# Calculation of PTR and the relationship between PTR and obscuration

Hello . I have two questions .

1. calculate  PTR

Refering to

https://www.analog.com/en/design-center/reference-designs/circuits-from-the-lab/cn0537.html#rd-description

PTR = Ipd/Iledx_pk/Rpd/ηled

(1)Ipd=Code*Q*(1/PULSE_COUNT)

Code = output of FIFO ; Q=1.15  (when R = 100k) ; PULSE_COUNT = 2 ;

(2)ILED_PK = Icourse*Ifine*Iscale

(3)Rpd=0.26（for blue）;0.42(Ifor IR）

(4)ηled =  ηN*K*C

for blue ηN=0.38 ; for IR ηN =0.22

for IR : k=1

for Blue :  k=A0+A1*ILED_PK+A2*(ILED_PK)^2+A3* (ILED_PK)^3;

for Blue: C = (Gain*ILED_PK +INTER)/ NORMINAL_SCALAR

Where:

Gain= 17*( Gain_COEFF-112)/256-17

INTER = 8*( Int_COEFF-128);

NORMINAL_SCALAR = 17*ILED_PK+622;

So I will calculate Blue PTR as a example .

(a) IPD=Code*Q*(1/PULSE_COUNT)= Code*1.15/2=0.575000*FIFO

(b) Icourse = 50.3+19.8*6;    //6  0x23bit[3:0]

Ifine = 0.74+0.022*12; //12 0x25bit[4:0]

Iscale = 0.1+0.9*1;     //1  0x23bit13

ILED_PK = Icourse*Ifine*Iscale=169.776398mA

(c) Rpd=0.26

(d) blue ηN=0.38

(e) k=9.8976e-01-5.1448e-03*ILED_PK+2.0287e-05*(ILED_PK)^2-2.9645e-08* (ILED_PK)^3

=0.555975  ( for ILED_PK , I should use mA or A here?)

(f) As I read from efuse register ,for Blue,

Gain_COEFF=0x22

Int_COEFF=0xC5

So :

Gain= 17*( Gain_COEFF-112)/256-17 = -22;

INTER= 8*( Int_COEFF-128) = 552;

NORMINAL_SCALAR = 17*ILED_PK+622 = 3508.198761;     ( for ILED_PK here , I should use mA or A?)

C=(Gain*ILED_PK + INTER)/ NORMINAL_SCALAR= -0.907326

So for Blue PTR = IPD / ILED_PK * (1/Rpd) *( 1 / (ηN*k*C) )  = -0.067954*FIFO

Similarly,for IR, Gain_COEFF=0xC5,Int_COEFF=0xAA, C=0.841255 , So IR PTR = 0.044633*FIFO

Obviously Blue PTR is less than 0 ，It‘s wrong . And I don't know if  IR PTR is  right or wrong .

If FIFO>1000,Then PTR is over 40 . As I see in the files you provide ,Is it to high?

the max PTR is 4.4

I think I have followed you instruction . Maybe I confused what unit  of ILED_PK should be used ?

2. convert PTR to obscuration

As CN0537 says, PTR =γ*β . So I think If I know PTR ,I can calculate β , which  is the obscuration level in ft-1.

You say "our device does not measure obscuration per foot. We measure what is essentially the inverse of obscuration per foot"

I dont quite understand it .I think that the higher FIFO is, the PTR is higher . It means the the smoke is thicker .

So the obscuration is higher .  Is it wrong ? So If you can expain it to me , or is there any  files I missed ？

Regards