arithmetic involving natural logarithm

[ekm]

In the arithmetic expression defining a calculated item, is it possible to take the natural log of a subexpression?

The application is to transform ambient temperature and relative humidity to dewpoint using a formula:

b ( ln(h/100) + a*t/(b+t) )

---------------------------

a - ln(h/100) - a*t/(b+t)

[ekm]

Just realized I did not share my solution.

The environment monitoring device is a Room Alert 11E. This device provides as input the temperature and relative humidity as SNMP objects, available as input items:

sensor3-1 = temperature (C)

sensor3-3 = relative humidity (%)

Using these inputs, I created two synthetic items:

sensor3-lnrh = natural log of relative humidity.

formula = last("sensor3-3")
preprocessing = JavaScript, return Math.log(value/100);

sensor3-edew = estimated dewpoint (C)

formula = 243.04*(last("sensor3-lnrh")+(17.625*last("sensor3-1")/(243.04+last("sensor3-1"))))/(17.625-last("sensor3-lnrh")-(17.625*last("sensor3-1")/(243.04+last("sensor3-1"))))

The formula above is an estimation using the method described by: rklarsen. "Calculating Dew Point Temperature from Relative Humidity." vCalc.com, which cites as source: Lawrence, M. G. (2005). "The relationship between relative humidity and the dewpoint temperature in moist air: A simple conversion and applications." Bulletin of the American Meteorological Society, 86(2), 225-233.

From these items, and referring to ASHRAE 2015 thermal guidelines for allowable humidity range, non-condensing, for a class A1 data center, I wrote moisture alert expressions:

warning:
{roomalert1:sensor3-3.last()} > 60
or
{roomalert1:sensor3-edew.last()} < -9
or
{roomalert1:sensor3-edew.last()} > 15

danger:
{roomalert1:sensor3-3.last()} < 8
or
{roomalert1:sensor3-3.last()} > 80
or
{roomalert1:sensor3-edew.last()} < -12
or
{roomalert1:sensor3-edew.last()} > 17

Because the outputs of the Room Alert 11E sensors are jittery, these should be rewritten to use more than just the last observation, to prevent alert spam. I have not tested the following expressions but they would probably do the trick:

warning:
{roomalert1:sensor3-3.min(15m)} > 60
or
{roomalert1:sensor3-edew.min(15m)} < -9
or
{roomalert1:sensor3-edew.max(15m)} > 15

danger:
{roomalert1:sensor3-3.min(15m)} < 8
or
{roomalert1:sensor3-3.max(15m)} > 80
or
{roomalert1:sensor3-edew.min(15m)} < -12
or
{roomalert1:sensor3-edew.max(15m)} > 17