# Smoke Digital value to Physical Value Conversion

How to convert the 16 bit Digital value into its Physical Value that is in obscuration per foot.

How to convert the 16 bit Digital value into its Physical Value that is in obscuration per foot.
[edited by: kumar@efftronics at 4:30 AM (GMT 0) on 11 Jan 2020]

## Top Replies

Parents
• Hello,

Obscuration per foot is the measure of how a beam of light is obscured by smoke during testing. Our device measures how the light scatters off of smoke present in the optical path; essentially the inverse of obscuration per foot. Where an obscuration measurement would be at 100% signal level with no smoke present (no obscuration), our device would be at 0% signal level since there is no smoke present to reflect off. So it is not as easy as just converting the output of the ADPD188BI into obscuration per foot. Instead, a measurement needs to be made to correlate the output of the ADPD188BI to the obsuration sensor which is measuring obscuration per foot at the same time, and at the same location in the room where the smoke measurement is being made, for example, at the UL facillity.

The output of the ADPD188BI should be converted to PTR (power transfer ratio) which is a value that corresponds to the amount of optical power recieved by the PD vs the amount of optical power provided by the LED. Then during testing you would determine the PTR level for a specific smoke that corresponds to the obscuration per foot measurement made by the obscuration sensor.

To convert the output of the ADPD188BI to PTR use the following conversion:

►PTR = IPD/ILED * (1/PDET_AMPS_PER_WATT) * (1/LED_WATTS_PER_AMP)

• IPD = (Output Codes * nA/LSB * 1/#Pulses). The nA/LSB factor can be taken from Table 2 of the ADPD188BI datasheet.
• ILED = the current setting of the LED
• PDET_AMPS_PER_WATT is expressed in Amps/Watt (or nA/nW) and is a function of wavelength. Use 0.26 for 470nm (Blue) and 0.41 for 850 nm (IR).
• LED_WATTS_PER_AMP = NOMINAL_WATTS_PER_AMP * DERATING_FACTOR * PART_TO_PART_SCALAR
• NOMINAL_WATTS_PER_AMP = 0.38 for Blue, 0.22 for IR
• DERATING_FACTOR accounts for the Blue LED non-linearity
• For Blue LED, calculate as :
• DERATING_FACTOR  = A0 + A1 ( ILED_MA) + A2(ILED_MA)^2 + A3 * (ILED_MA)^3
• ILED_MA is the LED Pulse Current expressed in mA
• A0 = 9.8976E-01
• A1 = -5.1448E-03
• A2 = 2.0287E-05
• A3 = -2.9645E-08
• For IR LED, DERATING_FACTOR = 1.0
• PART_TO_PART_SCALAR is how we normalize to device to device differences. This scalar is calculated from gain and offset values burned into an on-chip NVM. Read application note AN-2033 to understand how to apply this value.
• How the PTR is related to Obscuration per foot, Can you explain this with an example. and one more thing is i need to convert the Obscuration per foot value to  dB/m units.

• or else their is any procedure to directly convert the Smoke 16 bit digital value to db/m.

Reply
Children
• As I mentioned in the above email, our device does not measure obscuration per foot. We measure what is essentially the inverse of obscuration per foot. In order to understand how the output of the ADPD188BI correlates to obscuration per foot, this can only be done by empirical testing. You need to build your smoke detector and test it in the standards facility, e.g. UL smoke labs, so you can empirically measure what the output of the ADPD188BI yields at the obscuration per foot thresholds dictated by the standards.

Regards,

Kevin