Digit-TLH (Discontinued)

The Digit-TLH is a battery powered temperature, light, and humidity logging device, which collects data in a remote location, and stores it to internal memory.  When finished, the data can be downloaded via the convenient USB connector. Each data point is associated with a time stamp, so you know when it was collected.

Using the provided Otero software, starting, stopping, and downloading from the Digit is simple and easy.  Data is summarized in the program view screens, or can be exported to .csv for further analysis.  Temperature can be viewed in °C or °F, and real-time data is visible when connected over USB.

The Digit also supports field programmed firmware upgrades, just download the latest firmware file from the firmware page, and upgrade using the software package provided.

Digit-TLH Datasheet Additions

This information describes the Digit-TLH's humidity sensor additions and what features users are given on top of the Digit-TL device. This information should be used in conjunction with the information found on the Digit Datasheet page. Due to the Digit-TLH being discontinued, a copy of the original Digit-TL and Digit-TLH datasheet can be found in .pdf form.

Specifications

General

Vented Enclosure (H) IP53

Humidity (TLH only)

*Humidity accuracy can be improved to ±2% with high accuracy calibration chamber.  See Offset and Slope discussion below.

The Digit-TLH uses a Honeywell HCH-1000 to record humidity.  The 10-bit ADC and current source work together on the Digit to record the capacitance value of the HCH-1000, which is then adjusted in software to produce a calibrated humidity reading.  Due to the limited resolution of the on-board ADC, the humidity resolution is limited to about 0.75% humidity per step.

The humidity version of the Digit is called the Digit-TLH, meaning that it is capable of logging all 3 types of data: Temperature, Light, and Humidity. Touching the wires of the HCH-1000 sensor during a reading will cause the reading to be wrong, but this is only an issue when holding the unit in your fingers to plug it in.

The -TLH version of the Digit is factory calibrated to meet the above specifications.  If higher accuracy is necessary, it may be possible to improve calibration by adjusting the parameters listed below. There is typically no need to adjust the factory offset, or slope, they are provided for advanced users to improve accuracy.

There are two main parameters to adjust the calibration of Digit humidity. Use the calibrate humidity section in software.

1. Offset Adjustment: The offset adjustment can be used to change the factory calibration, or even improve on factory calibration for a specific %RH environment.  Saturated (or unsaturated) salt solutions, and certain other chemicals, can be used to generate an environment of a particular relative humidity in an enclosed space.  Create one of these enclosed spaces, and place the Digit-TLH inside until the chamber reaches equilibrium.  It can take many hours for equilibrium to occur, depending on the amount of air, and the effectiveness of the salt solution.  Download the data set from the Digit and compare the %RH result to the %RH of the chamber. Then simply adjust the offset using the CalOffsetH calculator in software.

2. Slope Adjustment: The factory slope can also be changed to increase accuracy.  To effectively alter the slope, 2 different fixed humidity chambers must be available.  After analyzing the raw data, it is possible to determine what the %RH per capacitance slope should be.  Use the Hslope calculator in software to adjust the slope.

To verify correct calibration, simply download the data after new settings are stored, and software will use the new calibration adjustments. The calibration images below illustrate humidity data collected by nine different Digit-TLH devices in two fixed-humidity reference chambers. The single-point calibration shows the factory calibration quality.

The factory calibration is a single-point calibration adjustment, but higher accuracy can be achieved with a two-point calibration as demonstrated in the image below.

Dew Point (TLH only)

The dew-point temperature can be calculated from the relative humidity and temperature.  The dew-point temperature is the temperature to which the air must be cooled to reach saturation. Basically when the temperature cools to the dew point, fog or dew can occur, because ambient water vapor will condense. A well-known approximation used to calculate the dew point, given just the air temperature, T and relative humidity (in percent), RH, is the Magnus formula:

Dew Point = (243.12*[ln(RH/100)+( (17.62*T) / (243.12+T) ) ] )  / (17.62 - [ln(RH/100)+( (17.62*T) / (243.12+T) ) ] )

RH = Relative Humidity (in percent)

T = Temperature (in Celsius)

Spreadsheet program users can copy and paste the above formula into their spreadsheet software to generate a column for dew point (substituting RH and T with corresponding cell contents). Alternatively, use an online calculator. There are some other more complex ways to calculate dew point, see the Wikipedia Dew Point topic if interested.

Logging Information

Battery life

Logging at a rate faster than one reading per minute doesn't decrease battery life dramatically, but it does have a small impact. Take note of the table below if you are using the Digit in hot temperatures. 

*3 year lifespan calculated: 140mAh / 0.0048mA = 31818hr = 3.33yr

Notice that at cold temperatures the power consumption is actually lower than at room temp, but some batteries have reduced overall capacity at low temperatures, which can offset the reduction in power usage. High temperatures have the most dramatic impact on battery life, as the processor and peripherals simply consume more power when hot.

Memory

Memory will not run out for at least 15 days, at a log interval of 10 seconds, while logging temperature, and light.  The memory lasts much longer if you select fewer log items, and change the logging to happen less frequently.  For example, if you log temperature and light once per minute, the memory will last for 90 days.

Ex: 260,000(readings) / 2(log items) / 1(per minute) / 60(minutes) / 24(hours) = 90.277 days of memory

Reference the table below to find how long it will take to use up memory space.

When the Digit runs out of space to store new readings, it will not wrap-around and erase older data, it will simply not store new information. When the Digit reaches 262,144 stored readings, the very last data point is repeatedly overwritten.

Files

Digit-TLH Datasheet

Digit Enclosure Assembly - How to avoid damaging humidity sensor