Measuring temperatures

Introduction

TiePie measuring instruments can be used to monitor slowly changing phenomenons over a longer time, like e.g. temperatures. This measurement example shows how the Multi Channel oscilloscope software can be used to measure temperature over a longer time and how it is configured to show the temperature directly, instead of a voltage.

Used equipment

This example uses a Handyscope HS3 as measuring instrument. The Handyscope HS3 is designed to measure a voltage. In order to measure a temperature, a temperature sensor is required that converts temperature into a voltage that can be measured by the Handyscope HS3.

This example uses a PT100 temperature sensor and corresponding adapter, that converts the signal from the sensor into a usable voltage.

This sensor and adapter are not available from TiePie engineering.

Temperature adapter

Setup the software

The temperature adapter provides an interface between a temperature probe and the input of the measuring instrument. The specifications of the adapter state that the temperature range of the adapter runs from -40°C to +250°C. The output voltage range runs from -40 mV to +250 mV. This implies that for each °C temperature change, the output voltage changes 1 mV and when the temperature is 0°C, the output voltage is 0 mV.

Converting the measured values

When displaying the signal of the temperature adapter directly, the oscilloscope will display volts, not °C. To display the measured temperature with unit °C, the measured values need to be converted.

To convert the measured values, a Gain/Offset I/O is used. Create it by right-clicking on I/Os in the object tree and selecting Gain/Offset.

Set the gain

As indicated before, the temperature adapter has an output sensitivity of 1mV/°C. To display the measured values directly in °C., they have to be multiplied by 1 / 0.001 = 1000. Right-click the Gain/Offset I/O and select Gain → User defined.... There is no need to calculate the proper value for the gain, the Gain/Offset I/O can do this by itself, so simply enter "1000", "1 / 0.001" or "1/1m".

Enter gain

Set the offset

In this example, the temperature adapter has an output voltage of 0 mV at 0°C, which means that no compensation is required here.

However, when the adapter would have a different output voltage at 0°C, this offset would have to be compensated. Suppose the output voltage at 0°C would be 500 mV, in that case the Gain/Offset I/O would have to compensate for this offset by subtracting 500 mV from all measured data. To achieve that, right-click the Gain/Offset I/O and select Input offset → User defined.... Then enter "-500m" to compensate for the offset of +500 mV.

Enter offset

Set the unit of measure

The measured values are now converted to the proper range, but the oscilloscope still shows Volt and not °C. To change this, right-click the Gain/Offset I/O and select Set Unit... and enter "°C".

Enter unit of measure

Set the alias

To simplify recognition of the various signals that can be displayed, it's possible to assign a name (alias) to an I/O. Right-click the Gain/Offset I/O and select Set alias... and enter "Temperature".

Enter alias

The Gain/Offset I/O is now fully configured to convert the measured voltages to a temperature. Connect the temperature adapter to Ch1 of the Handyscope HS3. Then, in the software, connect Ch1 to the Gain/Offset I/O by dragging and dropping in the object tree.

Measure over a longer time

Measuring temperature changes usually involves a relative slow but long continuous measurement. To achieve this, a measurement in streaming mode is required.

In streaming mode, a continuous measurement is started. During this measurement, chunks of measured data are transferred to the computer periodically. These chunks of data are contiguous, there is no time gap between them. The software can then place these chunks after each other, combining them to one continuous large record of measured data.

Setup streaming mode

To set the instrument to streaming mode, first make sure the instrument is not measuring by setting it to pause. Then click the Measure mode button Block mode / Streaming mode on the instrument toolbar. When the instrument is started now, it will generate a continuous stream of uninterrupted data.

Setup the time base

In streaming mode, the instrument will sample at a predefined rate and each time when a certain amount of samples are measured, this chunk of data is transferred to the computer. Temperature measurements are relatively slow and therefore we do not need to measure very fast, one measurement per second is a suitable number.

In this example, the data chunk size is set to 512 samples and the sampling rate to 512 Hz. This gives one chunk of data per second. The data chunk size is controlled by the record length. It can be set by clicking the record length indication on the instrument toolbar and selecting a value. The sampling rate can be set by clicking the sampling rate indication on the instrument toolbar and selecting a value.

Reducing the data stream

We now get a chunk of 512 samples each second, which we want to reduce to one sample per second. The Resampler I/O can be used for that. Create a Resampler I/O by right-clicking I/Os in the object tree. Connect the Gain/Offset I/O to the Resampler I/O by dragging it on the Resampler I/O.

Set the resampling ratio

The resampler will have to reduce the data stream by a factor 512. Right-click the resampler I/O and select Out/In ratio → User defined... and enter "1/512".

Enter ratio

Set the resampling method

The Resampler has two ways of resampling the input data. In Normal mode, the resampler takes the first sample in the chunk and uses that for output value. In Linear mode, the resampler takes the average value of all samples in the chunk and uses that for output value. This automatically gives low pass filtering on the data, reducing possible noise on the signal. This example uses linear mode, which is selected by right-clicking the Resampler I/O and then selecting Method → Linear. We now have one temperature measurement per second.

Collecting the data

To collect all (reduced) data from the stream, create a Data collector I/O by right-clicking I/Os and selecting Data Collector. A data collector receives chunks of streaming data and "glues" them together, forming one large record of continuous data.

Connect the Resampler I/O to the data collector by dragging it onto the data collector. The amount of data a data collector can contain can be set, in our example we use 2000 samples. Right-click the data collector and select Data size → 2000 from the popup menu.

Create a new graph and drag and drop the data collector on the new graph.

Temperature measurement