Chart Recorders – Temperature and Humidity

Chart recorders are microprocessor-based parameter logging devices often used to depict differential changes in reading. Chart recorders have a wide range of industrial applications, and amongst them, temperature and humidity chart recorders are the most popular. The advantage of circular chart recorders over digital meters is that these recorders can visually represent temperature or humidity changes on a circular chart in real-time. In contrast, a digital display can indicate and store the readings at a particular time. The recorders can transfer the data via analog (4-20mA or mV) and/or digital protocols (RS485 or TCP IP).

Applications of Temperature chart recorders include food processing, pharmaceutical, manufacturing, environmental applications, and wastewater treatment plants. In the food processing industry, chart recorders are used to measure and record thermal readings according to FDA storage maintenance requirements. Manufacturing sectors should avoid production lines “soaking” or storage of materials. Storing raw, by-product, or finished products at out-of-tolerance environmental conditions can lead to production inconsistencies or potentially lead to catastrophic failures.

Circular Chart Recorders – Operational Principle:

Thermocouples or RTD sensors that measure temperature are connected to the measuring environment. These sensors output the measured temperature reading as process signals, fed as input signals to the Circular Chart recorder. The signal is then processed and converted internally by the chart recorder using microprocessors, and the temperature readings are continuously depicted on a circular chart.

Conventional thermal gauges are mounted close to the measuring environment. They are often prone to thermal distortions and inaccurate readings, especially when used for a prolonged period without proper calibration and maintenance practices. In the case of chart recorders, due to the increased distance between the measuring environment (or the thermocouple/sensor), these are more resistant to thermal or humidity-related expansions and thus maintain the device accuracy throughout its service life.

Circular Chart Recorders – Types and Selection Criteria:

Based on the design and application requirements, Circular chart recorders either use a single or dual pen “hot stylus” set up to depict the parameter reading on the chart. A single pen can print one parameter on the chart, and a dual stylus set-up can accommodate two parameters simultaneously (ex: Temperature and Humidity variations on a single chart). Modern chart recorders are capable of erasing the data, and hence, they eliminate the need for maintaining multiple circular chart inventory.

Chart recorders are designed and certified for certain harsh applications with NEMA 3 or NEMA 4 protection grade, making them robust for harsh environments. Chart recorders of late contain an alarm output, which can be integrated with the third-party alarms or existing PCBs.

Maintenance Guidelines for Circular Chart Recorders:

1. Signal Noise suppression: Good wiring practices enhance noise protection of the chart recorders and maintain reading accuracy. On a chart recorder, the magnitude of electrical noise at the input is so high and suppressed at the source. This can be achieved by using “surge suppressors” mounted directly on the noise source or with resistance-capacitance (RC) circuits and/or voltage limiters.
2. Ferrite Filters kits: These kits act as a choke to suppress high-frequency electronic noise within the electronic circuits of the chart recorder. Hence, suppression filters must be installed on all wires connected to the chart recorder in one filter per circuit group order.
3. While accessing the internal electrical components of the chart recorder (PCBs or Communication channels), care should be taken to terminate the power supply before accessing the components to avoid the risk of electric shock.
4. Wrist straps used while repairing chart recorders should be connected to an earth group to avoid damage caused due to electrostatic discharge.
5. OEM Guidelines should strictly be followed to inspect the motor, replacing the membrane switch, removing/installing the display PCBs or electrical assemblies within the chart recorder.

Need for Chart Recorder Calibration – Why is it essential?

Digital Chart recorders require Input and Output calibration to be performed at regular intervals. The calibration frequency varies with the use case and the operator’s skill level.

Input calibration:

As the name suggests, input calibration focuses on maintaining the design accuracy of thermocouples, RTD, and input resistors. Why input calibration is essential for chart recorders:

• Thermocouple or RTD Should meet design accuracy. Generally, factory calibration is set at ± 1.2 °F plus ± 10 microvolts for thermocouples or ± 0.3 ohms for RTDs. When the reading exceeds this tolerance range, calibration is necessary. Thermocouple calibration can be performed using an ice bath, compensated calibrators, or manually measuring the ambient temperature at the input terminals and subtracting the voltage contribution of the input terminal junction.
• If the accuracy of the input resistor network is observed to be beyond the tolerance limit of ± 1.0 % for a 0-10VDC input, calibration is needed.
• Due to operational errors, recorder readings are not consistent, leading to erroneous readings or confidence in temperature measurement. Re-calibration is required for such chart recorders.
• Chart recorder re-calibration is essential after performing servicing activities such as replacing the input PCB.

Output calibration:

Output calibration focuses on Current output, Position proportional output, and auxiliary output.

• Current output: Calibration is required for the recorder to provide the proper amount of current over the desired range. The recorders generally provide an output current range of from 3.1 to 21 milliamperes and can be calibrated at 4 mA for 0 % of output and 20 mA for 100 % of output or any other values between 2 and 3.1 mA.
• Position proportional output: When the chart recorder has a position proportional control output, the recorder should be calibrated so that the increase and decrease relays operate properly with respect to the position of the external feedback slidewire.
• Auxiliary output calibration: The recorder needs to be calibrated so that the auxiliary output provides proper current over the desired range. A standard milliammeter (with a 0-20mA measuring range) is capable of performing this calibration.

Printing stylus calibration:

Generally, there is no need to adjust the chart printing characteristics as part of an input or output calibration procedure. However, calibration can be performed if the chart printing appears to be faded.

Calibration of chart recorders includes accessing hazardous live electric circuits and should only be performed by authorized and qualified service personnel. Also, more than one switch may be required to de-energize the chart recorder unit before the calibration. There, chart recorder owners can avail themselves of certified e2b calibration services to calibrate their equipment.

Though operating a circular chart recorder seems straightforward, the equipment needs to be calibrated and maintained as per the OEM operation guidelines. To comply with international standards, an annual input and output calibration process should be carried out to ensure repeatability and accuracy of circular chart recorders.

e2b calibration offers industry-leading ISO-certified chart recorder calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your recorders. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.