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

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.

There are two different hardware versions for the Digit-TL/TLH. The primary difference is that the current version has an easily replaceable CR1632 battery, and the old version has a solder mount battery which required a soldering tool for replacement.  Hardware version 1.11 is no longer in production, as v1.20 is a fully compatible improvement.  They are both compatible with Otero(software), and should be considered interchangeable.

Hardware version 1.20 on the left, and v1.11 on the right, the table below describes the notable differences.

*  The clear plastic enclosure includes both a vented and watertight base option. Unlike the aluminum enclosure, the clear plastic enclosure allows the device to collect light readings. The rugged aluminum enclosure can be purchased as an add-on item.

** Both hardware versions fit inside the aluminum and clear plastic enclosure variants, but the battery holder makes HW 1.2 a bit too large to fit both directions in the aluminum case. The aluminum enclosure is watertight and airtight, so response time of the Digit temperature readings is not impacted by orientation within the case.

Some comments about old hardware (v1.11)...

The battery in HWv1.11 was a 120mAh Panasonic BR-1632A, and can be found on DigiKey, part number P022.  With a soldering tool it is possible to both replace the battery, or install a different battery.  

Note: If extreme harsh temperatures are expected to last for a long time, a custom battery can be installed to handle the conditions. Acceptable battery voltage range is 3.0V to 3.6V.  Our top pick is a Tadiran TL-5101/P, which will work continuously for more than 3 years at even the most harsh of conditions (-55C to +85C).  The Digit shown below is stored inside our office refrigerator.

On old Digit-TLH devices, the humidity accuracy below 10°C was subject to battery voltage.  Often times the battery voltage would drop at cold temperatures, and the humidity readings were low if the battery voltage was less than 2.73V. Humidity readings on new hardware (v1.2+) operate normally over the full temperature and battery voltage range.

When the Digit is connected to a powered USB port, battery usage is suspended.  Therefore, it is possible to prolong battery life by connecting the Digit to a 5V USB adapter, or USB hub when it's not being used.  Another good method to prolong battery life is to disable all log items when the Digit is not in-use.  When nothing is being logged, it stays in a low-power sleeping state.

Use the instantaneous readings to grab a reading from each of the sensors while the Digit is connected via USB. The conversion equations used to convert each of these from a raw representation are described below. Alternatively, download the C++, or LabVIEW example code attached at the bottom of this page.

Converting raw temperature data

Temperature on the Digit is stored as an unsigned 12 bit value, this value needs to be converted into a signed number, and multiplied by a conversion factor before it will look like degrees C. The raw value is a 16-bit number, the 4 lowermost bits being system flags.

Raw Temperature 16-bits:
Xxxx xxxx xxxx rrrr

r's are reserved for flags.
x's are the temperature information.
X is the most significant temperature bit, discussed in step 2.

To convert raw temperature into °C:

1. Remove/Process the lowermost 4 bits (if non-zero). See flag descriptions below.
2. Decide if positive or negative by looking at the most significant bit.
   If the MSB is 1, temperature is negative,
   If the MSB is 0, temperature is positive.
3. (For negative temperature, MSB == 1)
   Subtract 1 from the value.
   Invert every bit of the value (change 0s to 1s, and 1s to 0s).
   Perform a logical right hand shift of the value by 4 bits.
   Multiply the value by -1, to get the sign right.
4. (For positive temperature, MSB == 0)
   Perform a logical right hand shift of the value by 4 bits.
5. Multiply the value from step 3 or 4 by the conversion factor: (0.0625 °C/count)

The TMP112 datasheet also explains this process. See page 8: http://www.ti.com/lit/ds/sbos473b/sbos473b.pdf

Invalid Measurement = 32768
If the battery is dead, the TMP112 has failed, or a communication line between the temperature sensor and the microprocessor is damaged. the Digit will return a value of 32768. A value of 32768 converts into -128C, but normal operation will never return -128C, so it's a unique indication of an invalid reading.

Flags in the temperature

bit0 (0x01) = Warning Detected
bit1 (0x02) = Power Failure Detected
bit2 (0x04) = Reset Detected
bit3 (0x08) = 'On USB' flag, to indicate processor heat influence

Converting raw light data

Raw light readings are saved as a number of timer counts in memory, ranging from 18-65535.

Raw Light 16-bits:
xxxx xxxx xxxx xxxx

In software the calibrated light value is generated through interpolation along a 3D surface (with some simplification). The temperature (to nearest 1°C) at the time of the light reading is used as an index, then the Digit's raw light value (timer counts) points to a region in-between Lux values, so interpolation is performed between the nearest 2 points in Lux.

Due to the simplification of 1°C resolution, the interpolation is merely done on a 2D surface, the whole 3D surface does not need to be analyzed in software.

To convert raw light into Lux:

If converting raw light readings sounds overwhelming, don't worry too much-just look at the C++ example code attached on this page. The 3D calibration surface is simply a header file[LightCalibration.h] with 3 static arrays. The light conversion code uses temperature as the first lookup index in the header file, which reduces the three large(1792 datapoint) arrays down into two 14 datapoint arrays that represent possible values of calibrated lux and raw light counts at the given temperature.

Next, the raw light reading (DGT_LIGHT_RAW) is compared to each of the raw readings in the 14 datapoint array that contains raw values. Simple linear interpolation between the two nearest raw values will yield a fractional index of where the raw reading resides relative to the available calibration points. Finally, this fractional index is applied to interpolate between the nearest Lux values which yields the light reading in Lux.

Invalid Measurement = 1
If the Digit is plugged/un-plugged from USB during a light reading in-progress, the reading is interrupted, and a 1 is saved instead of the timer value. A value of 1 converts to roughly 12000 Lux, but normal operation will never return a value of 1, so it's a unique indication of an invalid reading.

Converting raw humidity data

Humidity readings are performed by an HCH-1000, which is a capacitive humidity sensor. A constant current source and a 10-bit ADC on the Digit-TLH work together to read the capacitance value, which is the raw humidity reading.

Raw Humidity 16-bits:
rrrr xxxx xxxx xxxx

x - measurement of raw capacitance (humidity)
r - reserved.

Master Equation

%RH = (DGT_HUMIDITY_RAW - DGT_HUMIDITY_CAL_OFFSET + ( (DGT_HUMIDITY_CAL_T_SLOPE)(DGT_TEMPERATURE_LATEST_RAW) -
 (DGT_HUMIDITY_CAL_T_SLOPE)(-38) )(DGT_HUMIDITY_CAL_SLOPE)

To convert raw humidity into %RH:

1. Read the current temperature DGT_TEMPERATURE_LATEST_RAW*.
2. Read the values of the CAL registers listed above.
3. Read DGT_HUMIDITY_RAW.
4. Use the "Master Equation" to calculate the %RH humidity.

*In the case of downloaded data, use whatever temperature reading happened at the same time as the humidity reading (adjacent in memory).

Invalid Measurement = 1
If the Digit does not have a humidity sensor, or the humidity sensor is broken, the raw humidity reading will be a value of 1. Normal operation will never return a value of 1, so it's a unique indication of an invalid reading.

There is also a command (DGT_HUMIDITY_CAL_I_SOURCE) to initiate calibration of the constant current source, which is used during factory calibration, but it is not recommended to write to this register, unless you plan to re-calibrate the other parameters.

Everything Else

Use the _DATASET, and START_TIME parameters to interpret downloaded data. Logged data must always be saved in one of the following sequences: T, TL, TH, THL, so downloading code simply reads the logged items, and interprets data space in segments of unsigned 16s. There are no timestamps saved with the data. Instead, use the DGT_LOG_START_TIME and the DGT_LOG_INTERVAL_INDEX_DATASET to determine when data was collected.

Change the configuration of the Digit with the following registers. Changes are applied when data is erased with DGT_FLASH_bkERASE. It is recommended that the Digit RTC is synchronized with the host computer with DGT_RTCC_TIME immediately prior to applying changes to configuration (calling DGT_FLASH_bkERASE).

Alarm thresholds, changing factory settings, and upgrading firmware are not covered in this doc. If you need information about these topics please contact [email protected] for more details.