Hach WIMS allows the entry of <, >, and ND to signify a less than, greater than, or non-detect value. These values are typically derived from a laboratory test that was above or below the Method Detection Limit (MDL, aka Minimum Detection Limit). You may also setup additional allowable symbols using Symbol Setup. In order for a variable to accept the special characters, the Allow the use of <,>, and ND Entries field must be checked in Variable Setup. The question is: What value should be used in summary statistics, calculations, etc… when a special character is entered? WIMS stores both the entered value (ie <2, ND, >100, etc…) and the numeric value (2,0,100,etc…) in the database. In Variable Setup, the MDL Rule sets what value should be used.
FIELDS:
Allow Symbols: If checked, sets the variable to allow the entry of symbols (data qualifiers).
Symbol: The data qualifier from the list of data qualifiers (symbols) in Symbol Setup.
Numeric Value to Use: Sets what value to use when the Symbol is entered.
| Zero |
0 is used, i.e. <2 is 0, <10,000 is zero. |
| 1/2 Entered Value |
<2 is 1, <10,000 is 5,000. Cannot be used with symbols that do not have a numeric portion (i.e. ND. |
| Constant |
Uses the setting field as a constant numeric value. If setting set to 1,000, >50 is 1,000, >1200 is 1,000... |
| Twice Entered Value |
>2 is 4, >100 is 200. |
| % of Entered Value |
Uses the setting field to calculate a % of the numeric portion entered. If setting is 10, then <100 is 10, <1500 is 150... |
Setting: Used with Constant and % of Entered value to detemine numeric value to use.
EXAMPLES:
Calculate the loading based on a variable that allows data qualifiers
Calculated variables that refer to variables with a < use the value specified by the Numeric Value to use if < Entered field (i.e. the numeric value). The MDL rules for Effluent BOD are shown in the following figure.
The example below shows the calculated values for another variable that is setup to calculate the pounds of BOD (V11 = V1 * V4 * 8.34 where V1 is the Flow, V4 is the BOD).
| V1 |
V4 |
V11 |
Notes |
| 3 |
<2 |
50.04 |
<2 is treated as a 2 (Entered Value) in the calculation: 3 * 2 * 8.34 = 50.04 |
| 3 |
ND |
0 |
ND is treated as a 0 in the calculation: 3 * 0 * 8.34 = 0 |
| 3 |
5 |
125.1 |
Entries with no symbol are treated as the value: 3 * 5 * 8.34 = 125.1 |
| 3 |
>100 |
2501 |
>100 is treated as 100 (Entered Value): 3 * 100 * 8.34 = 2501 |
As you can see, calculations simply use the value specified in the Numeric Value to use fields if a <, >, or ND is entered.
Calculate a value that displays the symbol if the inputs contain symbols
Calculated variables can be setup to Cascade Symbols. On the MDL rules tab, if the variable is calculated it will prompt if you want to cascade the symbol when calculating. Also, you will be prompted if you want to cascade all symbols or cascade just the most frequent symbol. The following table illistrates the different techniques. (Note: V1, V2, and V3 are setup to Allow Symbol and use Entered Value for '<','>', and 'E' symbols. ND is setup as Zero.)
V4 = MVAVG(V1,V2,V3) with Cascade Symbol OFF.
V5 = MVAVG(V1,V2,V3) with Cascade Symbol On with Most Frequent Symbol.
V6 = MVAVG(V1,V2,V3) with Cascade Symbol On with All Symbols.
| V1 |
V2 |
V3 |
V4 (Cascade Off) |
V6 (On - Most Frequent) |
V5 (On - All) |
| 3 |
<2 |
<4 |
3 |
<3 |
<3 |
| 3 |
ND |
4 |
3.5 |
ND3.5 |
ND3.5 |
| <3 |
E5 |
<4 |
4 |
<4 |
<E4 |
| 1 |
2 |
3 |
2 |
2 |
2 |
Calculate the percent removal based on variables that use symbols
Percent Removal is calculated with the formula:
V11 - Influent BOD {mg/L} with Allow Symbol On, < is treated as Entered Value
V4011 - Effluent BOD {mg/L} with Allow Symbol On, < is treated as Entered Value
V4106 - BOD Percent Removal with Cascade Symbol on, equation of (V1 - V4)/V1 * 100
Notice that the symbol was REVERSED in the calculation. Since the Less Than value (<2) was subtracted, this would logically mean that more than 248 mg/L was removed on the 2nd. If you think of the <2 as 1.9 (a value less than 2) then we removed 248.1. 248/250 = 99.20, 248.1/240 is 99.24. Therefore, we know the Percent removal is greater the 99.20.
NOTE: When subtracting or dividing by a value with a < or > symbol the symbol is reversed in the result. If you do not want the symbol in the calculated result, uncheck the Cascade Symbols option in the MDL Rules tab of the variable.
See Also:
For handling of symbols in reports for averages, etc... see GAVGZ or STATZ.
To override the Numeric Value to use setting in a specific calculation, see MDLV.