Wet Labs ac-s, User Manual

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30 ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006
Primary Processing
When receiving binary packets from an ac-s, the first thing to look for is the packet
registration, which is four characters. The characters, in hexidecimal notation, are FF 00
FF 00. After successfully receiving these characters, the 2 byte packet length is the next
item to be received. The packet length indicates the number of bytes sent in the packet,
including the 4 byte packet registration. The packet length excludes the two byte
checksum and the pad byte. The packet length varies from meter to meter depending on
the number of output wavelengths.
The next byte is a packet type identifier used by WETView to distinguish between ac-s
and Histar data packets. It should be 3 or higher for the ac-s.
The next byte is reserved for future use.
The serial number is a standard four-byte long integer. The first byte of the serial number
denotes the instrument type. The remaining three bytes are the binary serial number.
The two bytes following the serial number are the A reference channel dark counts. It is
present mainly for diagnostic purposes.
The next two bytes are the A/D counts from the pressure sensor circuitry. If no pressure
sensor is installed, these two bytes may be ignored. When a pressure sensor is installed,
the depth in meters can be calculated from this number by multiplying the counts by a
meter specific scale factor and then adding a meter specific offset. The depth offset and
scale factor can be found in row 5 of the factory device file provided with the meter. The
offset is the first number in row 5, and the scale factor is the second number in row 5.
The following 2 bytes are the A signal dark counts for diagnostic purposes.
The next two bytes represent the raw external temperature voltage counts from across a
thermistor. The external temp is calculated by a polynomial that converts counts to
degrees C. Calculate the temp using the polynomial:
d + counts c counts b + counts a = y
2 3 × + × ×
where a = -7.1023317 E-13, b = 7.09341920 E-8, c = -3.87065673 E-3, and d =
95.8241397
The internal temperature is given as a reading from a thermistor. The manufacturer of the
thermistor provides a function to convert resistance to temperature. The two byte value in
the data packet represents the voltage drop across the resistor. To determine the resistance
of the thermistor, first convert the counts (16 bit unsigned integer) to volts using the
equation:
volts = (5
×
counts / 65535)