Onset HOBO User Manual

Chapter 4: Modules
HOBO Energy Logger Pro User’s Guide 17
TRMS (TRMSA) module
The TRMS (true root mean square) module (S-FS-TRMSA) is a configurable, two-channel input measurement
module for the HOBO Energy Logger Pro. It has an input range of 512 mV RMS full scale, making it fully
compatible with industry-standard current and potential (voltage) transformers (CT and PT), which output 333 mV
RMS full scale.
The TRMS module features extremely low-power operation, resulting in long battery life for unattended data
logging applications.
Sensors connected to the TRMS module can be configured in HOBOware Pro software. Configuration options
include channel names and scaling parameters. Sensors are connected to the module via a four-pin Phoenix-style
detachable screw terminal connector.
CAUTION: This device requires the use of CTs that
incorporate internal burden resistors. Do not use with
unburdened CTs!
Functional block diagram
Using the module
Set up the TRMS module as described below. When logging begins, the logger will acquire and store measurements
from the sensors attached to the module.
Important considerations
The input to the TRMS module is AC coupled. Moreover, the AC coupling capacitor effectively blocks any DC
component, and the RMS converter detects a waveform where the volt-sec area above ground equals the volt-sec
area below ground. This can cause seemingly incorrect readings when signals such as full-wave or half-wave
rectified sine waves are applied to the inputs.
The TRMS Module is based on a unique delta-sigma RMS-to-DC converter, allowing accurate measurements of
waveforms with crest factors up to 4 without user calibration. However, due to the nature of the delta-sigma RMS
conversion, input voltages near zero result in unstable output voltages. Therefore, the TRMS module will truncate
input signals below 5 mV to zero.
Finally, when using inputs with high crest factor, it is important not to violate the maximum ±1 V input. For
example, a 300 mV RMS signal is limited to crest factors under 3.3:
3.3
3.0
1
≈
V
V

Onset HOBO Specifications

General

Battery Life	Depends on model. Typically 1 year, but can be several years depending on logging rate, parameters logged, and other factors.
Humidity Range	0% to 100% RH
Data Download	USB, Bluetooth
Logging Rate	1 second to 18 hours (configurable)
Dimensions	Varies by model. Check specific model specifications.
Weight	Varies by model. Check specific model specifications.
Waterproof	Yes (IP67 rated for some models)
Accuracy	±0.2°C (temperature), ±2.5% RH (humidity, varies by model)