Temperature measurement limits of La Crosse WS2357
Thursday, February 4. 2016
Guess what happens right before hell freezes over? Your weather station indicates dew point of 136 °C. 
Kinda funny.
Good thing that Weather Underground allows you to edit by removing data points. So, there are couple gaps in my graphs now:
The reson for this weird behaviour can be found from the spec:
OMG! The lower bound of outside temperature measurement is -29.9 °C. In Finland that can be reached occasionally.
Fortunately I'm using open source software, Open2300. With very little debugging I found the code:
double temperature_outdoor(WEATHERSTATION ws2300, int temperature_conv)
...
return ((((data[1] >> 4) * 10 + (data[1] & 0xF) +
(data[0] >> 4) / 10.0 + (data[0] & 0xF) / 100.0) - 30.0));
double dewpoint(WEATHERSTATION ws2300, int temperature_conv)
...
return ((((data[1] >> 4) * 10 + (data[1] & 0xF) +
(data[0] >> 4) / 10.0 + (data[0] & 0xF) / 100.0) - 30.0));
There is a formula to convert raw data read from RS-232 -line to celsius.
My patch to fix this is:
--- svn/rw2300.h 2015-01-19 23:42:17.728311172 +0200
+++ JaTu/rw2300.h 2016-02-04 23:58:45.675123710 +0200
@@ -24,6 +24,7 @@
#include <math.h>
#include <sys/types.h>
#include <sys/stat.h>
+#include <float.h>
#define MAXRETRIES 50
#define MAXWINDRETRIES 20
@@ -56,6 +57,8 @@
#define MAX_APRS_HOSTS 6
+#define TEMPERATURE_OVERFLOW FLT_MIN
+
typedef struct {
char name[50];
int port;
--- svn/wu2300.c 2015-01-19 23:42:16.619287028 +0200
+++ JaTu/wu2300.c 2016-01-18 10:13:21.252092414 +0200
@@ -53,15 +53,18 @@
/* READ TEMPERATURE OUTDOOR - deg F for Weather Underground */
-
- sprintf(tempstring, "&tempf=%.2f", temperature_outdoor(ws2300, FAHRENHEIT) );
- strcat(urlline, tempstring);
-
+ tempfloat = temperature_outdoor(ws2300, FAHRENHEIT);
+ if (tempfloat > TEMPERATURE_OVERFLOW) {
+ sprintf(tempstring, "&tempf=%.2f", tempfloat );
+ strcat(urlline, tempstring);
+ }
/* READ DEWPOINT - deg F for Weather Underground*/
-
- sprintf(tempstring, "&dewptf=%.2f", dewpoint(ws2300, FAHRENHEIT) );
- strcat(urlline, tempstring);
+ tempfloat = dewpoint(ws2300, FAHRENHEIT);
+ if (tempfloat > TEMPERATURE_OVERFLOW) {
+ sprintf(tempstring, "&dewptf=%.2f", tempfloat );
+ strcat(urlline, tempstring);
+ }
/* READ RELATIVE HUMIDITY OUTDOOR */
--- svn/fetch2300.c 2015-01-19 23:42:17.728311172 +0200
+++ JaTu/fetch2300.c 2016-01-18 10:09:46.762108076 +0200
@@ -35,7 +35,7 @@
char tendency[15];
char forecast[15];
struct config_type config;
- double tempfloat_min, tempfloat_max;
+ double tempfloat, tempfloat_min, tempfloat_max;
int tempint, tempint_min, tempint_max;
struct timestamp time_min, time_max;
time_t basictime;
@@ -63,7 +63,11 @@
/* READ TEMPERATURE OUTDOOR */
- sprintf(tempstring, "To %.1f\n", temperature_outdoor(ws2300, config.temperature_conv) );
+ tempfloat = temperature_outdoor(ws2300, config.temperature_conv);
+ if (tempfloat == TEMPERATURE_OVERFLOW)
+ sprintf(tempstring, "To OVR\n");
+ else
+ sprintf(tempstring, "To %.1f\n", tempfloat);
strcat(logline, tempstring);
temperature_outdoor_minmax(ws2300, config.temperature_conv, &tempfloat_min,
@@ -79,8 +83,11 @@
/* READ DEWPOINT */
-
- sprintf(tempstring, "DP %.1f\n", dewpoint(ws2300, config.temperature_conv) );
+ tempfloat = dewpoint(ws2300, config.temperature_conv);
+ if (tempfloat == TEMPERATURE_OVERFLOW)
+ sprintf(tempstring, "DP OVR\n");
+ else
+ sprintf(tempstring, "DP %.1f\n", tempfloat );
strcat(logline, tempstring);
dewpoint_minmax(ws2300, config.temperature_conv, &tempfloat_min,
There I introduce a hard-limit of TEMPERATURE_OVERFLOW and check if that has been reached. If yes, the invalid value is not sent to WUnderground.
I've contacted the author of Open2300 about this, but haven't received a response.
