3 sentences

1. 保持一个恒定的压力。

   bǎo​chí yī gè héng​dìng de yā​lì.

   Maintain a constant pressure.

   Source: OpenSubtitles

2. 它的频率是恒定的。

   tā de pín​lǜ shì héng​dìng de.

   Its frequency is constant.

   Source: OpenSubtitles

3. 光在真空中的传播速率是恒定的。

   guāng zài zhēn​kōng zhōng de chuán​bō sù​lǜ shì héng​dìng de.

   The speed of light in a vacuum is constant.

   Source: Dong Chinese Dictionary