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A GSR sensor (galvanic skin response sensor) measures how easily a small electrical signal passes through the skin between two electrodes. The same measurement is called electrodermal activity (EDA) or skin conductance, and the device is the same whichever name a vendor or study uses. This page explains how the measurement works, the circuit and recording limits one published standard sets, and where the device sits in US medical-device regulation.
This page describes products and general use; it is not medical advice.
What a GSR sensor is
A GSR sensor is a skin-contact sensor that reports the electrical resistance (or its inverse, conductance) of the skin and the tissue path between two electrodes. The US Food and Drug Administration defines a “galvanic skin response measurement device” in 21 CFR 882.1540(a) as a device used to determine autonomic responses as psychological indicators by measuring the electrical resistance of the skin and the tissue path between two electrodes applied to the skin.
Who uses it: polygraph examiners recording electrodermal activity in field practice, and researchers recording electrodermal activity in psychophysiology studies.
This is a type of medical device in FDA terms. Related terms used across sources: galvanic skin response sensor, GSR sensor, EDA sensor and skin conductance sensor. They describe the same measurement, expressed in resistance or conductance units.
How it works: two electrodes, resistance or conductance
The sensor passes a very small current or voltage across two skin electrodes and reads the result. Which quantity is held fixed decides which quantity the output is proportional to. The Standard for EDA Signal Processing, a consensus standard for polygraph field instruments, describes both options in section 2.1.2:
- Constant current circuit: the output is directly proportional to skin resistance.
- Constant voltage circuit: the output is directly proportional to skin conductance.
The same standard allows values to be expressed in resistance, conductance or dimensionless units (section 2.1.3). Because the two circuit types report different quantities, a reading is only comparable with another reading once you know which circuit and unit produced it.
Types and variants: three recording techniques
The Society for Psychophysiological Research (SPR) committee guide “Publication recommendations for electrodermal measurements” (Boucsein et al., 2012, Psychophysiology 49(8):1017–1034) distinguishes three techniques. The table summarises each technique as described in the guide’s abstract and the polygraph standard.
| Technique | What is stated |
|---|---|
| Exosomatic, direct current | An external direct current is applied to the skin. The SPR abstract describes it as the most widely used technique, and the polygraph standard requires it for polygraph field measurement (section 2.1.1). |
| Exosomatic, alternating current | An external alternating current is applied. The polygraph standard says alternating-current recordings have not been widely validated for the polygraph application and shall not be used for field examinations unless supported by published validation studies (section 2.1.4). |
| Endosomatic | No external current is applied; the signal comes from the skin itself, including skin potential. The polygraph standard gives the same validation caveat for field use (section 2.1.4). |
Specifications that matter
These values come from the Standard for EDA Signal Processing (author Brent Smiley, November 2023, hosted by the American Polygraph Association). Its scope is polygraph field instruments, so they are not universal limits for every GSR sensor, such as one built for research or a consumer wearable.
| Specification | Value in the polygraph standard | Section |
|---|---|---|
| Constant current excitation | Limited to 10 µA per square centimetre of electrode area | 2.2.1 |
| Fixed constant voltage | Limited to 0.5 V | 2.2.2 |
| Adjustable constant-voltage circuits | Current limited to 10 µA per square centimetre across the whole range; applied voltage never above 5 V | 2.2.3 |
| Electrode pairs | Each pair the same size, shape and material | 2.3.3 |
| Electrode attachment | Secured to the skin, typically by Velcro straps or adhesive | 2.3.2 |
| Standard recording sites | Volar surface of distal or medial finger phalanges, thenar and hypothenar palm, and two inner-foot sites | 2.4.1 |
| Minimum sample rate | 25 samples per second for acquisition, storage and scoring; a live display may run at 8 per second if 25 per second is kept | 2.6.1–2.6.2.1 |
| Automatic-mode high-pass filter | Time constant of 10 seconds | 3.1 |
How to choose: criteria from the standard
Published standards do not rank products. These are the criteria they do state:
- Check whether the instrument is a constant-current or constant-voltage design, because that sets whether it outputs resistance or conductance (polygraph standard, 2.1.2).
- Check that the excitation stays within the standard’s limits above, if the intended use is polygraph field practice.
- Check that the manufacturer states its electrode recommendations; the standard says manufacturers should make known any recommendation of electrode type or size (2.3.1).
- For polygraph field instruments used in basic-science or academic research into EDA, the standard says they should conform to the SPR guidelines (Boucsein et al., 2012) (2.3.4).
Regulatory status in the US
Under 21 CFR 882.1540(b), a galvanic skin response measurement device is Class II (special controls). It is exempt from the premarket notification procedures in subpart E of part 807, subject to the limitations in 21 CFR 882.9. The regulation sits in part 882 (neurological devices), subpart B (neurological diagnostic devices). For background on what the classes and 510(k) route mean, see the CASRAI guides on FDA medical device classification, 510(k) and PMA and 510(k) premarket notification.
Safe use and handling basics
- Follow the manufacturer’s instructions for use for electrode placement and skin preparation; the polygraph standard says manufacturers should recommend a standard site preparation in their instructional materials (2.5.1).
- Allow time after site preparation and attachment before recording starts, so the skin can hydrate and the EDA baseline can stabilise (2.5.2).
- Use the electrode type and size the manufacturer specifies, and keep electrode pairs matched (2.3.1, 2.3.3).
- If a non-standard recording site is used, the standard expects the manufacturer to publish a justification and the use to be documented in the record (2.4.1.1).
Common mistakes
- Comparing resistance with conductance. Readings from a constant-current device and a constant-voltage device are different quantities; check the unit before comparing.
- Treating polygraph limits as universal. The 0.5 V, 5 V, 10 µA/cm² and 25 samples-per-second figures come from a polygraph field standard.
- Recording immediately after attaching electrodes. The standard says to allow time for skin hydration and baseline stabilisation.
- Mixing electrode types within a pair. The standard requires each pair to match in size, shape and material.
FAQ
What is a GSR sensor?
A GSR sensor measures the electrical resistance or conductance of the skin and tissue between two electrodes. FDA’s definition in 21 CFR 882.1540(a) describes it as a device used to determine autonomic responses as psychological indicators.
Are a GSR sensor and an EDA sensor the same thing?
The sources use galvanic skin response, electrodermal activity and skin conductance for the same family of measurements. The regulation uses “galvanic skin response”; the polygraph standard and the SPR guide use “electrodermal activity”.
What is a skin conductance sensor?
It is a GSR sensor whose output is conductance. Per the polygraph standard, a constant-voltage circuit produces output proportional to skin conductance, while a constant-current circuit produces output proportional to resistance.
Is a GSR sensor an FDA-regulated medical device?
21 CFR 882.1540 classifies the galvanic skin response measurement device as Class II (special controls), exempt from 510(k) subject to the limitations in 21 CFR 882.9.
What is the minimum sampling rate?
The polygraph standard sets a minimum of 25 samples per second for acquisition, storage and scoring in polygraph field instruments.
For monitoring and diagnostic equipment more broadly, see the CASRAI procurement guide on choosing diagnostic and patient monitoring equipment, or browse diagnostic, monitoring and imaging products.
Sources
- 21 CFR 882.1540, Galvanic skin response measurement device (eCFR text via Legal Information Institute), checked 2026-10-30. law.cornell.edu/cfr/text/21/882.1540
- Standard for EDA Signal Processing (consensus standard for polygraph field instruments, B. Smiley, November 2023), American Polygraph Association, checked 2026-10-30. polygraph.org PDF
- Boucsein W. et al. (2012). Publication recommendations for electrodermal measurements. Psychophysiology 49(8):1017–1034, PMID 22680988, DOI 10.1111/j.1469-8986.2012.01384.x (abstract checked 2026-10-30). pubmed.ncbi.nlm.nih.gov/22680988
