Personal Health Device Implementation Guide
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Personal Health Device Implementation Guide, published by HL7 International - Health Care Devices Work Group. This is not an authorized publication; it is the continuous build for version current). This version is based on the current content of https://github.com/HL7/PHD/ and changes regularly. See the Directory of published versions

Mder FLOATs and SFLOATs

Mder FLOATs and SFLOATs are the 11073 20601 means of encoding floating point numbers. The primary reason for this encoding is to indicate the precision of the measurement. The SFLOAT is 16-bits and the FLOAT is 32-bits. In the SFLOAT, the most significant 4 bits is the exponent and the remaining 12 bits the mantissa. In the FLOAT, the most significant byte is the exponent and the remaining 24 bits the mantissa. Both the exponent and mantissa are signed.

The exponent gives the precision. It indicates where the decimal point goes in the mantissa. A negative exponent of -N moves the decimal point to the left N places and a positive exponent of +N moves the decimal point to the right N places.

Thus, it is possible in this encoding to distinguish between the value 2, 2.0, 2.00, etc. Numerically, they all have the same value, but 2.00 indicates that the value is two but taken with a sensor that has a precision to the hundredths.

The table below gives some examples of SFLOAT values and how they shall be encoded into the FHIR valueQuantity.value:

SFLOAT Hex SFLOAT decimal exponent mantissa FHIR encoding
0x0002 2 0 2 2
0xF014 61460 -1 20 2.0
0xE0C8 57544 -2 200 2.00
0x1002 4098 1 2 20
0x2002 8194 2 2 200
0x00C8 200 0 200 200
0x04D2 1234 0 1234 1234
0x0B2E 2862 0 -1234 -1234

In the FLOAT case the above values would be encoded as follows:

FLOAT Hex FLOAT decimal exponent mantissa FHIR encoding
0x00000002 2 0 2 2
0xFF000014 4278190100 -1 20 2.0
0xFE0000C8 4261413064 -2 200 2.00
0x01000002 4098 1 2 20
0x02000002 33554434 2 2 200
0x000000C8 200 0 200 200
0x000004D2 1234 0 1234 1234
0x00FFFB2E 16775982 0 -1234 -1234

Special Values

The Mder encoding also has a set of 5 reserved special values which represent some type of error condition. They are as follows:

FLOAT Case SFLOAT case meaning FHIR encoding
0x007FFFFF 0x7FF Not a Number (NaN) .dataAbsentReason set to ‘not-a-number’
0x007FFFFE 0x7FE Positive Infinity (+inf) .dataAbsentReason set to ‘positive-infinity’
0x00800002 0x802 Negative Infinity (-inf) .dataAbsentReason set to ‘negative-ifinity’
0x00800000 0x800 Not at this resolution .dataAbsentReason set to ‘error’
0x00800001 0x801 Reserved for future use .dataAbsentReason set to ‘error’