own internal study. That is not a claim that SCRAM is unreliable. It is a statement of what has and has not been tested, and it is the kind of thing a lawyer should know before deciding whether to fight a reading or explain it.

When Each Device Is Imposed

The two devices are imposed under different authority, and the difference in that authority is larger than the difference between the devices.

Ignition interlock: bond

Under Code of Criminal Procedure art. 17.441, a magistrate shall require an interlock as a condition of release for a defendant charged with a subsequent offense under Penal Code §49.04, §49.05, §49.06 or §49.061, or with an offense under §49.045, §49.07 or §49.08. Read that second list carefully, because it is the half most summaries drop: DWI with a child passenger, intoxication assault and intoxication manslaughter trigger the mandate on a first offense. There is no second-offense requirement for those three.

Three more features of the article. The only escape is a magistrate’s finding that requiring the device would not be in the best interest of justice. Installation is due before the 30th day after release on bond, at the defendant’s expense. And where the magistrate designates an agency to verify and monitor, the defendant pays that agency a monthly reimbursement fee capped at $10. There is no indigency provision in art. 17.441 at all.

A first-offense interlock on a 0.15 case, or on a case involving a collision, is common and is usually ordered under art. 17.40, the general power to impose any reasonable bond condition related to the safety of a victim or the community. That distinction is worth preserving rather than blurring, because a condition set under 17.40 can be taken back to the magistrate and argued about. A statutory mandate cannot. More on bond conditions generally.

Ignition interlock: community supervision

Article 42A.408 is the probation statute. The court may impose an interlock on any §49.04 through §49.08 probation. The court must impose one if any of three things is true: an analysis showed 0.15 or more at the time of testing; the defendant is punished under §49.09(a) or (b), meaning a second offense or a felony; or the defendant has a qualifying prior conviction. It is also mandatory for anyone under 21 at the time of the offense, and mandatory on deferred adjudication for §49.04 or §49.06.

The device must go on before the 30th day after conviction and stay on for not less than 50 percent of the supervision period. The defendant pays, though a court finding an inability to pay may set a payment schedule running up to twice the length of the order. There is an indigency reduction under art. 42A.408(e-1): where a defendant on deferred adjudication for §49.04 or §49.06 shows indigency by the Transportation Code §709.001 standard, the judge may waive the installation charge entirely and cut the monthly monitoring fee in half. It does not reach straight probation, bond conditions or occupational licenses, and it does not touch fees charged for a positive breath sample, but on the cases it does reach it is worth hundreds of dollars and it has to be requested. More on DWI probation generally.

Ignition interlock: licensing

Under Transportation Code §521.246, the judge granting an occupational driver’s license shall impose an interlock restriction if the person is already under an interlock order, including one imposed as a condition of bond, or if the license was suspended after a §49.04 through §49.08 conviction. Since 2015 there has been a waiver: the court may lift the requirement on a finding that it is not necessary for the safety of the community and that a waiver is in the best interest of justice. The device stays on for the duration of the suspension unless the court finds good cause to remove it.

The license that goes with the restriction is the special restricted license under Transportation Code §521.2465, issued by DPS for a $10 fee, which conspicuously indicates that the holder may operate only a vehicle equipped with an interlock. The occupational license provisions run from §521.241 through §521.253.

Ignition interlock: after the suspension ends

Penal Code §49.09(h) applies to a person convicted of a second or subsequent intoxication offense committed within five years of the most recent preceding offense. It requires the court to order an interlock on each vehicle owned or operated, and requires that the device remain installed until the first anniversary of the ending date of the license suspension period. Not one to three years, and not measured from conviction. It runs a year past the end of the suspension, and the court keeps jurisdiction over the defendant until the device comes off.

SCRAM: no statute of its own

Here is what makes SCRAM different. There is no Texas statute that names it. Not one. There is no definition, no device approval process, no calibration standard, no vendor certification, no state agency with rulemaking authority over it, and no fee cap. It is imposed under general discretion:

• On bond, under art. 17.40, the power to impose any reasonable condition related to the safety of a victim or the community. SCRAM is more common as a bond condition on felony cases, intoxication assault and intoxication manslaughter cases, and cases with a serious collision or a significant prior history. On a personal bond there is also art. 17.03(c), which requires alcohol or drug testing where there is a finding of alcohol abuse related to the offense, and which carries something worth knowing: art. 17.03(d) provides that the State may not use the results of any test conducted under that chapter in a criminal proceeding arising out of the offense charged.
• On community supervision, under art. 42A.301, which lets a judge impose any reasonable condition designed to protect the community or rehabilitate the defendant, and which lists submitting to testing for alcohol and submitting to electronic monitoring among the discretionary conditions. Note what is not in that list: there is no enumerated abstain-from-alcohol condition in Texas law. Abstinence conditions rest on the general grant, not on a named statute.
• Where there is no vehicle, for instance after a total loss, an interlock cannot be installed, and SCRAM or a portable remote breath device is sometimes ordered instead so that some enforcement mechanism exists.
• As a remedial condition, added to an existing probation after an alcohol-related violation.

Two practical consequences of that asymmetry. Because no rule governs the device, everything about it comes from the vendor’s participation agreement, and those agreements routinely prohibit the use of any product containing alcohol, consumable or not, which is a broader obligation than most clients realize they have signed. And because the condition rests on discretion rather than mandate, it is more open to negotiation, modification and removal than an interlock ordered under art. 17.441.

Side-by-Side Comparison

Putting the two devices in direct comparison:

Living with an Ignition Interlock

The practical realities of an interlock-restricted life:

• Morning routine. Every start begins with a breath sample. Mouthwash, some medications, breath sprays and even a recent meal can produce a reading from residual mouth alcohol. Rinsing with water and waiting 20 minutes solves most of it.
• Rolling retests. Retest prompts during operation require attention. Pull over if you can do it safely. Note that NHTSA deliberately removed any instruction about how retests should be conducted from the model specification, on the view that this is a question for states and drivers rather than a device design question, so what you are expected to do is a matter of your order and your program.
• Service appointments. Every 30 to 60 days depending on your program, for download and calibration. The device warns you and counts down. Miss the countdown and the car will not start until it is serviced.
• Failed tests. Every sample is logged and reported. An isolated inadvertent failure that can be explained is usually survivable. A pattern is treated as a violation. Keep your own record of anything unusual on the day it happens, not weeks later when someone asks.
• Driving a vehicle without a device. As a general rule, operating any vehicle without an interlock during the restriction period is a violation. But there is a statutory employer exception, and it is real. Under art. 42A.408(h) and Transportation Code §521.246(e), a person may operate a vehicle without an interlock if the driving is in the course and scope of employment, the vehicle is owned by the employer, the employer is not owned or controlled by the restricted person, the employer has been notified of the restriction, and proof of that notification is in the vehicle. All five elements, every time. It does not cover a company you own, and it does not cover the commute. If your work requires driving, raise this at the plea rather than after the first problem.
• Passenger awareness. Passengers see the device every time you drive. Most people find it manageable; some find it genuinely difficult in professional or social settings.

Living with SCRAM

The practical realities of SCRAM are different in important ways:

• Visibility. The bracelet shows whenever the wearer is in shorts, swimwear or anything that exposes the ankle. Longer socks and pants are the usual accommodation.
• Water. SCRAM is water-resistant but not submersible. Showering is fine. Baths, pools and lakes are not.
• Skin. Some wearers develop irritation under the device. Strap tension adjustments and skin care usually resolve it. Report it early and in writing, because an untreated skin problem becomes a reason to want the device off, and wanting it off is not a defense to a tamper alert.
• Alcohol-containing products. Hand sanitizer, some skincare and hair products, some cleaning products, and incidental kitchen and workplace exposure can all reach the sensor. The confirmation process is designed to sort environmental exposure from consumption by the shape of the curve, and it usually does. But read your participation agreement: most of them prohibit any product containing alcohol, consumable or not, which means the burden of avoiding interferents has been placed on you by contract. If you work in a bar, a restaurant kitchen, a salon, a lab or a body shop, tell your lawyer before the device goes on rather than after the first alert.
• Transmission. Most systems need the wearer within range of a base station for a period each day. Travel and night-shift schedules have to be coordinated with the provider in advance.
• Removal. Only by authorized service. Never by the wearer, under any circumstances, for any reason.

When Both Are Required at the Same Time

In some cases, most often felony DWI, intoxication assault and intoxication manslaughter, and certain multiple-offense cases, the court orders both. The combination gives the court driving-specific monitoring and abstinence monitoring at once.

Living with both is more expensive, more visible and more restrictive than either alone, and the cost is where the argument usually lives. The Court of Criminal Appeals held in Mathis v. State that a trial judge may not, over objection, order an indigent defendant to pay for a SCRAM device as a probation term without considering the defendant’s financial ability. Since 2021, art. 42A.301(a) has also required the judge to consider the defendant’s ability to meet financial obligations when setting conditions in the first place. Those two together are the basis for asking a court to choose one device rather than stacking both, or to shorten the period of the second. On the most serious felony cases the combination is sometimes the realistic alternative to pretrial detention, and it is worth saying so plainly to a client.

Consequences of Violations

Consequences vary by the type of order and the seriousness of what happened:

• Isolated, explainable events. A single mouthwash-related interlock failure or a single hand-sanitizer SCRAM alert is logged, and if it can be explained and does not repeat it usually goes nowhere. Explanations are far more persuasive when they are contemporaneous.
• A pattern. Repeated failed samples, missed service appointments, or repeated transdermal detections are treated as violations. Consequences run from arrest on a bond violation to revocation of probation or of an occupational license.
• Tampering. Removal, obstruction or shielding is the most serious category on either device, and it is treated differently from a drinking allegation because it looks like consciousness of guilt. A skin problem you reported in writing is a very different record than a skin problem you mention afterward.
• Driving without a device. Operating a vehicle without an interlock during a period when one is required is a bond, probation or licensing violation, and under Transportation Code §521.252 the court shall revoke an occupational license for it unless the employer exception applies.

Two things about how this plays out in court, both of which favor early attention over later argument. A revocation is proved by a preponderance of the evidence, not beyond a reasonable doubt, which is a much lower bar than the criminal case the client is used to thinking about. And in practice, monitoring reports are very often admitted without objection, after which the only fight left is a sufficiency fight the defense usually loses. Whether alcohol monitoring records can properly be authenticated through a business records affidavit is a question a Texas court has expressly declined to reach for want of briefing. It is open. It will not stay open forever, and it will be answered in a case where somebody objected.

The takeaway: when you are subject to interlock or SCRAM, the device is part of your case. Treat it that way.

What to Do if You Are Subject to Interlock or SCRAM

Practical guidance for either type of monitoring:

• Comply meticulously. Every appointment, every protocol, every reporting requirement. The cost of non-compliance is dramatically higher than the inconvenience of compliance.
• Ask about the money before you agree to anything. On a deferred adjudication for DWI or BWI, ask your lawyer about the art. 42A.408(e-1) indigency reduction, which waives the installation charge and halves the monthly monitoring fee. On any condition you cannot afford, ask that the court’s consideration of your ability to pay be put on the record. Mathis exists because somebody objected.
• Ask about the employer exception. If your job requires driving an employer-owned vehicle, the five-element exception in art. 42A.408(h) and Transportation Code §521.246(e) may keep you working. It has to be set up in advance, with written notice to the employer and proof of that notice in the vehicle.
• Ask about a medical accommodation. If a lung condition makes a full breath sample difficult, 37 TAC §10.16 allows the minimum sample to be reduced to 1.2 liters on documentation from the court.
• Communicate with the provider, in writing. Medical issues, work travel, equipment problems, anything unusual. Then keep the confirmation. A phone call you cannot prove is worth very little six months later.
• Avoid exposure sources. With SCRAM, alcohol-containing sanitizers, skincare and cleaning products, and incidental kitchen or workplace exposure. With an interlock, mouthwash, breath sprays and some medications before testing.
• Document anomalies the day they happen. Time, place, what you had been doing, what you had touched, who was with you. A contemporaneous note is evidence. A reconstruction is an argument.
• Stay in close contact with your defense lawyer. Monitoring data goes to the court and the supervising authority whether or not anyone tells you. Your lawyer should hear about a problem from you first.
• Plan financially. Both devices are expensive over time, and the SCRAM daily rate adds up faster than people expect. Budget for it. It is still a fraction of the cost of a revocation.

The Bottom Line

Ignition interlock devices and SCRAM devices serve different purposes. The interlock monitors driving sobriety; SCRAM monitors abstinence. But they are not two versions of the same thing. The interlock is a regulated device imposed under specific statutes with defined triggers, defined durations, a device-approval process, an employer exception and an indigency reduction. SCRAM is an unregulated device imposed under a judge’s general discretion, running on a proprietary confirmation algorithm, with a body of research showing a long and variable lag, limited sensitivity to light drinking, and a false-positive rate that has never been independently established in the field. Neither of those descriptions makes a device good or bad. They describe two different kinds of argument available to a defense lawyer, and knowing which one you are in is most of the work. Negotiating appropriate conditions in the first place, and getting relief from inappropriate ones, is part of what a real DWI defense does.

NOTE: All Deandra Grant Law clients with alcohol monitoring devices are encouraged to complete the MyCAM Program in order to reduce the chances of a violation. It is offered free of charge to DGL clients.

Interlock and SCRAM Issues at Deandra Grant Law

Deandra Grant Law defends DWI cases across North and Central Texas including Dallas, Fort Worth, Plano, McKinney, Frisco, Allen, Lewisville, Denton, Rockwall, and Waco. We address interlock and SCRAM conditions as part of every case where they are imposed: negotiating bond conditions, advocating for appropriate scope and duration, managing violations and unexplained detections, and coordinating the monitoring period with the criminal case and the licensing process. Our team includes an ACS-CHAL Forensic Lawyer-Scientist with a Master of Science in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology, which means the transdermal and breath data in these cases gets read by someone who understands the instrumentation and the literature behind it rather than taken at face value.

If you are facing interlock or SCRAM monitoring in a Texas DWI case, or if you have a violation issue under either device, call Deandra Grant Law at (214) 225-7117 or visit texasdwisite.com to schedule a confidential consultation. And remember that the 15-day ALR deadline runs from the date of service of the notice of suspension.

Have a DWI question you want answered in this series? Submit it at texasdwisite.com and you might see it featured in a future Ask Deandra post.